ECM-B Fuse blowing

[stig]

Our 95 Sub is acting up.

Here's the history of the current problem.

A 3 weeks ago the Sub shut off while my wife was driving. Happened before and was the PMD, so I put on the spare DTech and nothing - no start. Had it towed home and did a search on the diesel page. Checked fuses and low and behold the ECM-B fuse was blown. One thread on the search stated that the ECM-B fuse problem on his truck was fixed by moving the grounds at the rear pass side on the intake manifold to the next stud up. Checked the grounds and found 4 on the rear stud (1 braid that went to the frame, 1 short set of 2 wires into one lug, 1 heavy gauge maybe 10 or 12g, and 1 light gauge maybe 16g). I moved the heavy and light gauge grounds since they had enough wire for them to be moved easily. I left the braid and the 2 into 1 on the rear stud, but I cleaned and inspected them. Looked fine. The 2 wires I moved, were bolted to the upper intake manifold since it was convenient. Plugged in a new fuse and the beast cranked. This worked worked for about 2 weeks.

Then the Sub died again. ECM-B blown again. Had it towed home again. Since my last fix had to do with the grounds, I returnedt to them. While checking them again, I managed to break off a few lugs and had to solder on some wire and lugs. Moved the 2 wires I had previously moved to 2 different locations around the intake manifold. Kept blowing fuses when cranking - GP light for less than a second and then fuse blown. Another search and found a thread where a 97 was blowing the ECM-B fuse. It was suggested to check the items on that circuit - OPS, lift pump. OK - so figure if you unplug an item and then try cranking and if the fuse doesn't blow, then there is an issue with that item. As you can tell I am not electrical.

Anyway, I removed the fuel lift pump relay and cranked. No start - fuse blown,as usual. So when I went to put in a new fuse - it blew immediately!! Before that was not a problem - it would only blow when cranked. So apparently I screwed something up when cranking without the lift pump relay in.

Something is shorting somewhere, I guess.

Any thoughts? Anyone near Athens GA that would like to tackle this? Should I disconnect the batteries?

[stig]

Any electrical gurus like to chime in?

[THEFERMANATOR]

ECM-B fuse powers a constant power to the PCM and the lift pump power supply. I would suggest unplugging the oil pressure sending unit and lift pump and try another fuse. If it blows again then you need to unplug the 3 connectors from the ECM and completely isolate the circuit. If it still blows a fuse with everything disconnected you need to start checking the wiring harness for chafes or cuts where it can short out. If disconnecting the OPS and lift pump stops the fuse from blowing then reconnect the OPS. If that causes it to blow then your problem is a shorted OPS to ground internally. If the OPS being in the circuit doesn't cause it to blow then it is most likely the lift pump doing it.

[sailun]

I think you were reading my old post..............

I just took a look at mine, the two-into-one wires are the ECM grounds: Black with white stripe, and light brown or tan, on my '97. So if you didn't move those off the rear stud, most likely you didn't fix the problem.

If you move all of these grounds to one stud, and clean up the braid where it goes to the frame, you should be in a better place.

The braid may need to be replaced, I also added an additional body-to-engine or body-to-frame ground, and I've never blown an ECM-B fuse since. It's been several years and many thousands of miles.

Hope this helps, let us know.

[tommac95]

Stig-
On the '95MY many have profited by renewing the ignition switch. The wiring seemed in flux before MY'96.
That said , grounding is critical. The InMan bolts 'loosen' over time since the gasket beneath dries (loses elasticity, maybe shrinks) .
On '95 many of the frame-ground screws on the psgr side by engine were stripped.
> I also added an additional body-to-engine or body-to-frame ground, and I've never blown an ECM-B fuse since. It's been several years and many thousands of miles.
Sailun's advice is good. I'd suggest do Both [Frame/Engine Battery/Engine], and have them join on the InMan by the rear engine wire harness ground , psgr side.
Renewing battery cables is not a bad idea under circumstances-- think of the heavy battery cable-connections as forming an electrical Foundation upon which the +/_ 5 VDC signal pulses travel....
A recent post by Robyn in the Members forum suggests that an aftermarket supplier is relevant ... either a welding shop or maybe J Kennedy has some. I got new dealer cables 8 years ago , but apparently they no longer have the goods....

[JohnC]

This is an older thread, so I wonder if the problem was ever corrected?

That said, blown fuses are almost never caused by bad ground connections; on the contrary, they are generally caused by a path to ground that is not intended. Moving the grounds on the back of the intake manifold should have no effect unless they were not good grounds to start with. The key there is that all the ECM wires and the engine be grounded together and the engine to body.

The most likely suspect for the ECM B fuse is the lift pump circuit. If the fault was in the ECM itself I'd expect you'd have a lot more issues, but that's really the only other possibility.

[tommac95]

Good post , John ... i very much appreciate the logic (which initially took me aback).

My first introduction to 'unintended circuitry' in the automotive context was occasioned by turn-signal and brake filaments operating dependently (unexpectedly) on my first car , a 6-VDC VW. Poor/inadequate grounding of the lamp socket(s) allowed a voltage divider to be established for the ground circuit .

This does not contradict your observation/assertion that fuses are not blown in these instances .

A single grounding location has a lot to recommend it.
-T m

[JohnC]

Yup, bad grounds in tail lights, or anywhere else that multiple circuits meet, are always interesting...

[Robyn]

I had a gound go bad in a 86 GMC 454 distributor coil once.

(the lead broke) reving the engine would make the wipers intermitently activate.

Now that one was a real shocker. The engine started running goofy and when I pulled the cap off to look thin gs over I discovered the broken ground.

Fixed the ground and the engine ran fine again and the wiper thingy stopped :eek:

Yess grounds can make things get real hinky

Robyn

[sailun]

When I was having my ECM-B problem, the person who recommended moving the grounds to another stud, said he was a GM mechanic, and had seen this before. (He saw my Burb die, and pulled over to help)

I don't have the technical knowledge to verify this, but he claimed that the rearmost stud, which is used as the ground, penetrated or came very close to a coolant passage, and was installed with a heavy dose of sealant, which may have compromised that stud's qualities as a ground.

Not starting a debate here, just passing along what I was told. Considering the just-barely-adequate-when-new design and build mentality of our beloved GM products, it seems likely.

Battery cables: If anyone makes the cable for pass-side batt to starter, with the same 'L'-shaped terminal end, I'll buy one. I can't find an acceptable replacement, I don't want to buy the crimp tool, and they're not available from the factory anymore. I previously replaced the pos-to-pos cable between batts, which had the invisible cancer (thanks, Robyn).