1999 F550 7.3 Melting Starters and Start Relays - Ford Powerstroke Diesel Forum
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post #1 of 3 Old 07-11-2019, 08:41 PM Thread Starter
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Question 1999 F550 7.3 Melting Starters and Start Relays

Hey All,

Got a 99 F-550 with a 7.3. I use this truck to haul cut trees and the other day I took it to a small town some 30 miles from home. Got a call from the local body shop guy who told me my truck was smoking real bad. He volunteered to open the door and get the hood open and remove the battery cables. When I arrived about 40 minutes later found the starter to be real warm and smelled like burnt electrical. Next day, got a new starter and installed it, but truck wouldn't start, no clicking nothing. Jumped the starter relay and truck was running. Later in the day I replaced the starter relay and all was well. Mind you, this all happened after a fairly heavy rain and my cab has a few dents that allows water to seep in around the cab lights and run down the A pillar onto the ECM or PCM whatever in located in the kick panel left side of the steering wheel. So for about a week and a half, all seemed good. Then last Tuesday morning while at work my wife calls and asks why my dump truck is running. No idea I said. I had her go out and try to get it shut off, but you can guess how that went.. It did finally shut itself off about 20 minutes later. So today I went out to check out the truck found and another starter melted down and a starter relay fried again. Been running tests all day trying to figure this thing out. I think I've narrowed the problem to the tan/red wire the signals the starter relay to close and start the truck. On any of my other 7.3 or even the gaser I have, there is no power to this wire unless you are attempting to start the vehicle. However on this truck, with the key off engine off, I read around 10.5 volts on this wire. With the key in the start position, the voltage drops to around 2.4 volts. Completely backwards of what it should be... I replaced the ignition switch which did nothing to help. Also swapped out the neutral safety switch with a known good one which did nothing. I also found that the new starter relay I had installed in the morning was no good by noon today. I was putting tools away when all of a sudden the truck just started trying to start after replacing the starter and relay. I've checked fuses, changed relays in the fuse box tried everything I can think of to no avail. SOOOooo What I'm looking for is some advice on the start circuit on this truck and if anybody has a good wiring diagram to follow the circuit with would be GREAT!!! I've spent 14 hours on this truck today because I really need it in the morning. I can get it running by jumping the starter relay, but need to get it fixed. Sorry so long winded, just trying to lay it all out there. P L E A S E H E L P!!!
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post #2 of 3 Old 07-22-2019, 06:26 AM
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This is a WAY off-the-wall GUESS:

Regarding the "backwards" voltage reading between the tan/red wire and ground. The starter relay is mounted to sheet-metal, which is the ground for the internal coil in the relay. IF this sheet-metal ground isn't a good ground path to the battery negative, you will see the voltage indication with key=OFF. With key = START, voltage runs into voltage, and you see the 0-volt indication.

Guess #2: The tan/red wire for START also goes into the GEM (rear of fuse block under dash). These fuse blocks / junction panels are notorious for developing corrosion internally from water leaks and doing some weird stuff. Like your truck starting itself. If your 550 is a manual trans, and it can crank itself without the clutch being depressed, the clutch safety switch has probably been bypassed. Good thing you leave it in neutral.

Thanks for the entertaining post. Hope you can find the problem(s).
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post #3 of 3 Old 07-22-2019, 07:03 AM
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Disconnect that tan/red and wire a simple switch into the cab from pos on battery to the post the trigger wire connection on the relay. Leave the tan red disconnected. This will get you running until you can troubleshoot. Many leave it like this. You still need the key to be at the on position before starting.
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