I'm trying to help Dr. Monstaa with the problem he's having with his 20 amp "I" fuse blowing. To facilitate the process, I have a few questions for the experts on this forum.

First, simply for my own knowledge, what is the PCM Power Diode in the wiring diagram found on this thread? It appears to activate the PCM Power Relay. I suppose it must get power via ignition...

Secondly, does anyone see any problems associated with me doing the following:

1. I would like to cut the Red wire coming off the PCM Power Relay and install an inline fuse.

2. I would like to cut the yellow wire that goes between Fuse I and C105 and install an inline fuse.

3. I would like to temporarily bypass fuse I with a wire or something similar.

This would enable me to determine whether the red wire and all of its faucets need to be diagnosed or not. Is this a plausible first step?

I should add that the problem is so intermittent that trying to force the failure by wiggling wires has not yielded any results. Dr. Monstaa was even able to drive the vehicle for a week before the problem resurfaced. I know it's not an ideal way to diagnose a problem, but I have a feeling it may yield results the quickest.

If this is an acceptable diagnostics procedure, what size fuses should I put on the two wires and does anyone have a better idea for getting fuses inline without actually cutting the wires themselves? I wondered if I could get the wires out of the fuse panel and use spade connectors to make a jumper wire without cutting anything?

Also, how do you remove those connectors from the fuse panel? I could not get a small screwdriver to release the tab even after modifying the screwdriver with a bench grinder.

Thanks for all the good information provided in the thread so far, it's much appreciated.

 

Power is supplied to the PCM Power Diode from the Ignition switch. The PCM Power Diode is there incase someone puts the battery in backwards. If they do you will not get any power to the coil of the PCM Power relay. That way the EEC-IV system will not burn up. This does not protect the cluster or anything mounted in the dash however.

C105 is the keep alive power to the computer memory. This is a very low power draw and if you fuse this circuit I would not use over a 1 Amp fuse if that.
Would not think the computer is your problem as if the one chip it is powering would blow apart before it could draw anything close enough to blow fuse I.
So the problem is on the red wire circuit.
This however does not say the that the computer does not have problems as the red wire also powers pin 37 & 57 of the computer (5 volt power supply & VP power in the computer).

Ues a AGC 20 AMP on the red wire.


I would remove the red wire from C101 and supply the C101 pin with a 20 amp fuse and toggle switch for testing.

It is not easy, have done it with the box off the truck and it was still very hard to do.






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Thanks subford. I started another thread about it this morning since I feel like I'm changing the focus of the thread off the fuel pumps. I really appreciate your help and wanted to let you know I saw your post and there is no need for you to reply on the other one if you don't want. Thanks again.

 

Here is a pin out of C101 (The engine plug) for a 1995 5.8L, I do not show one for 1993 but it should be the same.


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Hey thanks dpilot, those are some of the questions I had, and I think you are going in the right direction. Yesterday my boss and I took apart the fuse box just to see if we could find anything out of the ordinary, we did find that the ground wire for the EEC relay was badly corroded. Now, I dont know if that would be the reason the fuse keeps blowing, but we tested the resistance with a votlmeter on the top side of a non-corroded plug and it tested 0.2 ohms, then we tested the corroded tab and it read 1.156 mega ohms. So this may or may not be our problem. What do you guys think?

Ha ha, I just realized who you are dpilot, thanks for helping me out with this!

 

Lol, I am the boss you're referring to. I realized what I did wrong when we were measuring the resistance on that after you left. The corrosion should be addressed, but that's not what's causing our problem I'm sure.

The resistance that was indicated on the ohm meter was because I wasn't getting the probe thought the corrosion, not necessarily because there was a poor connection between the connector and the wire. The corrosion that is starting in various places on the fuse box will eventually cause problems and should be addressed, but that isn't the primary issue.

 

So are you thinking that if we cut those wires and put an inline fuse in, we can find out where our problem is, or do you think it will fix it completely?

 

I do not think that would cause a fuse to blow.
You might check the wiring going to the transmission plug on the passenger side plug as it has the red 361 wire is in it also.

 

It is not an ideal troubleshooting method. It will slowly lead us to the problem if we cannot find it by any other means. I want to look at it more closely before we cut up the wiring harness for no good reason, but if we can not find the short any other way, it's an option that will inevitably lead us to the root issue.

 

Alright, I'll take a look at that. But I thought I might mention that the pickup will idle just fine for days, but once you put it in gear and drive 5 or 10 feet the fuse blows. So I think it's gotta be a short somewhere and a bump or twist from the engine causes it short.

 

I've got time to work on this right now but I won't have much time this evening when Dr. Monstaa gets here. I haven't yet had the privilige of owning a Ford vehicle so I'm not familiar with where the transmission plug on the passenger side plug would be.

I've looked all over for harnesses or wires on the passenger side of the transmission and the passenger side of the engine compartment. The only harness I'm seeing on the transmission is on the drivers side. I examined that and moved it around a bunch while it was running and didn't find anything obvious. I don't see any plugs for harnesses on the passenger side of the engine compartment like I see on the drivers side..

On the drivers side I have followed every harness that comes from the fuse box and I've paid special attention to the one going to the engine. Nothing clearly a problem there either. As much as I am able, I have also moved the harness for the fuel injectors around and haven't found a problem there either.

Have I covered what I need to cover or have I still missed the transmission harness in question? Thanks.

 

The transmission plug on the passenger side is the plug for the solenoid pack and the red wire powers all the solenoids in the pack.
The red wire does not go to the plug on the drivers side.

The plug on the passenger side:




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Thanks. Thanks for the idea of using a forklift the next time I have to work on a transmission too. Why didn't I ever think of that? As long as you have side shift, it would work really slick I bet.

 

Just as an update, we spent a LOT of time carefully looking for shorts with the help of all the wiring diagrams that have been provided. Really thought we would be able to find something with enough persistence. No such luck. Finally Dr. Monstaa installed a fuse on the 361 circuit. We didn't think to look at your suggestion on where to put it subford, so we ended up putting it after the engine plug and before the engine. Maybe not ideal but I guess it's a start. We used a 15 amp fuse and we left the 20 amp maxi-fuse in the "I" position. He's driving it now. I guess we'll see what happens.

Didn't appear to be anything wrong with the connection on the right side of the transmission. Spent a lot of time moving that around while the engine was running as well.

Anyways, just wanted to give an update. Thanks again.

 

subford, now that I've re-read your suggestion on fuse placement, I had a question for you. How does the C101 plug compare to like a weatherpack or deutz connector? I didn't examine it with the intent of removing the wires from the plug. After reading your suggestion again, I wish I had. That would have done less damage to the harness. I think what we did will serve the same purpose, but your method would have resulted in a neater repair.

For what it's worth, I'm just an inquisitive farmer. I approach problems as logically as I know how, but my lack of formal education in regards to things of a mechanical nature as well as my lack of experience sometimes hinders my abilities. I thank you for being willing to provide the technical documentation required for us to pursue this issue as well as the suggestions pointing us in the right direction.