ECM 1 20 amp minifuse

[Chris Cuzynski]

The ECM 1 20 amp minifuse blows as soon as I turn on the key. 1996 Tahoe Vortech 5.7 no mods. Happened the second day of the weeks worth of rain we had here in S. California last week.

Ran fine got home, went out 30 mins later no start. It will crank but not start.
The 20 amp ECM 1 mini fuse blows the minute you turn the key on. Fuel pump does not run, ignition system is not powered. Took fuel pump out of the tank and tested it with battery, it runs. Unplugged everything that is downstream from the diagram to isolate problem. Crank sensor, oil pressure sensor, alternator, coil, ECM everything that might be bad but cause a short was unplugged.

This is a huge short to ground, the minute you turn the key, it blows the fuse.
All other curcuits and fuses fine.

My next step is to pull this fuse panel from its mount under hood and see if any shorts show on the backside. Then trace all the power wires out. All the power out appears to be pink wires to different items.

[Eric VanBuren]

The only things that should be receiving power directly from the ECM fuse are the injectors, coil and computer. The sensors that receive a 5V ref from the computer get indirect power from that fuse via the ECM but those probably wouldn't cause the fuse to blow, more likely to fry the voltage regulator in the computer. The fuel pump has it's own fuse and a relay triggered by the computer controls it.

[Chris Cuzynski]

We tested the coil with an Ohmeter. Some question based on the repair manual if its correct. I will follow up with Autozones site. Even with the coil wires from the loom to the coil unplugged it blew the fuse instantly when the key was turned on.

The second you turn on the key...bam the fuse blows.

I get the impression that this receives power before it goes out to evrything else downstream electrically. None of the pink wires that are supposed to receive voltage from this fuse is receiving any...and obviously won't till I figure out the problem.

[Bill usn-1]

Have you unplugged the ecm and other main connectors and look for water/corrosion?

[Eric VanBuren]

Reading the schematic shows the only things powered by the ECM1 fuse are the injectors, coil, coil driver (ign module), crank position sensor and the ECM turn on power. There are no additional fuses downstream from the ECM1 fuse just the mentioned items. If it still blows the fuse with all the above mentioned items disconnected then yes you need to trace the wire from the fuse output side to the above items.

[Chris Cuzynski]

Bill, yes unplugged the ECM, no corrosion
Eric, we unplugged all the items you mentioned and it still blows fuses.
I think we went through 10 or 15 of them trying to isolate the problem.

I am ging to start at the ignition switch and go to the fuse box first, then from the box out.

The fuse protects everything downstream of it, that has all been isolated.
Upstream of it is unprotected so the problem should be there.
The power wire comes from the battery over to this fuse box, across the top of the radiator shroud, I suspect all the hot leads into the vehicle come off the bottom of this panel and run out to things that need 12 volts from there.
Sorry about the upstream downstream references, I am a fireman, it helps me think of things relating to hose and water flow and correlate it to electricity. I am so ignorant about this stuff I need colored schematics to follow wires!

Will try to work backwards to the switch and see if that is it.

[Eric VanBuren]

The ignition switch has nothing to do with it, it is upstream from the fuse and cannot cause the fuse to blow, only things downstream from the fuse can cause it to blow. The upstream part of the circuit is protected by a maxi fuse or fuseable link somewhere.

[Chris Cuzynski]

Eric, now that you mention it we did not unplug the connector to the injectors on the upper intake.

Will try that and then see what happens.

Bill, the crank sensor was wet, but when we blew it out and let it set for a while, it still blew a fuse, with the sensor unplugged of course.

[Chris Cuzynski]

OK guys, its seems to be fixed. Unplugged the injector harness from the top of the intake, plugged it back in and it started right up! Ran a little rough, shut it off and restarted and it settled right down.

I think that I may have gotten water on the MAF sensor. When I checked the air cleaner box it was damp on the bottom and shows indications of grit in it.

My theory is during freeway driving enough water ran into the plastic intake ducting from the front of the vehicle (runs inside the passenger fender) that it got the air cleaner damp. When I pulled into my driveway (upslope) enough moisture ran down the intake tube to possibly get water on the heated wire of the MAF and short it out. Does this sound feasible?

I have a 93 TBI truck and Suburban. I've had the truck in so much mud that it has gone up that ductwork and through the plastic oval intake plenum into the hose to the snorkel of the air cleaner. So I know it is possible for water/mud to travel that distance. The Vortec engines intake tract is shorter and used a big round air filter right on top of the inner fender, so it does have the possibility to get enough water up that far.

I may just remove that inner fender duct and drill some drain holes in the bottom of it for good measure or remove it completely, like I did on the 93's.
I will lose some of the ram air effect of the inner fender duct but not much since the fender is closed most of the way anyhow. The air filter box has drain holes in the bottom too, so I will probably just enlarge them a little.

Any other thoughts, suggestions or theories are encouraged!

[Eric VanBuren]

Water in the MAF could cause problems but that would not cause the ECM 1 fuse to blow as it is not on that circuit, and the fuse protecting the MAF cir blowing would not have caused the computer to not turn on and fail to provide the prime pulse to the fuel pump relay.