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'97 GM 4.3 Crank/No Start (x-post from YouTube comment)

posted Jul 05, 2016 18:02:55 by Steve Harris (65Westfalia)
'97 Astro 4.3 Crank/No Start / Short to ground on ECM-1 and ENG-1 circuits
Question asked on YouTube Another great ScannerDanner video!

KOEO

Short fault on ECM-1 and ENG-2 Burned fuses.
No comm @ DLC(good pwr and ground at DLC)
No spark
No pwr to pump
Pump verified good with 12v via test lead @ under hood convience center location
Power to all other circuits good/working
No 5v ref. @ pin 12 (ECM WHITE connector)
No 5v ref. @ pin 4(5v ret.) on VCM(ECM WHITE connector)
No 5v ref @ any sensor
No 12v @ pin 4 (ECM BLUE connector)
12v on pin 21 power B(+)(ECM WHITE connector) coming from ECM-B fuse (not shorted)

Fuel and coolant temp gauges pegged out of range

No scope available


Questions:

1. What is the difference between ECM-B (pin 21 WHITE connector(good)) pwr feed to computer and ENG-1 (pin 4 BLUE connector(shorted)) pwr. feed to computer? Are different sections of the computer powered by different circuits?

2.How can I test ref circuit (5v) if my ECM circuit is shorted to ground(blown ECM-1 AND ENG-1)? Having a tough time locating short that is showing on both those 2 circuits.

Attempted fixes:
-Unplugged all sensors on affected circuits. Short still shows.

-Used inductive short tool to no avail. Tool indicates point in loom short exists, but inspection shows no short/bare wire/broken wire/touching wire.

-Inspected loom

- ECU swapped with known good unit. No change.

Failure was instantaneous while running. Have chased all power feeds to sensors on affected (pink) wires on ECM-1 circuit, and still shows shorted. Continuity of all wires on that circuit. Unplugged all ECM controlled components and information sensors on that circuit, still shorted.

I have Mitchell1 diagrams for "engine power" circuits. Oh, btw diagram shows fuel pump ground to frame that is not there on vehicle . <shrug>


Thanks for any help!!
[Last edited Jul 05, 2016 23:05:39]
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8 replies
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cheryl hartkorn said Jul 05, 2016 22:05:13
kinda confused are you saying the ecm 1 fuse is blown? and you don't have 5 volt ref? if the ecm 1 fuse is blown it has all the fuel injectors, crank sensor, vcm, ignition control module, and the coil on that fuse.
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Steve Harris (65Westfalia) said Jul 05, 2016 22:48:59
Sorry for the confusion. I laid out all the conditions. Some may be relevant, others may not.

Yes, that fuse is blown. I get that if the fuse is blown, it's normal to not have 5v at the sensors on that circuit. The computer should not be getting 12v since that comes through the ECM-1 fuse. Correct?

Where I am confused, is that there IS 12v at one of the computer connectors(pin 21 on the WHITE connector(battery(+)). BUT, there is no power on the other side of the computer (pin 4 BLUE connector). The diagram shows this power wire comes from the ECM-B fuse. It is line 1(orange) on both diagrams but pin 21 at the computer. 12v measured at the computer(pin 21) KOEO. Should not this make the system power up?

Not having power makes sense, since the ECM-1 fuse is blown. What does not make sense to me is why there is a second power source. I'm sure it's just my lack of understanding how the computer works/is built. There must be a reason.


The real issue is that the short is still is there. Where it is, is the issue. Ideas?
Thanks.

[Last edited Jul 06, 2016 00:25:04]
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Tyler said Jul 07, 2016 01:58:06
Hey Steve!

Not having power makes sense, since the ECM-1 fuse is blown. What does not make sense to me is why there is a second power source. I'm sure it's just my lack of understanding how the computer works/is built. There must be a reason.

That's a tough one to answer specifically, but I think you've got the idea. We have no idea how the power feeds end up getting split up and shared internally, so we don't really know who feeds what. Maybe pin 21 feeds the component drivers (injectors, solenoids, stuff), but pin 4 feeds the 5V regulator? Just speculating, of course.

I was able to find a diagram for the ENG-1 and ECM-1 fuses, just so I can get an idea of everything that's on it (and anyone else following along).

ECM-1


ENG-1


What, they had to run the whole engine off of two fuses? lol I hate finding diagrams like this when I'm short testing. Just so I'm clear, the fuses fails instantly when running? Also, do both fuses pop at the same time?

Both popping is weird, because it's the ECM-1 fuse that powers the ENG-1, both 20A. My first thought is to leave the ENG-1 fuse out, replace the ECM-1 fuse, and see if it pops again. If it does, then that branch is suspect, and you can leave the ENG-1 branch alone for now. If it does not pop, then you know the fault was in the ENG-1 branch.
[Last edited Jul 07, 2016 01:58:43]
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Steve Harris (65Westfalia) said Jul 07, 2016 02:25:52
Tyler,

Thanks for the suggestion. The truck was running and suddenly shut down. I found the ECM-1 fuse burnt upon inspection. Not exactly sure if the ENG-1 was burnt at the same time. I remember finding that later. Also, at one point in time the short to ground (both ECM and ENG)was fluctuating intermittently. This has now stopped and I can't get it to repete. Maybe all my harness messing has changed things. Someone on a Astro board suggested it was a connector at the firewall pass through. Extremely hard to get to though.

I'm just a diy'r and I hope I'm explaining what is going on clearly. I've got a PP3 and it shows one side of the each fuse is shorted to ground. I'll try again tonight when I get home, but I'm almost positive the ECM-1 will pop instantly. I'll perform the check like you said with the ENG-1 too. I'll post what I find.

Is it logical to unplug every sensor from each circuit and see if the short still exists or do I need to perform a sensor by sensor test for internal shorts. I thought I'd done that, but I may have missed a sensor. I have the mitchell diagrams which are a tad clearer than the factory ones you show, but none the less they show the same thing.

So I'm looking for a exposed wire touching ground (body or other ground wire) and not one that might be broken within it's sheathing, right? Used a ECT-20 (P-Probe) inductive short tester as mentioned. It led me to a place with perfectly intact wires.<sigh>

Just a thought, this 4.3 has that spider injector and not individual injectors. Could one short by itself? I know I unplugged the connector(one for all 6) and the short is still there.

I thought I'd struck gold with this vid The short tool showed this area was where the short is, but nothing but perfect wires :-( It's OK to laugh at my desperate comments on the video. LOL.

Found a way to link to the Mitchell1 power dist. diagrams 1-4

Pg. 1
1 OF 4
Pg. 2
2 OF 4
Pg. 3
3 OF 4
Pg. 4
4 OF 4

Hope those work without having to login to Mitchell1.



[Last edited Jul 08, 2016 03:01:21]
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Tyler said Jul 07, 2016 13:00:19
Ah OK, so the fuse popping was intermittent, but may be constant now. Yeah, you may have inadvertently corrected the short by doing your inspection. Not that you were wrong to do so, that's just the nature of these things.

No worries about your descriptions, sir, I'm just glad you're supplying us with information!

About the PP3, the 'ground' indication on it can be a bit misleading. It will, for example, indicate ground when connected the fuel pump test connector when the pump is not energized. Clearly this is not a ground, or even shorted, but it finds a ground through the pump itself. Tests lights will do the same thing. Not saying you don't have a short, just noting how the PP3 operates.

You probably are looking for bare copper wires rubbing ground, OR a power wire and ground wire touching each other. The short finder pointing to the harness makes me wonder about a previous wire repair that went wrong, but i see that you already inspected the harness there.

It sounds like you've eliminated shorted sensors and components by disconnecting them. If you've got everything disconnected and the fuse still pops, then I wouldn't worry about doing individual checks. The spider system actually does have individual injectors, but disconnecting the main plug on top of the intake will isolate all of them.

Let me know how the fuse test goes, and we can brainstorm further!
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Steve Harris (65Westfalia) said Jul 08, 2016 20:41:39
So I did the fuse/sensor tests, and as expected the ECM-1 pops instantly no matter what(all plugged in or all unplugged). The ENG-1 seems to be as you expected, the fuse is fine and the PP3 was showing a FALSE ground on one side of the fuse. Once I put a fuse back in, both sides of the fuse read 12.5v and the fuse did not blow. Where before the PP3 showed + 12.5v on one side and a ground on the other. Looks like I can eliminate that circuit. Slight bit of good news ;-).

Now, there is still the matter of finding the short on the ECM-1 circuit. Question: The fuel gauge and temp gauge in the cluster are both pegged out of range. The voltmeter apparently does not work either in KOEO condition. Does this have any significance in leading me to where the short might be?

Thanks!


[Last edited Jul 09, 2016 02:26:18]
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Tyler said Jul 09, 2016 18:06:11
So I did the fuse/sensor tests, and as expected the ECM-1 pops instantly no matter what(all plugged in or all unplugged). The ENG-1 seems to be as you expected, the fuse is fine and the PP3 was showing a FALSE ground on one side of the fuse. Once I put a fuse back in, both sides of the fuse read 12.5v and the fuse did not blow. Where before the PP3 showed + 12.5v on one side and a ground on the other. Looks like I can eliminate that circuit. Slight bit of good news ;-).


Awesome, progress! Crossing everything on the ENG-1 circuit makes this a great deal easier. Also, it probably doesn't seem like a good thing, but finding a dead short is actually a TON easier than finding an intermittent one.

Question: The fuel gauge and temp gauge in the cluster are both pegged out of range. The voltmeter apparently does not work either in KOEO condition. Does this have any significance in leading me to where the short might be?


I don't believe so, were the gauges this way before the short started? The instrument panel isn't anywhere on the ECM-1 fuse, so I'm tempted to say it's not related.

So, the ECM-1 fuse still pops with the injectors, crank sensor, ignition coil, ignition module and VCM unplugged? Then you really are chasing a grounded pink wire somewhere. Something I usually do with short testing is install a test light, or other small bulb, in place of the fuse. This way, any time the circuit is shorted, the bulb will light (current flow to ground), and any time you wiggle the right wire and 'fix' the short, the bulb will go out. Fewer fuses you have to buy, and you have a better visual indication of the problem.

I can't really point you anywhere specific for the short, but there are a couple tips worth mentioning. 'Heat and vibration' is what Paul mentions in one of his videos, and I've found it to be good advice. Keep your eyes open on the areas of the harness most exposed to heat, and the most subject to vibration. Also, any places it looks like a previous repair has taken place.
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Steve Harris (65Westfalia) said Jul 10, 2016 20:28:57
Tyler,

Thanks so much for all the advice and help! I'll keep looking and I'm sure it's just a matter of time and persistence in finding the short.
So, the ECM-1 fuse still pops with the injectors, crank sensor, ignition coil, ignition module and VCM unplugged?
Yes.

The PP3 is nice as it has a sound that, while annoying is good at signaling when the fault is found by becoming intermittent once you disturb the short. You don't have to have someone watch the light or have it within sight all the time.

Worst case senario, I'll take it to a speciality auto electric shop for them to give it a shot. I'll at least have done the majority of detective work for them. lol.

Oh, I forgot to mention, here is a post I put up over on the Astro/Safari site. A possible answer from another van owner. One guy mentioned he had the same symptoms and the issue was a connector on the cabin side of the pass through connector. Unfortunately, I could see no way of accessing it short of removing dash/crash bar. Did not really want to do that.

One thing I did was pull the main harness at the bulkhead connector. The short was still there. Would that eliminate anything touching ground on the other side of the bulkhead connector?

Thanks again, and I'll update this thread with whatever happens. Even if I end up using the nuclear option (5 gal of gas and a match) ;-P
[Last edited Jul 12, 2016 03:50:57]
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