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the new guy
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598 Posts
Discussion Starter #1
Where does the fuel pump get power? We put the replacement motor in the 92 and it just cranked and cranked. I poured some fuel in the throttle body and it immediately fired up until the gas was gone.

So, we checked power at the pump, nothing. Checked at the relay and nothing. Oil pressure sensor seems to be working, I guess, the gage on the cluster works. The fuse is good in the fuse box, all the fuses are good in the box.

Where does the relay get its power and what tells it to close the circuit to send power to the pump?


As I put 12v on the pump and it came right online. So I ran a test line to that grey plug by the distributor amd the trucks fires right up, like its perfect.

What do I need to check? So far, that pump circuit and a drive shaft center support bearing is all that keeps the truck from going and getting inspected
 

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Been there Done it
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6,664 Posts
You could have a bad ignition module in the distributor. If it's not sending a reference signal to the ECM the ECM won't turn on the fuel pump relay. Test the green and white wire into the relay for 12v when the engine is cranking.
If you aren't getting 12v during cranking it's either an ICM or ECM issue.
 

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the new guy
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598 Posts
Discussion Starter #3
that ignition module is actually in the distributor? like inside the cap? I am not getting that 12v signal you are talking about
 

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Hello,

Fuel pump power, I think, is on fuse "ECM B" so make sure that's good. Remove that fuse and re-install it, just to make sure there's no corrosion or connection issue with the fuse.

If you can power the gray test connector and the pump and truck runs, then there's nothing wrong with the ignition module.

You mentioned that there is no power at the orange wire (relay pin D), so that suggests to me that either the fuse ECM B is bad, or that there is a wiring fault.

The fuel pump relay and the oil pressure switch are tied together in parallel, at the fuel pump relay plug (I think). There may be two orange wires on pin D of the relay. If there is no power on relay pin D, then there is a wiring fault between the fuse box and the relay plug, or the fuse (ECM B) is bad.

If there is power on the orange wire at relay pin D, try powering relay pin B (gray). That goes to the pump. If that doesn't work then there is a wiring fault between the relay harness and the pump. There is a splice somewhere connecting pin B to the gray test connector, so if powering pin B doesn't work, I'd be suspicious of the wiring between the relay and the test connector.

If you have power on relay pin D, and powering relay pin B runs the pump, my guess is the relay is bad, but there are a couple more checks. Relay plug pin C should be ground, but verify that's good. If not, check the wiring and grounds at the back of the cylinder head.

Relay pin A (green/white wire) goes to the ECM. It should have power for 3 seconds when you first turn on the key, and should be powered while cranking. If not then there is a wiring fault between the relay plug and the ECM, or the ECM is bad.
 

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the new guy
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598 Posts
Discussion Starter #6
I have the old ecm from the engine we removed that ran fine, other than knocking, so I am going to swap that today. if nothing, I will try the other stuff but I know the ecmb fuse is good because we pulled and checked all the fuses first. we swapped it with another same amp fuse and nothing, also. I swapped the relay with the relay next to it since they seemed to be the same, I believe the ac relay is the one next to it on the firewall.

its weird since the truck ran fine on the other engine. this is another 2.8 so I figured it was plug and play but we had to change the tbi since one of the plugs was different, the throttle position sensor had a round plug on ours and this one was flat, and the sensors didn't mount the same so I couldn't just swap those.

but, we were happy to hear the truck start and idle perfectly
 

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Others can probably be a bigger help to you about the specific sensors involved and the wiring. I just want to add a cautionary warning. Don't drive it with the grey "test prime" plug wired to run the pump, or with any kind of mod that allows the pump to continue running after the engine stops. That's a very serious safety issue.

I had a friend who got in an accident with an old "beater" that someone had jumperred the fuel pump relay on so it ran the fuel pump when the key was turned on (even if the engine was stopped). He was in a moderate collision that broke a fuel line above the exhaust on his car. The impact also knocked him unconscious. The fuel pump continued to run and pumped gasoline into the fire, and he spent about a year in the hospital recovering from burns sustained in that fiery crash. However you repair it, make certain the fuel stops when the engine stops running, just like the stock setup.
 

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the new guy
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598 Posts
Discussion Starter #8
oh ya, that is why its not going for safety inspection and getting driven. I have it set so it runs when the key is in the on position so we can verify everything works and no other problems but I don't want to let my 16 year old start driving it without stuff being right. its his first vehicle so he can start learning to take care of it and how to do repairs so I don't want previous owner weird issues or alternate wiring to cause him trouble when he might need to use a wiring diagram when he heads off to the air force after high school. (he is a senior now and wants to go as soon as he turns 17, after he graduates)
 

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the new guy
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598 Posts
Discussion Starter #9 (Edited)
Ok, something is wrong.

what is the other oil sensor on the passenger side? I have one by the oil filter with a 3 wire plug and a single wire sensor above the starter.

Changed the deal in the distributor and still nothing.

None of the checks for the relay give me any numbers unless i jumper that grey wire.
 

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the new guy
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598 Posts
Discussion Starter #10 (Edited)
Also, is there a cold start primer or something that is similar to a choke? If i pour gas into the tb, it starts right up but if just try and crank it, it wont start on the fuel coming from the injectors.

This is new since the ecm got swapped. Previously, with jumpered grey wire, the truck would start with key turn.
 

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the new guy
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598 Posts
Discussion Starter #11
Another question. What is the wire that someone tried to splice in the pic? Its right next to the small wire that leads to the starter. No idea where it previously went as there is a ton of prior owner home repairs.
 

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the new guy
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598 Posts
Discussion Starter #13
i think I need a new multimeter, or new leads. I am getting inconsistent readings with mine so I have to hold until I get that taken care of.
 

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Ok, something is wrong.

what is the other oil sensor on the passenger side? I have one by the oil filter with a 3 wire plug and a single wire sensor above the starter.

Changed the deal in the distributor and still nothing.

None of the checks for the relay give me any numbers unless i jumper that grey wire.
The single wire sensor by the starter should be the knock sensor. That should be 1 wire, and squeeze-to-unplug. The sensor itself has a round connector body.
 

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Also, is there a cold start primer or something that is similar to a choke? If i pour gas into the tb, it starts right up but if just try and crank it, it wont start on the fuel coming from the injectors.

This is new since the ecm got swapped. Previously, with jumpered grey wire, the truck would start with key turn.
There's no cold-start injector, etc. on the TBI trucks. The only source for fuel is the two injectors in the throttle body.

Is there fuel coming from the injectors with the new ECM while cranking? The ECM knows you're trying to start it by seeing voltage from the starter solenoid on ECM pin C9 (purple/white). That pin should have battery voltage on it while you're cranking, and should be open otherwise. If the ECM doesn't see that signal then it won't enrich for starting. If the other ECM works and nothing has been disturbed under the hood, I would suspect a bad input on this ECM. There are other possibilities (for example, a TPS reading full-throttle would cause it to not fire the injectors at all since it think's you're trying to clear a flooded engine), but if the old ECM starts okay and this one doesn't, I would bet on the ECM.
 

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Another question. What is the wire that someone tried to splice in the pic? Its right next to the small wire that leads to the starter. No idea where it previously went as there is a ton of prior owner home repairs.
From the photos I'm not sure where that goes. There are a few wires going to the starter. The battery cable goes to the solenoid, and there are either one or two (I don't remember) wires on the crank side of the solenoid that are powered when you turn the key to start. There are also a couple more wires on the solenoid that have fusible links in them (these look like plastic cylinders in the middle of the wire). These will go up to the distribution block on the firewall. I don't remember the exact routing but the blower motor relay and the alternator are fed from either the starter solenoid itself, or from that junction block on the firewall. I would trace the alternator charge wire, the blower motor relay wire, and the wires on the junction block on the firewall to see where they go. It could also be leftovers from something the previous owner installed (lights, stereo, etc.)
 

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the new guy
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598 Posts
Discussion Starter #17
ok, so that knock sensor I need to replace. this one has a different type of plug then the harness has so I need to buy the one for the year of the truck. got it on the list now, thanks.

as for the cold start enrichment, there is a good cone of fuel from the injectors but I guess its not enough of a cone but I believe it has to do with the wires from the starter and distribution block on the firewall. there is the wire that goes to the battery and the alternator as I had to reinstall that to get the starter to crank so that and the one from the ignition switch are the only wires on the starter right now. this makes me think those other wires might have something to do with the signals I am not getting. that power distribution block on the firewall has two posts, one with a number of red wires and one with a single red wire that comes up from behind the engine. that single one must be the one that goes to the starter, which I have dangling somewhere unconnected that needs to be found. and maybe that is the one that was spliced and is now missing its end so that's why I was wondering why I thought the starter had two small wires on it instead of one wire because I sure thought I was missing a wire.

ok, thanks for everything so far, its been really helpful. I hope to be able to jump back on the truck sunday as I have to go to san Antonio today after work so I will be back Saturday afternoon/evening, I will update once I get back in there though to let you know how it goes.
 

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the new guy
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598 Posts
Discussion Starter #18
alright, great news. I found a wiring diagram for this engine and year and sure enough, the weird wire is going to the junction block on the firewall. once I hooked that wire up, everything started working properly. the relay gets power like its supposed to and the truck started immediately. we finally got to drive the truck without a knocking engine.

now I will fix the gripe list and it will be cleared for safety inspection and stickers so my youngest son can drive it for school. he was very excited when I told him to turn the key and it fired up with no jumper wire for the fuel pump and we went for a test drive.
 
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