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Discussion Starter #1
OK folks, let's talk about the controversial OBD types we find on these 2 years of S-series trucks. Some call them OBD1.5, some call them worse!

I am not the OBD guru but I think we need a repository of information on this and how to work with it.

We know that with the older OBD1 type with the ALDL under the dash you could get the codes out with a paper clip.


We also know that the OBD2 type you must have a scanner to get codes.

In 94-95, GM began a transition on many vehicles into a more comprehensive OBD system, but stopped short of OBD2 in that even though it has an OBD2 style connector, an OBD2 scanner will not work.

Much misinformation is floating about regarding the vehicles with these systems. Some say if the PCM is on the coolant reservoir under the hood it's OBD2. Some say the distributor cap with horizontal wires is OBD2.

Let's use this thread to only post what we know for a fact about these vehicles, and not just what we have heard.

For my own 1995 Jimmy I had to buy a scanner that specifically would work with 94-95 GM vehicles. Found it on ebay for $250, Actron CP9150. Of course a professional automotive scanner like a Snap-on can scan these with the proper software.

For the record, my truck ("W" engine code, CPI injection) has the PCM under the hood,


and has a distributor cap with horizontal wires,


and has the OBD2 style connector.


The sticker under the hood says OBD1 certified.


As far as I'm concerned, if I cannot use an OBD2 scanner on it, it's NOT OBD2.

My truck has one pre-cat O2 sensor, and one post-cat O2 sensor. My truck has a crank sensor.

My scanner shows "N/A" as far as EVAP monitoring is concerned, so I guess that means not yet!

Also in these years were trucks with the TBI injection, the vertical TB with an "old school" type air cleaner like a carb engine. I believe that these trucks still used the old OBD1 ALDL system, but correct me if I'm wrong.

So let's pool our knowledge here and help those poor souls who bought one of these things and doesn't know what to do.
 

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I'll add to this.

I have the under hood PCM, just like OBD2 however this is a OBD1 PCM. I crossed the numbers on a Fullsize Chev forum. Alot of the info info I have found for these mix years , is from the Fullsize forums. there are alot of older guys there who play with these.

1994 GMC Sonoma TBI 4.3l 5speed Manual 4x4
Diagnostic connector OBD1

PCM OBD1 in OBDII location

PCM Connectors

Dizzy Same one since the 4.3tbi came out Vertical.


Additionally, unlike the old OBD1 where you could get away with a paperclip, these do not respond so well. I have been told by numerous Full-sized PCM tuners that using a paperclip, "can" short out and kill the PCM.

Here is a link to where i read up on this, http://www.fullsizechevy.com/forum/general-discussion/performance/tbi-tuning-87-95-obd-i-ecm-pcm/... mind you this is more biased for the TBI 350, it still covers all the basic PCM questions and details.
 

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For the record, my truck ("W" engine code, CPI injection) has the PCM under the hood,

The sticker under the hood says OBD1 certified.


As far as I'm concerned, if I cannot use an OBD2 scanner on it, it's NOT OBD2.
.
See if I saw that I would assume OBD2, only because I know that the OBD1 has a removable prom, or at least mine does.
You got the good PCM, I have to buy a flash chip, or Prom burner to change anything on mine. That one should already have a flash chip.
 

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Only V6 trucks came in obd1 and 2 versions afaik. I had never seen an obd 1 with an under hood mounted pcm until the pics posted above. Might not been available in all states. Some of the early obd 2 vehicles were not full function [no evap monitor] so were not certified as obd2, but exceeded obd 1 criteria.
 

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Basically, what I have is a 93 PCM, inelegance and all. the fueling is the same, the emissions are the same, aside from some misc connectors its a 93. I would rather have the behind the glove box than this thing.
I'm getting into tuning it more than I was in the past, so picking up a cable and flash prom from Moates.net. hopefully I can get on the fly fueling changes lean it out a bit on highway voyages.
 

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Even though your label says obd 1, it has an early obd 2 system. This system was not fully functional [no enhanced evap monitoring] so could not be certified as obd 2. I went to gm classes on these obd 2 systems. had the s10 obd 2 manual supplement, but a roof leak in the shop pretty much ruined it. GM never used the term obd 1.5 that i know of. Sorry if some of this post repeats my post above, but the first post did not show up, so I made another post.
 

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Interesting, I never noticed the removeable PROM on the TBI's. And GM didn't label it as OBD1.5, that's something that others have used to differentiate it. I don't know if this is true, but someone once said that you can flash an OBD1.5 computer to be OBDII compliant. Might be worth looking into for at least a Code reader standpoint.
 

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Lol mine doesn't have any lable or what not to indicate OBD anything. aside from the plug and the lack of a for and aft O2 sensor it could be mistaken as OBD2. Only real way I knew was to cross the number, came up as an OBD1 eprom in an early OBD2 housing.... Either way its OBD1diag port, codes, and intelegance.
 

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Ok everything on the op's truck is the same as mine can this be repinned to to OBD2. or am i stuck with an unreadable truck. i took it to autozone last night to check the ABS light but their scanner wouldnt read it.
 

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Ok everything on the op's truck is the same as mine can this be repinned to to OBD2. or am i stuck with an unreadable truck. i took it to autozone last night to check the ABS light but their scanner wouldnt read it.
If you have the OBD1 setup with the OBD1 pcm under the hood. I hate to tell you the pcm isn't intelligent enough to pick out the ABS problems.
 

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so is this obd1 or obd2 stuff






 

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That's the OBD 2 style of computer, but it's an OBD1 type of programming. But it's different than OBD1 so you can't use the jumper trick to flash codes either. And you can't just put a newer computer in, as the engine controls won't work correctly unless it had a fully custom tune and programming. I'm not really sure what options you have, but they are limited and OBD1.5 is the worse, which is what you have.
 

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Discussion Starter #14
You will it really hard to get codes out of these years. As stated, you can't use a jumper and your OBD2 scanner won't work. I had to search around quite a bit to find one. The OBD2 scanners are cheap and plentiful but not these.
 

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so if it has the full trifecta... says Certified OBD-II under the hood, has the ECM under the hood, and has the 16pin under the dash is that full OBD-II?

I just bought a 95 Blazer 4x4 with CPI w/ 130k from a friends father for $300 and it's been sitting for a year and a half... needs brake lines, a couple body mounts fixed from rot, and 4 tires. but if i can get it running right (rough idle and ses light) it's going to be my winter ride.
 

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Discussion Starter #16
IF it says OBD2 under the hood, it IS OBD2.
If it's a 95 and it says OBD1 under the hood, it is a later version of OBD1 with some of the features of OBD2 but is still not up to the OBD2 standard.

You will need a scanner that works specifically on 94-95 OBD1 GM vehicles, no paper clip trick will get you codes.
 

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I'm digging this up, for those who want a scanner, that does indeed work with these PCM's. the one AWD V8 has stated to work, does indeed work. I received mine in the mail today.

here is a Vid, it was for a differetn purpose, but non the less displays it working
 

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I had a 95 s10 with TBI injection, it had the underhood ecu and was obd1.5
 

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Interesting, I never noticed the removeable PROM on the TBI's. And GM didn't label it as OBD1.5, that's something that others have used to differentiate it. I don't know if this is true, but someone once said that you can flash an OBD1.5 computer to be OBDII compliant. Might be worth looking into for at least a Code reader standpoint.
I wondered this, computers (desktops and laptops) can be rewritten all the time to run different software, so it would seem possible that a good tuner/programmer could re-write the OBD1.5 trucks to run on OBD2. I just have to find someone that can..

My set up looks exactly like Jons1995s10's
 

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I wondered this, computers (desktops and laptops) can be rewritten all the time to run different software, so it would seem possible that a good tuner/programmer could re-write the OBD1.5 trucks to run on OBD2. I just have to find someone that can..

My set up looks exactly like Jons1995s10's

If you have the newer PCM, it is possible, the older ones take more money and parts to make them work as such, but again it is possible.

All the new sealed units have Flash memory, basically like an SSD, where as the old ones have a PROM chip, they are only programmable once and take a really long time to do so.

There is a company that deals with reprogramming PCM's, they have the tools and software but it isn't cheap, upwards of $400 just for a basic setup.
 
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