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Starter diode (OTIS/Intellistart) keeps blowing. Zero information found.
I have a no crank/no start issue Ive been battling for months, and I’ve found the problem. I just can’t find a solution. Late in the 2008 model year, Ford added the One Touch Integrated Start (OTIS), or Intellistart feature to the F150. It’s a one amp diode located in the small, secondary power box under the hood on the inside of the driver side fender. The diode blows intermittently. I replace it. The truck starts normally for a few days, and then it doesn’t. I know there’s a short in one of the two wires the diode connects. I don’t know for certain where either comes from or where they go because I’ve found no wiring diagram that shows the diode as part of the starter circuit, and I’ve spent hours looking for this info online. There’s a blue/orange wire on one side of the diode, and a green wire on the other. I know I’m not the only person ever to have this issue, but I haven’t found much useful information on that one component of the system. If anyone has a diagram with that info, or any experience with the diode blowing or melting, I’d appreciate it. Currently I have a push button rigged to jump the starter relay from the driver seat so I can start it. I replaced the neutral safety switch (twice, the first replacement resolved the issue for a week), the ignition switch, the starter relay, and the PCM relay prior to finding that the diode was blown.
Last edited by SiskCFRN; Jun 12, 2021 at 08:43 PM.
Reason: Grammatical errors
You've probably got a wire that is nicked or chafed and is intermittently shorting to ground.
The two prime suspects would be the DB-OG wire that goes from the start diode in the auxiliary relay box to the DTR sensor and the TN-RD wire that goes from the DTR sensor back to the start relay in the CJB. Schematic is attached.
I've seen one previous report of a successful diagnosis of this issue where the short occurred where the harness ran over the bracket for the transmission dip stick causing a nick that was intermittent as the harness moved during operation. See https://www.ford-trucks.com/forums/1...-at-start.html post #10.
You could use a meter with a continuity beeper and perform wiggle tests to see if you can locate the suspect section of the harness.
Last edited by projectSHO89; Jun 13, 2021 at 07:57 AM.
Found it. The bundle of wires that plug into the right side of the PCM were wedged between the rear right side of the engine and the firewall heat shield. The sheath was gone where it made contact & I could see one bare wire (yellow/pink stripe). I’ll have to clean them off & go through them, but I’m sure the wire you mentioned will be the same way. No idea how I’m going to get to it to do all that, but I have the fender liner off & I can fix it where it is if I have to & hopefully reroute it. Thanks a lot for the info & the diagram.
Taken from the right wheel well with the fender liner removed.
Last edited by SiskCFRN; Jun 13, 2021 at 04:11 PM.
Reason: Error
Keep in mind that the colors of the insulation tends to fade with exposure to heat and whatever. It may be tough to determine the actual color code of the wire without stripping back coverings to expose insulation with its original colors.
Keep in mind that the colors of the insulation tends to fade with exposure to heat and whatever. It may be tough to determine the actual color code of the wire without stripping back coverings to expose insulation with its original colors.
It was aggravating to deal with, but it’s fixed now. It’s a 4.6 liter.
The starter diode was getting hot because there was a short in the wire. The wiring harness had fallen between the engine block & the firewall on the passenger side, and had melted the coating off a couple wires in the harness, one of which was on the same circuit as that diode. Eventually that wire broke, and the problem went from intermittent to constant. To see if that's your issue, remove the front wheel and fender liner on the passenger side. When you do that you'll have a clear view of the right rear corner of the engine & it'll be obvious if the wire harness is against it. Check for damage to the outside surface of the harness. I repaired two damaged wires & zip tied the harness so it wouldn't fall back into that location.
