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Strange injector issue.

cuervo

cuervo

Runn'n down a dream!
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Wife was driving the suburban the other day and when she went to put it in reverse it died on her and would not start.


I grabbed a timing light and multi meter and went to check it out.


I have a fuel pressure gauge on the fuel line and it was still reading 18 psi but there was no fuel spray from the injectors.

I put the timing like on and check for spark.

So I had spark and fuel pressure but no spray. I figured it was one of two things. Either the ignition control module or the ECM. I don't have a NOID light set so I couldn't check to see if the injectors were getting a pulse signal.

Before pulling the ICM out I figured I check fuses but didn't think I was going to find any blown.

Well, I check the ones I thought it might be. ECM1, ECM2 and neither blown. I started pulling the other ones and came acrossed one label Trans that was blown. It is a 10 amp fuse and I replaced it and the damn thing started right up.

I haven't chased down that wire yet to see what it goes to but I thought it was very strange that a fuse label Trans would stop injector pulse.

This is on a 7060 ECM and a 4L80e trans 3/4 suburban, 1991.
 
I'll have to check those out, Mark.

I have the GM shop manual and all the picks for the fuse box do not show this "Trans" fuse. They all say crank in that position.

I'm the second owner of the burb and it hasn't had any mods to that part of the wiring.
 
I can see the Cans a bouncin! Just have to reply with quote to read what you posted!

:haha:
 
Your shop manual is for the '91? I have that year too, I'd be happy to look at mine. You know what page offhand?

Odd if the wiring manual doesn't show the trans fuse labeled correctly. I've seen mistakes in print before, just not in the wiring manual.

No matter what the case, the fact that the fuse was blown is probably cause for concern. I guess throw a pile of 10A fuses in the rig if it's intermittent, until it becomes constant. I used to grab handfuls out of the wrecking yards when I visited more often for this reason. :)
 
Well, I think I have most of it figured out now.

In the wiring diagram it shows "crank" fuse and that is for the 4 spd manual trans. In the upper right hand corner it shows the RPO code for a 4 speed auto trans. You can see where the pink wire goes to some other distribution block of some sort the joins with a purple wire. That block also ties into the brown wire which goes back to the ECM B fuse.

I assume when the trans fuse is blown it fails a circuit for the ECM and stops the injector pulse. On a 4 spd manual trans the pink wire runs to a what looks like a clutch pedal switch to prevent cranking. On the automatic it ties into the ECM. I'm not sure why it's like this. I figure the normal safety neutral switch on the trans would be all it needs.

fuseblock.jpg


transwire.jpg
 
I found this wiring diagram online. It shows one of the wires coming from the crank/trans fuse going to ECM Pin C3.

burb914.jpg
 
FWIW, it also shows a 3A fuse. Hot in start only if I'm reading that diagram right, not sure why the trans or ECM would need a hot in start only input.

I should look at my manual, mine is definitely not like yours, maybe it's a 1990? I could have sworn it was a '91.
 
Yeah, I think that 3A fuse is used in the 4 spd manual applications and the 10A is the automatic. I know there a couple other 1991 3/4 suburban owners here. I should ask them to check the fuse box and see which one they have. Also have them pull that fuse and see if they start.
 
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Hey man,

PM'd you . I pulled my 10A trans fuse, and truck still starts.

Swap out the Ign module.

When you do, check the wiring harnesses to the module. One comes from somewhere (ECM?) and the other goes to the coil. I replaced 3 ign modules before i figured out that my wire insulation was cracked off, and shorting out between the wires.

Cut out the bad sections, and patched them back together with shrink wrap marine butt connectors. No more issues!
 
Well, after I replaced that fuse mine started and ran fine.

I'll check out the wires once it warms up here! lol

Thanks for the help.
 
Did a quick check and pulled the fuse out. The Suburban wont start with that 10A Trans fuse pulled out.

This is going to require some investigation to find out why our trucks are working differently.
 
cuervo said:
Did a quick check and pulled the fuse out. The Suburban wont start with that 10A Trans fuse pulled out.

This is going to require some investigation to find out why our trucks are working differently.
Click to expand...

Well, that narrows it down....

One of our trucks is wired wrong :doah:

Mine starts without it, so i'll assume the issue is with yours :haha:


In all seriousness, let me know if you need me to do anymore testing on mine. They should be identical.
 
Mine IS a 1991 manual, but it's GMC. Can't believe it has a totally different cover lol.

Page A14 isn't what you need. If you look in the upper right corner of some of the pages, it will tell you the Engine/trans the diagram is applicable to. MT1 I believe is the HD 4 speed auto, so that would be the diagram to look at.

They aren't making this easy, you would think there would be multiple fuse block layouts on following pages, but that's not the way they have it. I will have to keep digging.

Edit: Check out page A55. There are some "MT1 only" lines on circuit 6 I believe it is, tied into ECM crank.
 
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I got the page number wrong, A11 has RPO code MD-8 Automatic 4-speed Trans and LO-5 5.7 gasoline V8 engine VIN k.

I'm not sure which is which when it comes to MT1 and MD8, but I would think you are correct. MD8 is the 7004r and the MT1 is the 4L80e.

Thanks.
 
Maybe EagleMark will chime in. I don't recall ever seeing an ECM that used a cranking input, always just a constant and ignition 12V. I don't understand what the 4L80E would change in that regard, unless there are two circuit boards or something in the PCM's?
 
dyeager535 said:
Maybe EagleMark will chime in. I don't recall ever seeing an ECM that used a cranking input, always just a constant and ignition 12V. I don't understand what the 4L80E would change in that regard, unless there are two circuit boards or something in the PCM's?
Click to expand...

I don't know about two circuit boards or not, but there are two large plugs that go to the ECM. I believe one goes to the trans and the other runs the normal stuff.
 
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