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I have a 2004 ford f350 diesel and some very perplexing problems: The ignition won't turn over the starter yet the lights shut off when engaged (usually means the ignition switch is ok) but also the starter works if I jump the hot to the ground. I've swapped relays, checked fuses apart from replacing the ignition switch and starter how else can I diagnose this? Also it seems the solenoid isn't engaging when I try to hot jump it now (starter just spins).

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First thing I'd do is break out the multimeter and measure voltage at the solenoid to ensure you actually get 12V-ish when the key is in the start position. –  Timo Geusch Dec 26 '12 at 19:33
    
Which hot are you jumping to which ground? –  mikes Dec 26 '12 at 19:41
    
No voltage on the solenoid but fuses seem fine, maybe the ignition switch? –  nwmcsween Dec 26 '12 at 21:09
    
@mikes hot on the starter to the ground on the starter just spins the starter with ignition switch jammed in start –  nwmcsween Dec 26 '12 at 21:11
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The solenoid is really a big switch. It has two functions, send power to the starter and control the Bendix. The Bendix is the gear assembly that engages the flywheel. If the starter spins when you bypass the solenoid the starter is working. By jumping out the solenoid terminals both of which are "hot" by the way, you have tested the starter motor. You should be able to see at least one smaller gauge wire and the battery cable attached to the starter. With the key in the start position you should see about 12V on the small wire. If you have the 12v at that wire the solenoid is the most likely fault. If you don't have 12v at the small wire, you need to get a wiring diagram for the truck. You then have to follow the circuit back to find out where you do have 12v and where it stops. It could be a relay, the ignition switch or any wire in between. This is why the diagram is so important. Many times the engine control module is looking for several inputs to generate that 12v on the solenoid. It may be from the chip in the key, the I am in park switch, etc. Without the wiring diagram you are guessing.

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Yeah I'm going to take a few simple guesses before running the gamut, It's not the neutral safety switch, not the relay (I swapped them and tried), 12v on the solenoid is dead, fuses are fine so I'm going to take a peak at the ignition switch then trace some wires. –  nwmcsween Dec 26 '12 at 21:56
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