So I have a 64 Chevy 1/2 ton pickup with an '85 305 TPI V8 (according to the block casting code) with a '92 computer (7730) and a 16151348 chip. It is new to me, but had a pinhole radiator leak so I set about pulling the radiator to get it fixed, new hoses, etc. When I went to hook up the negative battery cable (the battery tray had to come out to get the radiator out) I got a big spark--12.5 volts between the negative battery terminal and ground. Proceeded to disconnect the alternator, pull every fuse, etc. trying to figure out what was shorting to ground. It turns out it was a wire running from the BAT terminal of the starter solenoid to the fuel pump relay. I figured I could just jump a wire around this, which I did (and this eliminated the ground fault) but now when I turn the key to "run" the fuel pump doesn't come on. All of the wires are in an aftermarket wiring harness, but I think power also came from this wire to the ECU, and that this ECU has the triggering circuit for the fuel pump relay in it (as well as other fuel pump control circuits). My questions are:

-Does this make sense?
-I assume the ground fault (big spark when connecting the negative battery cable) is NOT normal for these TPI engines?
-Is it likely that I have a short in the ECU somewhere?
-Are there posted instructions for testing this ECU, or if it may be bad is it just time to buy a new used ECU?

Thanks so much.

 

In a stock GM wiring harness there is a separate wire that runs from the battery plus to the ECM, fuel pump relay, and oil pressure switch for the fuel pump. It has a 20 amp fuse inline with it.

It provides constant power for the ECM and power for the fuel pump when the relay is activated.

To check the ECM, observe the SES/CEL light and:

Do a key-on, engine-off, the SES/CEL should turn on briefly, blink off, then turn on solid. Can repeat the test after a minimum of 10 seconds of key-off.

If the blink occurs the ECM is running from the PROM.

RBob.

 

I'm nervous about doing a key on check, as I have to connect the battery cable to a known 12v ground, and I'm a little worried about having something catch on fire. Having said that, nothing should have changed and the truck was running fine (so assume, to answer the question below, that the ECM is running from the PROM, and that it has not thrown any codes).

One final question--if I have a bad ground (meaning something that is allowing significant current flow with the key off) is it a bad ECM, and bad PROM, or can you tell. I was thinking about just replacing the ECM to see if that works, but I was thinking that I don't understand the system well enough to know in which part of the "computer" the bad ground might be.

I'll appreciate any answers.

 

So you had 12.5 V when connecting the multimeter between the battery negative terminal and the ground cable? If so, that is normal. What you are measuring is the voltage potential. You would read the same on the positive side. What happens when you disconnect the negative terminal is the chassis shows a positive charge, because the circuit is now open, but the positive is still connected to the vehicle. The positive charge will not stop at a component or a light bulb, electricity isn't that smart. It will flow through a conductor as far as it can (not accounting for resistance, yadda, yadda)

Connect everything as it was, and you should be back to running. Without seeing the vehcile in person, I would say you likely disconnected the main positive feed to at least the ECM, possibly more circuits.

Large sparks are not normal, but I have seen this on more than one occasion. This happens more frequently when large stereo systems are installed, especially ones that have stiffening capacitors added. What the large spark is in those cases quick charging the cap, which can potentially damage the cap, but not always.

 

Just to be clear (for me) it sparked with every fuse out of the fuse box, every wire off the alternator, all wires disconnected from the ignition switch, and the feed to the fuel pump relay disconnected (and no big stereo--speakers run off of whatever little amp is in the head unit). So the potential is somewhere in the ECM/PROM (as everything else was disconnected). As long as this is normal, I'm good.

So is this normal to get this potential from the ECM/PROM?

 

The problem is that your definition of a large spark may not be the same as mine. I've seen some people get freaked out by what I call a tiny spark.

One other thing that I forgot about in my last reply is that if a door is open the dome light can cause a hefty current draw especially when re-connecting the battery.

I have to think that if it was running the way it was before, then it should run the same way after reconnecting the battery and everything else as it was.