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Blowing ECM "B" fuse

Re: Blowing ECM \"B\" fuse

Questions:

Are you using a factory harness or aftermarket?
Do you have the oil pressure switch wired into the harness?

Andy
 
Re: Blowing ECM \"B\" fuse

It is all factory. 1990 GMC Jimmy. It has a oil pressure gauge. And it works. There is an aftermarket alarm wired into my ignition but it has been there since I got the truck. I am trying to figure out how to disconnect it.
 
Re: Blowing ECM \"B\" fuse

I don't mean oil pressure gauge, I mean an oil pressure switch that runs to the ECM... I don't even know where it would be on your truck. This switch allows the fuel pump to run in the event of a relay failure... I need to see how it's wired in. Maybe the 90 didn't have one. The only issue is that it might be something else to check on but that depends how it's wired.

Or it might not have it at all.

Andy
 
Re: Blowing ECM \"B\" fuse

Where wold I find this info. The schematics in the chilton are so vague. It is ridiculous. I cant find any on line either. I am going to call the chevy dealership i n town at lunch and see what they have. I dont know if I have a switch. When you say to allow it to run if the relay fails? Is this the fuel pump relay or oil pressure relay (is there one). I have heard before that a failed oil pressure sensor will cause the fuel pump not to run? How is this? If it causes the pump to lock up would that make the pump work as a giant resistor blocking the current therefore blowing the fuse? I am not sure about this?
 
Re: Blowing ECM \"B\" fuse

There are 3 things on the ECM B circuit on GM TBI trucks. Oil pressure switch, fuel pump relay and fuel pump. On one S10 I used to own all the wires to the oil pressure switch melted on the exhaust and started shorting out (switch is mounted near oil filter). Other people I have heard from have had bad grounds at the fuel pump cause this problem. So both are good places to look.
 
Re: Blowing ECM \"B\" fuse

I didnt know about the oil pressure switch. I will check that first. So I am looking for a sensor (?) that plugs in near my oil filter. Then the wires run from there to????
-Shaun-
 
Re: Blowing ECM \"B\" fuse

Stupid question, but you said you replaced the fuel pump, did you also replace the fuel filter?

Another place to look at is the ECM, we had a truck come in at work for the same exact problem, well we put another ECM in it with a new PROM and the other little chip in there and it fixed it. It turned out being the other little chip in there, not the PROM, would cause the ECM B fuse to blow, maybe thats what your problem is. Especially since your battery went crazy.
 
Re: Blowing ECM \"B\" fuse

I hope it isnt that chip. How much would that cost to replace. Can do it myself? Yes the fuel filter is new. I could replace the whole ECM if I had to. Its 90 bucks but I am geussing through GM (which is probably the only place you could get that chip if you can) that chip will be spendy?
 
Re: Blowing ECM \"B\" fuse

Not to be confrontational but the ECM is not causing your problem here.

Start with the fuel pump relay. I'll dig out the schematic to see how the oil pressure switch is wired up in a few minutes...

Andy
 
Re: Blowing ECM \"B\" fuse

Thank you so much. I just got off the phone with a friend that works at the local gm dealership and she is going to check into getting me a bunch of scats.
 
Re: Blowing ECM \"B\" fuse

Here ya be:

747ecm4.jpg


Start at the little pitchfork looking thing (chassis ground) and work up from there. You see it goes to the fuel pump right away, then to the oil pressure switch and the output of the relay.

When the oil pressure switch sees 4PSI it turns the fuel pump on. As long as there is 4PSI your fuel pump *cannot* turn off, remember this.

So, now what? It's the same situation as I explained in an earlier post... but now the possibility is that wire with +12 that runs to the oil pressure switch is shorted out on something (like the exhaust as someone already suggested).

Andy
 
Re: Blowing ECM \"B\" fuse

[ QUOTE ]
but now the possibility is that wire with +12 that runs to the oil pressure switch is shorted out on something (like the exhaust as someone already suggested).



[/ QUOTE ]

This is what I was thinking. But now this will all have to wait another day b/c I have to stay overnight at work. Man sometimes I hate my job. Thank you guys for all the help and if there are any more ideas please post up.
 
Re: Blowing ECM \"B\" fuse

[ QUOTE ]
Not to be confrontational but the ECM is not causing your problem here.

Start with the fuel pump relay. I'll dig out the schematic to see how the oil pressure switch is wired up in a few minutes...

Andy

[/ QUOTE ]

Not to be an ass, But yes I have seen an ECM cause a blown ECM B fuse. It is possible and Yes the ECM does control the fuel pump relay. We replaced the ECM with a ECM that we knew was good and the problem went away, and to check our work we put the old ECM back in and it still blew the fuse. Ill see If I can find my spare ECM we used so I can show you the chip that was causing it.

Oh and squash im a Tech at the local Chevy dealer.
 
Re: Blowing ECM \"B\" fuse

Dude, chill out! I said *his* problem isn't the ECM, not yours. And for that matter I don't care who you work for, I know more than 90% of the dealer tech's I've ever talked to. Running around exclaiming that you're a dealer tech is the quickest way to get ignored.

Andy
 
Re: Blowing ECM \"B\" fuse

I will just try the simpler of the two which is going to be a ground and if I havent fixed after tracing everything in that circuit then I will replace the ECM. It cant hurt right. /forums/images/graemlins/dunno.gif
 
Re: Blowing ECM \"B\" fuse

It's not an ECM issue. If you simply turn the key to run and the fuse blows then the oil pressure switch isn't allowing the ECM to draw any current from this fuse B. If there were any other internal shorts in the ECM it would blow one of the other fuses, not the fuel pump fuse.

It's not a grounding issue either. It's possible to blow a fuse with crappy grounds but to blow a 30 amp fuse repeatedly just isn't logical.

Andy
 
Re: Blowing ECM \"B\" fuse

There are two oil pressure sending units on your truck, the one that looks like a bell (big ole thing, doesnt really look like a bell but whatever) runs the gauge I believe, and the other (for the life of me i cant remember where it was on my sbc) runs to the computer. If the one running the computer is f'ing up, or if the wiring to it is screwed up, it will shut the truck down, by killing the fuel pump which could be causing it to be a resistor therefore killing the fuse. Check that sensor. On my sbc i know the gauge bell looking one was up near the distributor, and for some reason i am thinking there was a T with the other sensor hooked in there, but I am not positive on that, it may have been my bbc. Also as I recall the two oil pressure sensors are between 25-35 bucks and the ECM is around 150-200, its been awhile since I thought my ecm was bad lol.
 
Re: Blowing ECM \"B\" fuse

[ QUOTE ]
It's not an ECM issue. If you simply turn the key to run and the fuse blows then the oil pressure switch isn't allowing the ECM to draw any current from this fuse B. If there were any other internal shorts in the ECM it would blow one of the other fuses, not the fuel pump fuse.

It's not a grounding issue either. It's possible to blow a fuse with crappy grounds but to blow a 30 amp fuse repeatedly just isn't logical.

Andy

[/ QUOTE ]

It can still be an ECM issue because if you follow the circuit up above the oil pressure switch is only a back up power source for the fuel pump with the fuel pump relay being primary. Easiest way to know if your relay is bad is a delayed start while cranking (basically waiting for a little oil pressure to build up to activate the fuel pump).

Checking/cleaning the grounds is a good idea anyway and cheap if not free. After that I would look at your oil pressure switch wiring for heat damage and possibly test the switch as well. The relay's are also fairly cheap. ECM is probably the most expensive part and hard to test short of replacement but I would trace wiring looking for an intermitant short before spending the money if it were me.
 
Re: Blowing ECM \"B\" fuse

[ QUOTE ]
It can still be an ECM issue because if you follow the circuit up above the oil pressure switch is only a back up power source for the fuel pump with the fuel pump relay being primary. Easiest way to know if your relay is bad is a delayed start while cranking (basically waiting for a little oil pressure to build up to activate the fuel pump).

[/ QUOTE ]

It cannot be the ECM in the context of the discussion so far(relay being removed). If the relay is removed and there is no oil pressure the ECM can't pop that fuse.

I want to know what happened to the battery...

Andy
 
Re: Blowing ECM \"B\" fuse

[ QUOTE ]
I want to know what happened to the battery...


[/ QUOTE ]
I removed the inner fender wells (the bottom bolt of the battery tray bolts onto this and I forgot) 7 or 8 days later (the day I broke my truck) I was jumping it and after the third one the tray (supporting the weight of my battery) broke free of the inner fenserslamming against the radiator. It then proceeded to leak and the positive terminal was somwe how loose.

Well I dont have to stay late anymore so I am going to go home and try and figure this out. I will start with the oil pressure sw, checking its wres and grounds. Where is this second oil pressure sensor? Is it in the block somewhere. I will the trace the wires to my relay and test the relay. What is the best way to do that? Then I will progress on to the ECM. But some of you feel it cant be my ECM. So could my oil pressure sw have failed? HOw do I test it? Or do I just replace it?
 
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