1989 Chevy V3500 popping ecmb fuse when turn key

[that_wide_square]

I’ve got a 1989 1ton with the 350 tbi system, popped a fuse on me driving, didn’t think much of it and put another one in, started back up, later in the night coming home it popped the ecmb fuse again, now it pops every time I turn the key. Someone please help!

 

[gmbellew]

the power to ECM and power to fuel pump all goes through ECMB fuse. power to the fuel pump also splices off and goes through the oil pressure switch so your engine doesn't die if the relay fails. it is constant hot. it runs on an orange wire in the main harness along the firewall on its way from the fuse block to ECM, oil pressure switch, and fuel pump relay. it is prone to rubbing on the various brackets and if it shorts to ground it blows ECMB fuse and then the ECM and the fuel pump have no power. not sure why it blows only when you turn the key, unless the short to ground is downstream of the fuel pump relay.

 

[that_wide_square]

It doesn’t pop the fuse when I unplug the ecm though. I checked the wiring on the firewall and it looks good.

 

[SirRobyn0]

It doesn’t pop the fuse when I unplug the ecm though. I checked the wiring on the firewall and it looks good.

Well then I would be thinking the ECM is shorted.

 

[that_wide_square]

Anyway to fix it or should I just order a new ecm?

 

[SirRobyn0]

Anyway to fix it or should I just order a new ecm?

Not practically, if you have experience with electronics and circuit board you could try, but most of us would want to just order a new one.

 

[Bennyt]

For 10-15 years I owned an '89 Jimmy (K5) and if my memory is correct, it would pop the ECM fuse every few days, and occasionally would immediately pop one as soon as installed. I was having to buy packs of 25 of that particular size about every month for 3-5 months while having this issue.

It was intermittent so very difficult to diagnose but would happen on one particular turn on my commute home with a small bump in the road.

Anyways, I finally found one wire that ran over the transmission, I think it was to a 02 sensor, and the clip that held it to the body and come loose and the wire would bounce onto trans. After probably many years of doing this, it wore the sheathing and would ground.

 

[that_wide_square]

For 10-15 years I owned an '89 Jimmy (K5) and if my memory is correct, it would pop the ECM fuse every few days, and occasionally would immediately pop one as soon as installed. I was having to buy packs of 25 of that particular size about every month for 3-5 months while having this issue.

It was intermittent so very difficult to diagnose but would happen on one particular turn on my commute home with a small bump in the road.

Anyways, I finally found one wire that ran over the transmission, I think it was to a 02 sensor, and the clip that held it to the body and come loose and the wire would bounce onto trans. After probably many years of doing this, it wore the sheathing and would ground.

I actually unplugged my o2 sensor so I’ll give it a look at today.

 

[gmbellew]

if the issue doesn't happen with the ECM unplugged, it could still be something in the wiring between the relay and the fuel pump. I'd plug the ECM back in, unplug the fuel pump relay, and then turn the key on. if it doesnt blow the fuse, it probably wont be the ECM, and is likely something in the wiring to the fuel pump. the ECM is what closes the relay to send power to the fuel pump.

 

[that_wide_square]

if the issue doesn't happen with the ECM unplugged, it could still be something in the wiring between the relay and the fuel pump. I'd plug the ECM back in, unplug the fuel pump relay, and then turn the key on. if it doesnt blow the fuse, it probably wont be the ECM, and is likely something in the wiring to the fuel pump. the ECM is what closes the relay to send power to the fuel pump.

So I plugged the ecm back in and unplugged the fuel pump relay then turned the key and it didn’t pop the fuse

 

[gmbellew]

So I plugged the ecm back in and unplugged the fuel pump relay then turned the key and it didn’t pop the fuse

so there is likely a short to ground between the relay and the fuel pump.

 

[SirRobyn0]

so there is likely a short to ground between the relay and the fuel pump.

I would agree with that. So scratch the ECM off the list.

 

[gmbellew]

you may be able to crawl under and listen as somebody turns the key and find the spark quicker than just tracing the wire and inspecting. but a bare wire must be sitting on bare metal right now....

 

[Stumann666]

I had similar troubles some year back. It turned out to be the big wiring harness behind the engine. It had rubbed a wire bare between the engine and the firewall and would randomly short out and blow a fuse. Try reaching behind there and wiggling that wire harness wit the key on and see if that blows the fuse.