ECM fuse blowing.

[wtd]

Today out of the blue, my daily driver 92 Cavalier died as soon as I got home and right before I put it in park. It shut off like you turned off the power to it.

I checked the fuses and found that my ECM fuse was blown. I installed another fuse and I guess it blew as soon as I turned the key on because the car just cranks and doesn't start. I checked the fuse again and it was blown.

Car has 230,000 miles on it and I assume the original ECM in it. I bought it with 76,000 miles on it. I've had no issues until today.

Is there any way to test the ECM to see if it's bad? What else can I test? Thanks for any information.

 

[MrMoody]

It's more likely a short in the wiring harness somewhere, maybe around the steering column or console.

 

[eljefino]

The ECM has "quad drivers" that run relays: basically for your car it'll be the fuel pump, radiator fan, torque converter clutch solenoid, AC stuff if you have it. These pass the most current and are most suspect.

But as said above anything can drag it down. Some sensors need a regulated +5 (?) volt feed from the computer so if any sensor or wiring goes bad it can take the whole shebang down with it. You can wiggle your under-hood harness to see if and when another fuse will blow, or replace the fuse with a little light-- when the light goes out, your short has gone away.

If you can get the ECM to live long enough to read codes you can see if it's got an internal checksum error, IDK the OBD-I code for that. You can also look for burned pins, crack it open and look for cracked solder joints. Something that blew the fuse might have left a mark inside.

New PCMs are surely cheap on ebay, but get the MEMCAL chip correct for your exact car. And if your wiring is not up to snuff you could cook that replacement quickly too.

 

[wtd]

Well, I believe I have it kind of isolated to the fuel system because when I pull the fuse for the fuel pump and then turn the ignition on, the ECM fuse does not blow. The fuel injectors, fuel pump relay, and the fuel pump switch/oil pressure sender switch is on fuel system circuit. Now I have to figure out which one of those could be causing the problem.

The fuel injectors have been replaced in the past but I don't know if a bad injector would cause this issue.

I have replaced the oil pressure sender switch if this is the one that screws into the block and shows the oil pressure on the gauge in the instrument panel. It's been quite a few years ago though. I guess I need to check the wiring that goes to it but it could have an internal short.

I can try switching the fuel pump relay with one of the other relays to see if that makes a difference.

It's 18 degrees out right now so I'm not going to be doing too much diagnosing outside at the moment.

In your experience, is any of those things mentioned the more likely culprit?

 

[wtd]

Looking at the wiring diagrams some more, the fuel pump itself is on that circuit. The fuel injectors do not appear to go through the fuel pump fuse so I don't think it's the fuel injectors. The fuel pump fuse does not blow so what does that mean?

 

[wtd]

Here are some pictures of the wiring diagrams from the factory service manual that deal with the ECM and Fuel Pump fuses. I'm confused why when the fuel pump fuse is installed it will blow the ECM fuse but not the fuel pump fuse if there is an issue on the fuel pump side of the circuit.

I'm not very good at reading these diagrams so can someone look at these and tell me what part of the system could be at fault given the fuse situation. Thanks.

This car has the 2.2L/auto drivetrain.

 

[zrxkawboy]

If I'm seeing that right, looks like they're both 10A fuses. If so, either one could blow first. I'd guess the ECM fuse is blowing first because there are other loads on it in addition to the fuel system.

 

[StevieC]

Try pulling the fuel pump relay and see if the fuses blow. If they don't then it's either the relay (most likely) or the pump itself. If the fuse blows again the problem is elsewhere.

 

[wtd]

I unhooked the fuel pump relay and the ECM fuse still blew. I also replaced the oil pressure switch because it was leaking oil pretty bad and it had a lifetime warranty so it was free so I know it's not that.

The ignition control module and one coil was replaced about 5,000 miles ago.

Could it be the ECM that is bad? Is there a way to test it? Thanks.

 

[MrMoody]

Without seeing the whole diagram, it sounds like the short is somewhere between the fuel pump fuse and relay, or something that feeds from there.

 

[StevieC]

Originally Posted by wtd
I unhooked the fuel pump relay and the ECM fuse still blew. I also replaced the oil pressure switch because it was leaking oil pretty bad and it had a lifetime warranty so it was free so I know it's not that.

The ignition control module and one coil was replaced about 5,000 miles ago.

Could it be the ECM that is bad? Is there a way to test it? Thanks.

You can disconnect the ECM but IMO it's highly unlikely that this is the cause and you don't really want to go messing around with its connectors if you don't have to because it can be hard to get apart depending on corrosion/oxidation around the plastic connectors on the metallic casing and there is a high risk of bending the pins if you aren't careful. I would disconnect other things fed off that fuse and if it still blows then check for shorts in the wiring.

Also have you looked at the other side of where the fuses are for wires that are melted together that feed the fuse block. I've seen this sort of stuff melted together before in GM vehicles.

 

[wtd]

Originally Posted by MrMoody
Without seeing the whole diagram, it sounds like the short is somewhere between the fuel pump fuse and relay, or something that feeds from there.


Diagram 1 above runs into Diagram 2 and are on adjacent pages. Diagram 3 shows everything that is connected between the ECM and the Fuel pump fuses.

All the other stuff on the page in diagram 1 is what it tied to the instrument gauges fuse and the other stuff in diagram 2 is the fuel injector wiring for the V6 powered cars and the wiring related to the turn and back up light fuse.

There are no other diagrams that are related to both the ECM fuse and the fuel pump fuse. Those three are it.

 

[wtd]

Originally Posted by StevieC
Originally Posted by wtd
I unhooked the fuel pump relay and the ECM fuse still blew. I also replaced the oil pressure switch because it was leaking oil pretty bad and it had a lifetime warranty so it was free so I know it's not that.

The ignition control module and one coil was replaced about 5,000 miles ago.

Could it be the ECM that is bad? Is there a way to test it? Thanks.

You can disconnect the ECM but IMO it's highly unlikely that this is the cause and you don't really want to go messing around with its connectors if you don't have to because it can be hard to get apart depending on corrosion/oxidation around the plastic connectors on the metallic casing and there is a high risk of bending the pins if you aren't careful. I would disconnect other things fed off that fuse and if it still blows then check for shorts in the wiring.

Also have you looked at the other side of where the fuses are for wires that are melted together that feed the fuse block. I've seen this sort of stuff melted together before in GM vehicles.

I unbolted the connector that bolts on to the firewall inside the engine compartment that I assume goes to the fuse block and nothing looks burned or melted. I have not looked at the wires that feed the fuse box that are inside the car.

I did hook up the fuel pump prime connector to direct battery power and the fuel pump did come on and it did not blow the ECM fuse so I'm going to assume it's not a problem on the fuel side of things. I did double check all of the rest of the fuses in the fuse panel and none of the other fuses were blown.

I guess I'm just going to have to look at everything I can and see if I see anything obvious. Thanks for the ideas.

 

[wtd]

I pulled the fuses one at a time for the heat/AC, the radio, and the fuel pump and the ECM fuse blew each time I tried starting the car. So now it's blowing with the fuel pump fuse pulled.

Is it possible that an internal short in the battery could cause this? The battery is almost 10 years old and I have not tested it yet but it's been starting fine.

 

[MrMoody]

OK, the ECM fuse powers the ECM, the ignition module, and the fuel injectors. Follow the wires from the fuse to these things, looking for a spot where the wires have rubbed through or gotten against something hot and melted. The power wire is pink with black stripe.

Also try unplugging the ignition module, this is somewhat more likely to have failed and shorted out than the ECM. If it does turn out to be bad, replace the coils too.

Fuel injectors can short out but the duty cycle should be too short to blow the fuse unless the ECM is bad ...

 

[wtd]

I just got done testing the battery and it says it's bad. It has 12.34 volts and 457 cca's. When "bad" shows in the display, it means the battery cannot hold a charge or has at least one short circuited cell.

I'm going to replace the battery but I have my doubts that this is what the problem is because I have never had a bad battery blow an ECM fuse but maybe I will get lucky.

I got my use out of this battery. It's a Super Start Extreme from O'reillys manufactured in May of 2009.

 

[wtd]

Originally Posted by MrMoody
OK, the ECM fuse powers the ECM, the ignition module, and the fuel injectors. Follow the wires from the fuse to these things, looking for a spot where the wires have rubbed through or gotten against something hot and melted. The power wire is pink with black stripe.

Also try unplugging the ignition module, this is somewhat more likely to have failed and shorted out than the ECM. If it does turn out to be bad, replace the coils too.

Fuel injectors can short out but the duty cycle should be too short to blow the fuse unless the ECM is bad ...


I will try doing that tomorrow. Like I mentioned above, the ICM only has about 5,000 miles on it because it went out last year but the car would still run but only on 2 cylinders. The one that went out 5,000 miles ago was the replacement for the original ICM that went out. Neither time when the ICM went out did it blow the ECM fuse. I have another 92 Cavalier that we had to replace the ICM on it too and it never blew this fuse either.

Unfortunately, the ICM on this car is bolted on the back side of the block and very hard to get to and not fun to replace.

 

[MrMoody]

A bad coil can overload the ICM and cause it to fail prematurely.

 

[StevieC]

I would assume it's not on the fuel side and would disconnect ignition components and attempt to turn the key on and see if the fuse blows. If it doesn't connect one thing at a time until it does blow to narrow it down. If it still blows with ignition components disconnect you are most likely looking at a wiring fault somewhere or melted pins in connectors somewhere.

I've seen bad ignition coils fry an ECM before because they backfeed to the ECM. There is some protection in the ECM for this but it can't hold out forever and certainly not if the spike being backfed is extremely juicy.

 

[wtd]

I did install a new battery because the parts store tester also showed it being bad. Of coarse it did not fix the problem.

I also installed a used ignition switch to see if that was the problem and it wasn't so I put the other one back in. This switch had been replaced in the past.

My next step will be disconnecting the ICM and see if the ECM fuse still blows. The ICM's on these cars are kind of temperamental and is why I've always made it a point to keep all my grounds clean on this car. It's possible that the one older coil that I left on when I replaced the ICM and the other coil recently, could have failed and taken out the ICM. Always in the past I have replaced both coils when replacing the ICM but I didn't have any justification to warranty the older coil I left on. The other coil had a crack in it which they warrantied with no problem.