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08 F-150 Electrical mystery

 
Old 01-15-2019, 01:57 PM
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Default 08 F-150 Electrical mystery

Hi everyone, new user but have been using this as a resource for quite sometime (thanks google). I'm not sure if I'm posting in the right forum, but I've got a hell of a weird one. Hopefully someone has encountered it or has an idea.
I have a problem with the OTIS on my truck. Essentially, after 3 starts it will blow the diode in the auxiliary fuse box under the hood. Every time. Without fail. The result is the diode doesn't blow shut(as one would assume), it goes neutral. This makes it so that the truck will not start without jumping the relay in the fuse panel, unless it's been sitting for several hours. The truck has been to the dealer and my own garage floor many times trying to sort this out. Everything on this circuit short of the wiring has been replaced. Fuse panel, PCM, Ignition switch, Stereo, TCM, All fuses/relays, that particular diode has been replaced more than a dozen times. The remote start has been disconnected, the last thing I did was remove the wires for that diode from the fuse box under the hood and bypass it on the off chance that the connections in said box were somehow screwed. None of it has worked. As best as I can figure, there is a short in the wiring between that diode and the PCM. But I have no clue how to test or trace it given that the wire goes directly into the main harness that goes from that side of the truck to the bundles of wires connecting to the PCM and I would really rather not blindly replace the entire wiring harness if I can avoid it.

I've been driving for a while now literally with a switch wired up to jumpers in the main fuse box so my truck starts reliably. If anyone has thoughts or suggestions, they would be very welcomed.
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Old 02-06-2019, 10:29 AM
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It seems this is stumping everyone as much as me. I have done a little more testing and found that when I connect the mutlimeter it gives the expect 0L reading on each wire when the diode is removed. However if I start the truck, turn it off, remove the diode again, the wire that goes from the diode then splits to PCM and fuse box actually produces a reading. I would think that it should just read 0L still and after a while it does. Not sure if that helps.
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Old 03-01-2019, 03:56 PM
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Well in case this mystery befalls someone else, I think I 'may' have found the issue. After doing more digging around and coming up completely empty, I went back to the fuse panel(which has been replaced). I re-checked all the mini-fuses and as usual they were all fine. I then noticed the odd cartridge fuses at the bottom of the panel. Can't say as I've ever seen this style before, but given the consistency with the diode blowing and the weird time frame for the truck being able to start again with the diode blown. I thought it might be worth investigating. Sure enough there is 1 for the starter and 1 for the ignition. Neither of these seemed blown by visual inspection. I figured they're pretty much the only things that haven't been replaced and if one of them had a small fault, it may explain the apparent 'cool down' time needed if the diode is already blown and why it would always go after a specific number of starts. So I swapped out both and so far a month later, there hasn't been a single problem. For perspective, since the issue started the diode has blown(meaning truck would not start with the key) after every 4th start. Without exception.
Hopefully this does mean the issue has been resolved and at the very least may help someone else down the line with troubleshooting.
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