UsedCarManTN

Any help would be greatly appreciated!
We recently did an engine change in this truck but, I'm not sure if this problem existed before then. Let me begin by saying that this is a pretty nice 1995 Chevrolet Silverado Ex-Cab / 5.7 / 2 Wheel drive / 5 Spd. and the engine swap was prety much bolt for bolt. We are still using everything factory (tbi fuel injection and stock everything else) and when we got it, it had a couple of valves bent from an over rev because the previous owner tried to siphen some gas out of another vehicle back last fall when gas prices were so high. Needless to say he ended up with a tank full of muck and his response was to rev it up as high as it would go to try to "blow" out whatever was causing it to run bad (not a real genius).
So, after emptying the fuel tank of everything, changing the fuel pump, and changing the engine, when we went to fire it up it has no power to the fuel pump but, it will run when we poured a little gas down the throttle body. At first it blew the ECM-B fuse every time we put one in it and after checking every wire we could for being pinched in the bell housing or something we loaded it on a trailer to take it somewhere else where we had more equipment to deal with it, then it wasn't blowing fuses but wouldn't do anything. That's when we found out the MIL light on the dash wasn't working.
Now, we did some homework and after checking the relay, the continuity of all the wires leading to & from the tank/pump/relay/ and fuse panel (ECM-B) and tirelessly checking every ground we could find, we have come to the conclusion that somehow the ECU is dead. When we turn the key on, there is no "Service Engine Soon" (MIL) light nor a CHECK GUAGES light but, all the rest of the idiot lights come on. We have another 95 chev truck here to try some part swaps but it is a 4WD Automatic and the ECMs are totally different but, when we turn the key to the on position without starting it, both these lights and the other normal lights come on. So, we feel pretty sure we're on the right track. We have no blown fuses and have even taken the ECM out and removed it from it's case and with a thorough inspection visually can find nothing wrong with it.
I know that things can be wrong with electronics without looking bad but, without ALLDATA (which I sorely miss) I am kinda in the dark now as to where to turn next without trying to order some expensive computer from the parts house that if it does not fix it, I can't take back. Any suggestions anyone? Usually, I'm the "go to" guy that everyone asks these kinds of questions to but, this time I'm stuck.
Thanks

Raylo

You may have answered your own question. Try to get hold of a spare PCM and pop it in there to see what happens. They used to be readily available in the aftermarket (at least for Camaro 5.7s of thast vintage) and not too expensive. I don't have a source but local salvage yards or google should get you there.

pat mcgroin

Have you checked the fusible links in the engine compartment?
It is possible that one of those could be blown.

mack1

If it is your truck and you are going to keep it, I would reccommend that you list it on AllData and get some good prints for it. I have prints on 4 and 6 cylinder vehicles of that year and it shows that the ECM B fuse does not power the PCM at all. The only input from that fuse to the PCM is a signal telling it that the oil pressure if up, thus the PCM knows to pulse the fuel injectors.

I would suspect that you have shorted the wire to the pump motor when you changed it out. A good test would be to unplug the connector at the tank, then locate the pump relay and feel or listen to it as a friend switches the ignition from off to on (not start). If the PCM is working, it should pull in that relay for about 2 seconds, then release it. One click for each off/on sequence at the ignition switch.

In any event, I would do more checking before buying a new PCM. Like you said, they don't like to take them back, even at wrecking yards.

Very best regards,
Mack1

UsedCarManTN

Thanks for all the input guys. The only fusable link I know, is the one at the alternator. I checked at the starter like back in the day but, there is only a main power wire for the starter and one for the solinoid. <br><br>
The truck is for a close friend. We did test run the pump prior to install and all was good. I'm following you pretty good that the one wire going to the ECM is "oil pressure, min 4 lbs". I haven't pulled the tank back down to unhook the pump which seemd the only way I'm gunna get to the connector, it's just such a pain in the a$$. I did however losen the panel under the hood to access the back and also removed the cover on the relay. But, we've tried a couple different relays that I know are good. I also from here have done a couple other things. 1, tried the remote fuel pump prime (it's small & red single wire about 4 inches back in the harness going back toward the engine with a little black cover on it) 2, taken a hot wire to the wire that should be getting +12 close the relay coil (I think is green w/whitw tracer but, I know one of them did close the contacts). So, hypathetically I've tried to force the pump on anyway. Still no noise in the tank. <br><br>
None of this changes the fact that I still have no SES light or CHECK GUAGES but the "brake'.'ANTI_LOCK", and other red ones do come on for a few seconds. The motor is grounded with the mesh strap, the big black one in the loom that bolts to the back of the head, we at one time cut checking but we now reattached it, and the battery.plus the 2 eyelets on the front that attach to the thermostat housing bolts.>br><br> I looked around at a couple yards 2day but I'm pretty sure it's gotta be 5 spd 2wd and 95 cause its the first year of the style and the harness is different from a 94. No relays on the firewall on the pass side. Looks like Advance will order one for somehere between $82 & $122.<br><br>
P.S. I did check for power at the ecm and have 4 constant hots in various places.

UsedCarManTN

By the way guys. It was cool to find a bullitin board with some real professional sounding people to associate and pick brains with. Thanks for having me. Hopefully in the future I can be of some help to someone else.

mack1

Hi UsedCarManTN,

Sorry I forgot to bid you a warm welcome on your first post. From what you have posted, I can see why you are the "go to man" for mechanical solutions.

In your first post, you said that the engine would run if you put fuel into the intake. Sounded like the PCM was working at that time and I'm not sure that the SES light was susposed to flash unless there was a sensor problem. If so, it would then flash on, off, then back on to stay. Later models, I think it flashes each time you turn on the ignition. Probably for lamp test.

I have three Haynes repair manuals for GM vehicles. The years range form 94 to 2005. (don't have one that covers Silverado for your year) The wire color coding seems to be consistant through all those years. The wire going to the fuel pump is gray. One shows the Red Fuel Pump Prime wire you mentioned. On that print, it shows that the fuel pump relay must be in place and not energized for that path to the pump to work. I would caution you that if you have a short in the gray wire, putting 12 volts to it might burn some wires up. Hopefully if you have a short, it is outside the tank. I've heard some say that it is easier to remove the bed than to empty and lower the tank...four bolts holding it on and all the wires unplug. You'll need three folks to help you pick it up and remove it. You might find that you can remove two bolts and loosen the other two and jack the bed up enough to reach the pump wiring.

If you have a volt/ohm meter, you might measure the resistance of the gray wire to ground. I don't know what the motor winding resistance is, probably a few hundred ohms. If you still have the old pump laying around, measure it to get an idea of the resistance you should see on the gray wire. If it is a lot lower (near zero), then you have a short.

Again, good to have you here. If you see a post where you can help, by all means, jump right in and put your two cents worth in.

Very best regards,
Mack1