Anybody have their PCM fuse blow repeatedly? Mine has been blowing once every couple of months, so I've been carrying a few extras with me. Now they blow as soon as I replace it and try starting the car again. Anybody seen this before? It is one of the big cubical 20A fuses in the distribution box that cost $3 each.
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PCM Fuse Blowing
- Thread starter maddog440
- Start date
year of car and miles? has to be a short ... sounds like you have a pre98 ... and if that is the case you wiring harness is most likely bad ...
Linkman
CEG'er
Yes, I've seen it before 
Doesn't have to be a pre98, mine is a Sept. 97 build date 98 model year. PCM fuse blew an hour and thirty minutes and 110 miles into a 130 mile trip. Towed home. Believe the short is in the main wiring harness that runs from driver side strut tower across back of engine to the PCM connector.
Not a fun job at all to figure it out, I'm not certain I've actually figured out, and I still need to pull that harness and actually find the trouble spot. Car's been in the garage for longer than I'd care to admit.
Good luck finding the problem. The recommendations I got were to start with the harness connectors at every engine sensor because just about every engine sensor is powered through that fuse. Then start probing with a multimeter. If you replace the harness with a new one, it is NOT inexpensive.
I'll post more once I get back into working on my 'Tour. I've just ignored it all year so far.
Doesn't have to be a pre98, mine is a Sept. 97 build date 98 model year. PCM fuse blew an hour and thirty minutes and 110 miles into a 130 mile trip. Towed home. Believe the short is in the main wiring harness that runs from driver side strut tower across back of engine to the PCM connector.
Not a fun job at all to figure it out, I'm not certain I've actually figured out, and I still need to pull that harness and actually find the trouble spot. Car's been in the garage for longer than I'd care to admit.
Good luck finding the problem. The recommendations I got were to start with the harness connectors at every engine sensor because just about every engine sensor is powered through that fuse. Then start probing with a multimeter. If you replace the harness with a new one, it is NOT inexpensive.
I'll post more once I get back into working on my 'Tour. I've just ignored it all year so far.
Sounds like a blast. My car is a '96 SE 2.5 MTX. I'm sure the wiring harness was never done before, and I've suspected it was faulty wiring somewhere - it's been an intermittent problem until this point, so it would be kind of hard to find without a lot of searching. We had all the wiring, computers, and engine replaced in my girlfriend's '95 under warranty a couple of years ago- all related to the wiring issue, so I realize the scope of the job. I would junk the car before tackling an entire harness.
If you could let me know what you find linkman, that would be excellent.
If you could let me know what you find linkman, that would be excellent.
OK. I decided to pull the 104 pin connector off the PCM and start checking all pins for ground. My thought behind this was that power flows into the PCM and is distributed to various sensors, injectors, etc. If power to one of them goes to ground, I would have a short and blown PCM fuse. If I check resistance of all sensor inputs, I should get close to zero ohms to ground.
These are the pins that were close to zero ohms:
pin 31 (W) - power steering pressure switch - 1.3 ohm
pin 36 (BR/BL) - mass air flow sensor signal return - 0 ohm
pin 41 (P/BL) - high pressure cutout/fan switch - 4.1 ohm
pin 71 (GN/Y) - vehicle power (hot in start and run) - .3 ohm
pin 92 (O) - brake on/off (BOO) switch input - 1.3 ohm
pin 97 (GN/Y)- vehicle power (hot in start and run) - .3 ohm
pin 103 (BK/Y) - ground - 0 ohm
The PCM fuse was out of the car when I did the check. Do any of these seem out of wack?
These are the pins that were close to zero ohms:
pin 31 (W) - power steering pressure switch - 1.3 ohm
pin 36 (BR/BL) - mass air flow sensor signal return - 0 ohm
pin 41 (P/BL) - high pressure cutout/fan switch - 4.1 ohm
pin 71 (GN/Y) - vehicle power (hot in start and run) - .3 ohm
pin 92 (O) - brake on/off (BOO) switch input - 1.3 ohm
pin 97 (GN/Y)- vehicle power (hot in start and run) - .3 ohm
pin 103 (BK/Y) - ground - 0 ohm
The PCM fuse was out of the car when I did the check. Do any of these seem out of wack?
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projectSHO89
Hard-core CEG'er
A faulty assumption.
Power is distributed directly to all the sensors and actuators. The PCM supplies a ground to activate the actuators, etc.
The exception is the +5Vref supply in the PCM. However, when it gets shorted to ground, it doesn't blow any fuses, it just shuts down the supply.
Steve
So is the fused power going to the PCM only for running the PCM itself? What could cause the fuse to blow? Is there something else running off that fuse?
Aren't the GN/Y (hot in start and run) wires the power supply coming from the fuse? If I am reading a short to ground (.3 ohms) on those pins on the PCM connector, wouldn't that imply a short between the fuse and the PCM, or any other spliced power coming from that same fuse? I guess I thought that the GN/Y was power to the computer, and was also primary sensor power and that the ECU checked supplied voltage to ground vs. sensor voltage to ground to get the resistance through the sensor?
Aren't the GN/Y (hot in start and run) wires the power supply coming from the fuse? If I am reading a short to ground (.3 ohms) on those pins on the PCM connector, wouldn't that imply a short between the fuse and the PCM, or any other spliced power coming from that same fuse? I guess I thought that the GN/Y was power to the computer, and was also primary sensor power and that the ECU checked supplied voltage to ground vs. sensor voltage to ground to get the resistance through the sensor?
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projectSHO89
Hard-core CEG'er
To clarify: I should have correctly stated that power is supplied to the PCM AND to the various actuators, etc.
Here's the schematic to refer to: 1996 Contour Power Distriution.pdf
You have found the circuit with the short to ground.
First thing is to eliminate all the possible load devices:
1) Disconnct C100: That's the big square connector between the air filter box and the firewall. See if the short is still on the PCM connector's side of the circuit (load) or is on the supply side of this connector. Pull the HO2S fuse for a convenient access to that side of the circuit.
If on the supply side:
1) Pull relays for high and low speed fans, fuel pump, A/C WOT and disconnect the CANP valve to see if the short goes away. If so, that component is shorted to ground.
2) If short persists on supply side, the fault has to be in that harness.
If the short wasn't in the supply side of C100, time to check the load side.
Following are all connected: VSS, MAF sensor, EVR solenoid, PCM, IAC valve, IMRC box, and the six injectors. Disconnect each component to see if the short goes away. Again, if it doesn't, the short circuit is in the harness and you'll have LOTS of fun finding it.
Steve
Here's the schematic to refer to: 1996 Contour Power Distriution.pdf
You have found the circuit with the short to ground.
First thing is to eliminate all the possible load devices:
1) Disconnct C100: That's the big square connector between the air filter box and the firewall. See if the short is still on the PCM connector's side of the circuit (load) or is on the supply side of this connector. Pull the HO2S fuse for a convenient access to that side of the circuit.
If on the supply side:
1) Pull relays for high and low speed fans, fuel pump, A/C WOT and disconnect the CANP valve to see if the short goes away. If so, that component is shorted to ground.
2) If short persists on supply side, the fault has to be in that harness.
If the short wasn't in the supply side of C100, time to check the load side.
Following are all connected: VSS, MAF sensor, EVR solenoid, PCM, IAC valve, IMRC box, and the six injectors. Disconnect each component to see if the short goes away. Again, if it doesn't, the short circuit is in the harness and you'll have LOTS of fun finding it.
Steve
Linkman
CEG'er
Yeah, that's where I think I am.
It's been long enough since I've looked at it though, I think I will repeat some of the diagnostic steps just to make sure I didn't overlook something in my frustration.
I will say that during the diagnostic I learned a LOT about the wiring harness, how things go together, where things are, how the various power and grounds interact, etc. As long as you have a second vehicle, and someplace to keep the 'Tour where it won't get towed or provoke the neighbors, it's a great learning experience. Or so I tell myself. :crazy: