Mechanics

91 ELECTRICAL PROBLEMS

1991 Chevrolet Suburban • V8 2WD • 250 miles

My 1991 Chevy Suburban keeps blowing the ECMb fuse. Last Saturday the car started and ran for a minute or two. It stalled when I turned on the windshield washer. I couldn't get it started again, but the starter did continue to turn when I turned the key. I replaced the fuse but it blew again on the first attempt to start the car. I checked the distributor and replaced it and the rotor because two of the posts were spinning. A new fuse and it started. I let it run in the driveway for about 10 minutes. I turned the car off and tried to start it again and the fuse blew again. Called a local shop that specializes in electrical work. They told me it sounds like a short between the ignition switch and the fuse. I asked about using a stronger fuse just to avoid a tow charge and drive it in. They told me it would be fine. It has since blown 15 and 20 amp fuses (rated for 10amp). I've checked all the wires I can reach, both under the dash and under the hood. I don't see anything worn. Any ideas anyone? I have very little experience working under the hood other than changing a belt or the oil. Thanks in advance.


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BrianJansen
December 5, 2010.



When fuses are shorting out, NEVER use a larger capacity fuse. It can result in the electrical wirings starting a fire or melting and causing all the wires to be fused together.

The ECM-B fuse provides power to the fuel pump relay, fuel pump switch and PCM. Any of these components or their wiring can be faulty causing the shorting.


KHLow2008
Dec 5, 2010.
Thanks, KHLow2008. I appreciate your help. 2 things - can you define PCM a little more? I've tried Autozone and O'Reilly and neither website recognizes that as a part. Second - is there somewhere online I can go to get pictures of an engine so I can see where all these parts are located? That would help tremendously.


Tiny
BrianJansen
Dec 11, 2010.
You're welcome.

PCM = Power Control Module also known as ECM (Engine Control Module ) meaning the engine computer. It is located under right side of dash.

The ECM would be less likely to be the cause as they seldom shorts out.

Which engine variant do you have, 5.7 or 7.4 L?

The fuel pump circuit would be more likely to be the cause. As the wires runs from fron to rear of vehicle, there could be a possibility of some wires being damaged to external elements such as abrasion or rodent damages. Intermittent shorting is never easy to detect and a stroke of luck helps.

Fuel pump relay is located on right side of firewall in relay box.

I could not find any detailed diagrams and description of the relay box or other components.

Unplug the fuel pump relay.
Use a 10 A fused wire apply battery voltage to terminal B.
Alternatively you can use the fuel pump test connector locatred near relay box to connect the wire.
Start engine.
If fuse blows, you have a short in the fuel pump circuit.

Another components wiring that could cause the shorting of the fuse would be the fuel pump oil pressure switch, located at left rear of engine for 5.7L and at front side of engine block near oil pan for 7.4L.


KHLow2008
Dec 12, 2010.
Thanks again. I have the 5.7L. I'll run the wires tomorrow and let you know what I find. I truly appreciate your help. I'll keep looking. I just dread that by the time I finally find the problem I will have replaced a bunch of parts that didn't really need replacing.


Tiny
BrianJansen
Dec 12, 2010.
Don't replace any parts until they are confirmed to be faulty.

Shortings usually are due to wiring faults and the only component that can cause it would be the fuel pump.

Btw if your vehicle is equipped with dual fuel tanks, switch to the other pump and retest.


KHLow2008
Dec 12, 2010.