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post #1 of 84 Old 06-15-2017, 10:28 PM Thread Starter
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PCM swap problem

I was having spark advance issues so I swapped the pcm from a '96 Blazer into my '96 SS. It solved the advance issue and the tramsmission provides a nice kidney punch when it shifts. The problem is that the abs and brake light on the dash are on. I tried to clear them with my scanner but no luck. Any ideas?



And no.....black tape won't work...lol


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post #2 of 84 Old 06-15-2017, 11:12 PM
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Re: PCM swap problem

I'm Not sure on the older black pcm but on the newer ones they need to be vin matched,a while back my friend pulled a junk yard pmc and sent it to wait4meperformance, when he told them that it wasn't from his blazer on the phone they said they needed to encode it for his vin or something. You might want to try calling them i think they don't even do mail in anymore and just do a core charge.
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post #3 of 84 Old 06-15-2017, 11:21 PM
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Re: PCM swap problem

^^^ sounds like a passkey thing your talking about.

I know some (most actually) vehicles with ABS modules do communicate with the PCM. I had the same scenario with an '05 saffari but I replaced the ABS module/controller and it had to be programmed. The ABS light on that van was actually controlled thru data lines to the cluster from the PCM. I can't imagine that your 96 works the same way but possible. Reflashing may be the cure.
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post #4 of 84 Old 06-15-2017, 11:25 PM
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Re: PCM swap problem

Try putting the old PCM back in and see if the ABS and Brake light go out. Could just be a coincidence that they came on at the same time. A common problem, as you know, is the soldering on the boards in those EBCMs. That will make both brake lights come on. I would think any problem with the PCM would only cause the ABS light to come on. But you never know? Does your scanner give any ABS codes?
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post #5 of 84 Old 06-15-2017, 11:33 PM Thread Starter
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Re: PCM swap problem

EBCM? are you talking about body control module? I don't have one. No codes. It runs stronger than ever since the pcm swap. I'll try swapping it out again and see if the lights go away. If worse comes to worse ill find someone that can either fix the light issue, or see if they can figure out why my spark advance is so rtetarded....literally. I don't have the expertise do do a 411 swap or else I would consider it.

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post #6 of 84 Old 06-16-2017, 09:53 AM
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Re: PCM swap problem

EBCM = Electronic Brake Control Module.
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post #7 of 84 Old 06-16-2017, 10:53 AM
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Re: PCM swap problem

I bet its just a single pin difference for the brake input wire between the blazer and the truck. putting the original pcm back in might confirm this.
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post #8 of 84 Old 06-16-2017, 01:46 PM Thread Starter
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Re: PCM swap problem

Quote:
Originally Posted by manbearpig View Post
EBCM = Electronic Brake Control Module.
Is this part of the abs unit?

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post #9 of 84 Old 06-16-2017, 01:48 PM Thread Starter
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Re: PCM swap problem

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Originally Posted by joedoh View Post
I bet its just a single pin difference for the brake input wire between the blazer and the truck. putting the original pcm back in might confirm this.
I'm going to swap them out again to check. The abs harness has it's own connector on the pcm. There are four pins in the connector...I'll check the connection pin count.

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post #10 of 84 Old 06-16-2017, 03:07 PM Thread Starter
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Re: PCM swap problem

I was wrong on the pin count...both have five pins, and the old pcm made the lights go away. That suggests to me the problem is the pcm. I'll check with the dealer and see what a fresh program d/l would cost. For the time being...electrical tape it is over the lights...lol.

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post #11 of 84 Old 06-16-2017, 03:36 PM
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Re: PCM swap problem

sounds like you got a 97 pcm. scroll down to the 96-97 piece at this link.

http://www.currentperformance.com/s1...custom-wiring/
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post #12 of 84 Old 06-16-2017, 04:11 PM Thread Starter
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Re: PCM swap problem

I mis spoke about the pins....both ecm's have 5 pins, the harness however only uses three of them.

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post #13 of 84 Old 06-16-2017, 06:48 PM
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Re: PCM swap problem

for clarity I wasnt basing the 97 pcm thing on your pin count, I was basing it on that link I posted. the 96 pcm has abs functions built in, the 97 pcm will plug in but doesnt have abs functions. a follow up search says your issue, abs and brake lights on, is likely a problem from having a 97 pcm installed in a 96, because others have had the same issue.

I know you probably know this but its worth saying, a production date of say >6-96 will actually be a 97 model year vehicle, they start selling the new models in august. I owned a unicorn once too, a 98 4 cyl, the column wiring is slightly different by a couple pins and it sucks trying to find a gauge cluster. I tried swapping to a tilt column and had to reuse all the turn switch and ignition harness.
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post #14 of 84 Old 06-16-2017, 07:11 PM Thread Starter
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Re: PCM swap problem

I'm at a loss why the 97 pcm would have the connection for the abs plug. I'll get if figured out. The dealer was less than helpful. They won't program unless the vin matches. What happens if the pcm isn't repairable...what do they use then? The biggest problem is there doesn't seem to be much support for the black box, and I don't have the skills to swap to the 411

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post #15 of 84 Old 06-16-2017, 08:33 PM
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Re: PCM swap problem

this is what I think you are asking.

Quote:
The ’97 truck uses the same computer as the ’96, but it has a separate computer that controls the ABS which is a different unit than the ’96. The ’97 truck needs the 5th (small) connector from the stock harness retained when the stock truck computer is kept. This connector can simply be removed from the stock harness and plugged in when the new harness arrives. The 5th connector has a single wire in it that simply loops around from one hole to another and pretty much does nothing except tell the computer that it’s there.
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post #16 of 84 Old 06-16-2017, 11:11 PM
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Re: PCM swap problem

Gotta love engineers.....
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post #17 of 84 Old 06-16-2017, 11:38 PM Thread Starter
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Re: PCM swap problem

Says who....

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post #18 of 84 Old 06-17-2017, 08:21 AM
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Re: PCM swap problem

Do the two pcms have the same part number?
What scan tool do you have?
Have you tried the ss pcm on the blazer?
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post #19 of 84 Old 06-17-2017, 08:50 AM Thread Starter
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Re: PCM swap problem

I don't believe the part numbers are the same. I'll post pics later today. The engine in my truck came from a 96 Blazer along with all of the accessory parts. Now that I think of it, I have the harness also. I'll check the abs connector on it also. Headed down to Vancouver in a little bit and will check when I return later today. My scan tool is from Harbor Freight. I didn't feel the need to break the bank on a scanner. It does abs...just not mine...

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post #20 of 84 Old 06-18-2017, 12:54 AM
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Re: PCM swap problem

Quote:
The ’97 truck uses the same computer as the ’96, but it has a separate computer that controls the ABS which is a different unit than the ’96. The ’97 truck needs the 5th (small) connector from the stock harness retained when the stock truck computer is kept. This connector can simply be removed from the stock harness and plugged in when the new harness arrives. The 5th connector has a single wire in it that simply loops around from one hole to another and pretty much does nothing except tell the computer that it’s there.
Typical GM, in 98 they removed it. So that 5th connector was a 1 year only thing. Only the 97 had the 4 big (red,blue, white, and black connectors and the little jumper wire.

EBCM is the brainbox part of the ABS pump that the wiring connects to.
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post #21 of 84 Old 06-20-2017, 09:06 AM
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Re: PCM swap problem

I was working on the wiring on my 98 ZR2 6.2L swap this morning and had a thought about your problem. On the black box PCM's the knock sensor is actually two parts. The sensor on the block and the sensor module which is located in the bottom of the PCM. It is a removable chip under a small cover held on by 2 T-20 machine screws. If yours went bad it could cause the timing to retard as much as 30°. Since swapping the Blazer PCM cured the timing issue, but caused an ABS problem, try taking the knock module out of the Blazer PCM and pop it into the truck PCM. If it fixes the problem, fine. If not, at least it is worth a try. The KS module is essentially a filter for the AC signal riding the 5 volt KS reference voltage. It is actually only 1.3 volts when no knock is detected.

I became aware of this when I found out I couldn't just reprogram my 4.3 PCM to the 5.7 program. I also had to find a 5.7 knock sensor module because they are like the knock sensor and have to be matched to the specific engine. Some are the same. Some aren't. In my case both were different in 98 on the 4.3 vs the 5.7. I ended up buying a junk 16250279 PCM off ebay just to take the KS module out of it because no one still makes the correct 5.7 module. 4.3 modules are available all over the place for $35-40. I guess I did OK because the junk PCM only cost me $20 including shipping. Actually the PCM could be OK, it just sat outside a couple years so the guy sold it as is. Actually looks pretty clean.

You said that you were going to check the pinouts. Do you have them on paper or were you just going to compare the two? lextech could send you a printable one.
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post #22 of 84 Old 06-20-2017, 09:58 AM Thread Starter
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Re: PCM swap problem

Now that's an interesting thought. It looks like the pcm locks into the bracket. I tried removing the pcm from the bracket just for the fun of it, but it must need a magic password because I didn't have any luck. What's the trick?

I was just going to compare the two for position/color.

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post #23 of 84 Old 06-20-2017, 11:18 AM
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Re: PCM swap problem

You have to remove the bracket then the PCM snaps out of the bracket from underneath. Well that's how my 98 comes out.
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post #24 of 84 Old 06-20-2017, 11:37 AM Thread Starter
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Re: PCM swap problem

That's what I thought...no copy...I'll take another crack at it

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post #25 of 84 Old 06-20-2017, 12:26 PM Thread Starter
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Re: PCM swap problem

Well...there is a difference in the harness.The white plug is my harness, and the gray is from the Blazer pcm that's in the truck now. I think the easiest solution would be to swap the kc chip as you suggested...




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post #26 of 84 Old 06-20-2017, 12:39 PM Thread Starter
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Re: PCM swap problem

I found that if you try hard enough, you can break the locking clip it off of the bracket.lol. The good news is I did separate the bracket and the pcm and removed the knock sensor module. Going back down to try and not break the locking clip on the other bracket and swap the module....we'll see.

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post #27 of 84 Old 06-20-2017, 01:19 PM Thread Starter
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Re: PCM swap problem

It was a nice thought...but it didn't work. Blazer chip in the SS pcm...still retarded no lights. SS chip in the Blazer pcm..advance is 20* lights still on. Headed to Home Depot for electrical tape to cover the lights until this gets resolved...lol

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post #28 of 84 Old 06-20-2017, 03:23 PM
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Re: PCM swap problem

Look up spareecm on ebay. I bought a used PCM from them a few years ago. They program to your VIN so everything works right. IIRC it cost $80.
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post #29 of 84 Old 06-20-2017, 05:11 PM
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Re: PCM swap problem

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Originally Posted by Rhotpursuit View Post
It was a nice thought...but it didn't work. Blazer chip in the SS pcm...still retarded no lights. SS chip in the Blazer pcm..advance is 20* lights still on. Headed to Home Depot for electrical tape to cover the lights until this gets resolved...lol


I think maybe you just proved your replacement pcm is a 97 pcm, the abs is built in the 96 pcm and uses the extra wires. the 97 pcm the abs is external to the pcm and just has a loop between two pins. that 5th plug on the 97 was just a jumper between two pins according to the link I posted above.
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Re: PCM swap problem

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Look up spareecm on ebay. I bought a used PCM from them a few years ago. They program to your VIN so everything works right. IIRC it cost $80.
Thanks...I'll take a look. I know it's been awhile, but was I mistaken when I bought the motor and was told it was a '96? Also...if you get a chance, could you pull the 5th plug on your pcm and see how many wires are in it and where they're positioned?

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post #31 of 84 Old 06-20-2017, 06:45 PM
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Re: PCM swap problem

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Well...there is a difference in the harness.The white plug is my harness, and the gray is from the Blazer pcm that's in the truck now. I think the easiest solution would be to swap the kc chip as you suggested...

The plug on my 97 looks the same as this one.
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post #32 of 84 Old 06-20-2017, 07:42 PM Thread Starter
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Re: PCM swap problem

I started looking at the eBay site and the part number confirms it's a '97. Once I get the correct pcm, do you want this one as a backup?

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post #33 of 84 Old 06-20-2017, 08:17 PM
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Re: PCM swap problem

Yuup, you definitely have 2 different years there. The one with 2 pins (C5 C&D) is 97 only. After that there was no C5 connector. As far as what year the engine was, other than the semi-external ABS both engines are the same.
What serv. no is the 96 computer? Should be either 16244210 or 16208546. Unfortunately I don't have either in my stash.
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post #34 of 84 Old 06-20-2017, 08:23 PM Thread Starter
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Re: PCM swap problem

16244210 is the '96

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post #35 of 84 Old 06-20-2017, 09:06 PM Thread Starter
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Re: PCM swap problem

I just ordered an ecm from Spareecm on eBay. Should be here monday. I guess the tape will stay on the dash until then..

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post #36 of 84 Old 06-21-2017, 11:59 AM
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Re: PCM swap problem

Quote:
Originally Posted by Rhotpursuit View Post
I started looking at the eBay site and the part number confirms it's a '97. Once I get the correct pcm, do you want this one as a backup?
It's a long story, but I actually already have two extra ones. lol

Quote:
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I just ordered an ecm from Spareecm on eBay. Should be here monday. I guess the tape will stay on the dash until then..
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post #37 of 84 Old 06-21-2017, 12:06 PM Thread Starter
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Re: PCM swap problem

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Originally Posted by JustAnSS View Post
It's a long story, but I actually already have two extra ones. lol



It will be on the shelf should you need it.

On another note...was down in the 'couve last weekend. My son is having a new house built north and east of 119th & 78th. Should be done by the end of august.

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post #38 of 84 Old 06-23-2017, 07:46 PM Thread Starter
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Re: PCM swap problem

Well...I got the new ecm today...installed it and checked spark advance.... -20. Can someone with a 96 4.3 check spark advance and get back to me with the numbers? something just doesn't seem right that it would be that far retarded, when all of the other s10's I've checked are on the + side of the scale.

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post #39 of 84 Old 06-23-2017, 11:11 PM
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Re: PCM swap problem

Are you reading the spark advance with your scanner? Is the truck running that poorly? A 40° swing should kill the power.
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post #40 of 84 Old 06-23-2017, 11:23 PM Thread Starter
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Re: PCM swap problem

Yes it's with the scanner. And just to verify, I used a friends scanner and it read the same thing. Those numbers are not static...they fluctuate depend in throttle position. We also tried both scanners on his '03 s10 and the advance was on the positive side 17-30*, again depending on throttle position. Same with my wife's '02 Blazer. I'm at a complete loss. When I put the new engine in my truck, I had the transmission rebuilt with a Beast sun shell, Redline bands and clutches, with extra clutches and a Corvette servo. With the Blazer ecm in it, this thing gives some nice kidney punches with no lag between shifts. With the original ecm...it's just ok, so I'm thinking the + advance really wakes this thing up. I also went on a 250 mile road trip...milage went from 17.5 to just short of 21. If I could find someone to figure out how to turn the offending dash lights off on the Blazer ecm, or fix the advance on mine, I'd be a happy camper.

I'll takes screen shots of the scanner with both ecms

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post #41 of 84 Old 06-24-2017, 07:46 AM
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Re: PCM swap problem

How about trying both ecm's (96 and 97) and copying if you can or writing down all the other live data. There must be some variation that they are seeing differently. Are you doing both runs with the engine fully warmed up and are both operating in open loop? The 96 has to be seeing knock sensor feedback or fuel trim too lean or some other reason to be retarding the timing.
PM scriptedmind he knows a lot about computer stuff.
Oh, " the offending lights" are telling you that your ABS isn't working. With all the rain in your area they may save your life in a panic stop on wet roads. If you go and kill yourself I'd have to bury the Rockauto magnet on my tool box with full military honors. That would be a PITA, so get the ABS working.
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post #42 of 84 Old 06-24-2017, 09:19 AM Thread Starter
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Re: PCM swap problem

Actually I can print out the data. Truck is at temp and open loop. Going on short road trip today so I'll get the data. I wonder why the new pcm is showing retarded advance same as the old one...something strange there. I'm going to contact the seller and ask him about it also.
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post #43 of 84 Old 06-24-2017, 09:24 AM
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Re: PCM swap problem

Found this link while searching the web. Sound familiar?
https://blazerforum.com/forum/2nd-ge...erences-73590/
Doesn't really answer your problem, but confirms that others have run into the same problem.
The comment about late 95's having the same computer w/o abs was interesting.

I also downloaded the 96 and 97 4.3 wiring diagrams from here:
http://www.autozone.com/repairinfo/r...96b43f80cb0c2a
There are quite a few differences between the 2.
I opened the wiring diagram gifs in Microsoft paint and saved them as jpegs which are much easier to rotate and enlarge or shrink.
If you look at the area for the "E" connector on the 96 you'll see that all the ABS functions are done by the ECM. The 97 must be the first year with the self contained EBCM which sits on top of the ABS pump and does all the abs stuff. Eliminates all ABS from the ECM.
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post #44 of 84 Old 06-24-2017, 09:35 AM Thread Starter
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Re: PCM swap problem

The second link mentioned driveability problems...none here with the 97 ecm. I will check the AC since we're taking a short trip and temps hitting mid 90's today.

Turns out the AC doesn't work with the 97 ecm...time for another quick swap.

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Last edited by Rhotpursuit; 06-24-2017 at 03:50 PM.
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post #45 of 84 Old 06-25-2017, 12:18 AM
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Re: PCM swap problem

I've got a 97 factory manual, but not a 96. In the manual there are pinout diagrams for all four large connectors. The only 2 wires that are connected to the A/C are white (C3) pin 25 dk grn/white A/C request and red C2 pin 9 dk grn/white A/C relay enable. Sounds like the ecm simply gets the request from the heater control panel and send the signal to the relay. I think the only reason it goes through the ecm is so that the ecm knows when the A/C is on and can compensate the necessary engine functions such as idle speed. I would imagine if you ran a jumper from C3- 25 to C2-9 the A/C would work but it might idle a bit slow, with the 97 computer.
According to this website the wiring pinout is the same for 96 and 97 4.3,5.0, and 5.7. the site doesn't mention the ABS connections at all.
http://troubleshootmyvehicle.com/gm/...96-1997-page-3
On the link to the Blazer forum he was having driveability issues with the 96 ecm which cleared up when he tried the 97 ecm but then his ABS light was on and no A/C. Same as you are experiencing, but his sounded worse with the backfiring, etc. with the 96 ecm.
Maybe lextech knows something more about the differences between the 96 and 97's. The 96 must be a real pain to switch over to an 0411 because of the ABS function being in the ecm.
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post #46 of 84 Old 06-25-2017, 12:57 AM Thread Starter
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Re: PCM swap problem

I put the 96 back in for the trip today...no abs/brake lights on the dash...nice cold AC but runs like a turd trying to swim upstream. I have noticed if I let off the gas and let the engine do the braking that I can hear engine pops...for the lack of a better term. It's full on backfire thru the exhaust...but it's there. None of that with the 97 ecm. The shifts have gone back to soft and sloppy also. I'm sure it all has something to do with the negative advance. I have a 96 factory manual, but quite honestly...wiring diagrams elude me. I did find reference to timing at 21*.

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post #47 of 84 Old 06-25-2017, 01:25 AM
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Re: PCM swap problem

do you still have the 96 motor? I wouldnt be afraid of swapping over all the sensors and ignition components and MAF. cam sensor, crank sensor, distributor, ICM, oil pressure, coolant temp, IAT, EGR valve, MAP. because the 97 PCM is working with the 97 motor but not the original 96 PCM or the reprogrammed 96 PCM. its got to be just a sensor mismatch. I did a little research on rockauto to see if any of the parts were different but they are not, there was a one year only throttle body but I cant imagine that is the problem.

the ONLY other thing I can think of is your distributor is one tooth off still and the 97 PCM is correcting for it but the 96 is not. this was the mid 90s and the technology in cars/trucks was 5 years old, so processor improvements were exponential in every update. its possible they are both seeing the difference in timing but only the 97 can do anything about it.
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post #48 of 84 Old 06-25-2017, 01:30 AM Thread Starter
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Re: PCM swap problem

At this point anything is possible. I'll check the distributor again

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post #49 of 84 Old 06-25-2017, 01:34 AM
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Re: PCM swap problem

just advance it a tooth and see what happens! what could go wrong? it already runs bad.
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post #50 of 84 Old 06-25-2017, 02:25 AM Thread Starter
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Re: PCM swap problem

That's the plan...

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