Blowing ECM B fuse

Nyle · Nov 11, 2011

Alright guys, I have tried searching already, but I have not been able to find a definite answer thus far, so I'm hoping someone can point me in the right direction.

A few months ago I picked up a 1997 K3500 Crew Cab Long bed, SRW 6.5 VIN F with 160K miles, relocated PMD, and 4" DiamondEye exhaust. A day after getting it home the ECM B fuse blew about 2 seconds after key-ing the ignition to the "run" position. I put a new fuse in and didn't have a single problem for about 2 1/2 months. But about 3 weeks ago the fuse blew again (while waiting for the WTS light to go out), leaving me stranded, but by the time I got it towed home and replaced the fuse the truck started right up. After looking at some wiring diagrams and at the advice of another mechanic I replaced the lift pump just in case that was the issue.

Truck ran great for the last 3 weeks, but today while leaving a local burger joint the truck died right in the middle of a 2 lane road(the other motorists were not pleased I blocked all westbound traffic). And once again, when I get it home and replace the fuse, I can start the truck up without issue.

Where would you guys suggest I start looking for a problem at? Is this a common problem that I have somehow overlooked in my searches?

edzzed · Nov 12, 2011

Is this a common problem ? I doubt it. In 8 years I have blown absolutely ZERO fuses anywhere on the truck. You must have a short somewhere, or the fuses were made in China. Have you tried replacing the fuse with a slightly higher amperage fuse. Ed

Nyle · Nov 12, 2011

I have tried using a 25A fuse just to see what would happen, and it popped that fuse just as quickly as it blew the 20A fuse.

Nyle · Nov 12, 2011

I have the wiring diagram for the ECM B fuse in front of me right now, and the only things I can find on the circuit are the lift pump, fuel pump relay, OPS, and water in fuel sensor. I have already replaced the lift pump and the fuel pump relay, so could the OPS or the fuel in water sensor be shorting out and popping the fuse?

And I apologize if any of these are simple or repeatedly asked questions, I'm reading alot of info on this site and doing my best to learn as I go.

HeavyChevy95 · Nov 13, 2011

Nyle said:
I have the wiring diagram for the ECM B fuse in front of me right now, and the only things I can find on the circuit are the lift pump, fuel pump relay, OPS, and water in fuel sensor.

So its NOT the fuse/circuit the WIF sensor is on.. Thats why I questioned the "blown ECM B fuse?" and went on to say "If its the same fuse the WIF sensor is tied into, another potential suspect would be the fuel heater, possible internal malfunction/short. IIRC, the GPR, fuel heater, WIF sensor, and EGR solenoids (if so equip'd) all shared the same fuse/ switch'd circuit."

Nyle said:
The fuel heater, WIF, MAF, GP relay, and wastegate solenoid are on the ENG 1 fuse. The FP relay, LP, and OPS are on the ECM B fuse. I have the engine performance wiring diagrams sitting right in front of me and double checked just to be sure I had the circuits and components correct.

Last night, I didn't have a schematic in front of me and I couldn't recall the exact fuse ID off hand, I thought the WIF sensor was tied to an ENG fuse but... I just remember the circuit configuration involving the WIF sensor. Had a fuse problem with that circuit myself a few years back.. Anyhow, sounds like you've got it figured out...

Good Luck..

Tommygunner · Nov 12, 2011

The OPS has always been troublesome in these trucks. I would try replacing that and checking all the wireing and grounds on tht circut. Also it might be wise to carry a box of spare fuses in your truck so you dont get stranded.

Nyle · Nov 12, 2011

I have about 30 spare 20A fuses to hopefully be able to limp it home if it happens again before I get the problem fixed. And from what I can see I'll have to pull the intake to replace the OPS, correct? Or is there an easier way to replace it?

edzzed · Nov 12, 2011

I am not sure but you could just bypass the OP's It is only a safety feature to shut off fuel in case of a car accident

Nyle · Nov 12, 2011

I personally like to keep most every safety feature intact whenever possible, but if I do get stranded with a blown fuse again, I'll try unplugging the OPS and see if it blows a new fuse or not. That should really help narrow down if the OPS is at fault or not.

Steve83 · Nov 12, 2011

Buy a 20A circuit breaker with a wired adapter to fit the mini-fuse block.

If you post the wiring diagram, I might be able to help you spot likely causes of the short, but unplugging things is the right way to find it. A larger fuse is the WRONG way...

If you can get to the point that the fuse ALWAYS blows immediately (not intermittently), it'll be even easier to find: instead of putting a fuse in, put a light bulb across the fuse socket terminals. As long as the short circuit exists, the bulb will light. When the short disappears, the bulb will go dim or out. Try it on a circuit that's NOT blowing a fuse to see. How dim the bulb gets depends on how many active loads there are on the circuit, and the bulb's wattage, so go with a small bulb (like 194). Once the bulb is staying fully-lit, start GENTLY working the wiring harness, and then unplugging things. If the light flickers, you're near the fault. If it goes off, you just unplugged the section that contains the fault.

THEFERMANATOR · Nov 12, 2011

In 97 the OPS is not needed as teh PCM triggers the fuel pump on before cranking for a prime purpose, and also triggers the relay once it is running. They simply left it in there since the wiring was already there like they did for gas engines of the same year. When it pops the fuse just unplug the OPS as the ECM will still trigger the fuel pump to come on. About the only likely culprit is a bad OPS or a shorted wire somewhere to pop that circuit since you alreay replaced the lift pump.

Nyle · Nov 12, 2011

Thanks for the advice everyone, when I get off work tonight I will fire the beast back up and drive around for a while to see if the fuse blows again. If it does I will unplug the OPS and try a new fuse to see what happens. But either way, it's getting a new OPS before winter hits just to be sure.

racer55 · Nov 12, 2011

Check the WIF sensor wires for any chaffing as well.

Steve83 · Nov 12, 2011

These pics show how to pull the OPS without pulling anything else:

Nyle · Nov 13, 2011

Did some poking around under the hood this evening, and I couldn't find any chaffed or rubbed through wires for the FIW sensor. I took a nice long test drive and the fuse didn't pop once, tried unplugging the OPD and the truck still ran just fine. So I called my buddy at the local parts store and told him to get me a new OPS for the truck(AC Delco, of course). Will get it installed tomorrow and keep my fingers crossed that there won't be any kind of electrical gremlin showing up later.

HeavyChevy95 · Nov 13, 2011

A simple and cheap faucet/basin wrench makes easy work of R&R'ing the OPS, without having to remove the lower intake manifold.

As for your blown ECM B fuse? If its the same fuse the WIF sensor is tied into, another potential suspect would be the fuel heater, possible internal malfunction/short. IIRC, the GPR, fuel heater, WIF sensor, and EGR solenoids (if so equip'd) all shared the same fuse/ switch'd circuit.

Good Luck..

Nyle · Nov 13, 2011

The fuel heater, WIF, MAF, GP relay, and wastegate solenoid are on the ENG 1 fuse. The FP relay, LP, and OPS are on the ECM B fuse. I have the engine performance wiring diagrams sitting right in front of me and double checked just to be sure I had the circuits and components correct.

acesneights1 · Nov 13, 2011

This happended to me. It was the harness under the intake going to the IP. I could not see the damage until I removed the intake and removed the harness and washed it throughly with degreaser.

Steve83 · Nov 13, 2011

Another way to pinpoint it would be to add an inline fuse for the OPS; the lowest you can find will be more than enough, but it'll blow before the ECM B fuse, and tell you that the OPS is the culprit. If ECM B still blows, it's a wire - there's no way the FP relay itself can go THAT bad, and the new LP isn't likely to have the exact same failure as the original, so they're both eliminated.

BTW
I hope you kept the old LP - there's probably nothing wrong with it.

Nyle · Nov 13, 2011

I kept both the old lift pump and FP relay, and I will more than likely put an inline fuse on the OPS harness while I am replacing the OPS either tomorrow or Monday.

Again, thanks for all the input and ideas everyone. I don't know what I would do without the help.

dmax3500 · Nov 13, 2011

youve got a dead short,try moving the wiring around and see if the eng dies,try brake torquing it to see if the eng moves around,,i had a customers s-10 that keep blowing a main fuse,,looked all over ,only to find the eng harness was only getting shorted out when the truck was in reverse[then the colume shifter lock bolt dug into the harness-shorting it out]

Nyle · Nov 13, 2011

This morning before heading out on a few errands I climbed under the hood to shake and wiggle every wire harness i could see or get my hands on, and the fuse still would not pop. Tried it with the key to "Run" and with the engine running. But I have my new OPS in hand right now, just need to find a few minutes this afternoon to run out and switch it over.

Nyle · Nov 14, 2011

Got the new OPS installed a short time ago, and while removing the old one the plastic body snapped clean off the metal base. That is a pretty good sign to me the OPS was literally falling apart and needed replaced. Now to just keep my fingers crossed that my issue is solved for good.

Nyle · Nov 22, 2012

Sorry to dig up my old thread, but my blown fuse issue has surfaced again almost a year to the day of posting my fix from the last time it happened. Was on the way to a Thanksgiving lunch when I crossed a set of railroad tracks and the engine immediately died. I come to a stop, open the hood and find the ECM B fuse blown. Tried swapping out a new fuse and unplugging the OPS, and it ran for about 5 seconds before dying again.

So I come to the 6.5 gurus of DieselPlace once again in hopes that someone can help me with a permanent fix this time. Any ideas guys?

FloridaHorseman · Nov 22, 2012

If you unplug the OPS there's nothing to power the LP, so the engine dying becomes a different problem than the ECM B fuse blowing... followed by the engine dying.

When you changed the OPS last year did you install a AC Delco unit? And the other part of the equation is the LP itself. If it has an internal short or a power wire grounding out it will also blow the ECM B fuse. Check the LP wires and then run 12v direct to the LP and open the water drain valve to make sure it's pushing fuel and not just vibrating while doing nothing. Also check for the LP generating excessive heat. ~FH

Nyle · Nov 22, 2012

The OPS was replaced with an AC Delco unit, and the lift pump was replaced at the same time. I have already done a visual inspection of the wires for the OPS and LP, as well as the "wiggle" test to see if there is a short that only appears when the wire harness flexes and nothing will blow the fuse with the OPS and LP disconnected.

Glagulator · Nov 23, 2012

The PCM is also powered by the ECM-B fuse. The two orange wires in that wad of wires that pass through the firewall on the pass. side. You might check behind the glovebox for an orange wire shorting on one of the many sharp metal edges in there. Since the short came back when you hit a bump in the road my guess would be a short, wire to wire or wire to ground. And since it went away for a year after working on various areas of the harness,
those areas are most likely to be where the problem is. Find a really bad bumpy road and see if you can re-create the problem.

Blowing ECM B fuse

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