Hi All:

I have a problem to start to f150 truck (2002, v6, automatic transmission, equipped with remote anti-theft module) . Whenever I turn the ignition to run, the #21 fuse in the driver side fuse box blows. After initial diagnoses, I found there is a short between ignition switch and digital transmission range sensor.

After looking at Haynes repair manual ( I bought better manual but it is in the shipping process ), there are:
1. starter interrupt relay, which connects to remote anti-theft module
2. clutch pedal position switch.

Can anyone tell me where is the starter interrupt relay and clutch pedal switch? I want to look at these two components first before looked the wires in between.

Thanks a lot. It is my first time to trouble shooting electric problems. Any help and suggestions are highly welcome.

 

If you have an automatic, you don't have a clutch pedal switch.

 

If you have had exhaust or transmission work done recently, look at the wiring harnesses in that vicinity. It loops over the top of the transmission and gets pinched.

A blown F21 (when the key is turned to START, not RUN) is usually the result of a pinched wiring harness in that general vicinity.

The 2002 F150 did not leave the factory with a starer interrupt relay. If your aftermarket remote system added one, you'll have to figure it out on your own.

 

Thanks for quick reply. For pinched wiring harness, do you refer to the wiring harness inside the car or under the car? I found wring harness connected to the transmission range sensor difficult to access. Should I remove the whole wire harness to test lines?

 

Under the truck.

You didn't address the other potential causes I mentioned...

 

Thanks ProjectSHO89. I just bought this used truck three month ago, really not sure whether it has a starter interrupt relay. My only knowledge is from Haynes repair manual as they drew it there. I did see an anti-theft module in the truck.

So if there is no starter interrupt relay, I can assume that the #21 fuse directly connect to the transmission range sensor?

Thanks a lot.

 

If there is an aftermarket system installed it likely has a starter interrupt relay added. You will have to research that in person. Most convenient location is usually under the dash near the steering column where the signal comes off the ignition switch. However, F21 is after that part of the circuit so you really need to dig into there and see what you have. You also need to crawl under the truck and perform a visual exam of the wiring harness to and from the TRS and look for signs of the things I mentioned.

The original circuit is what I've called "stupid-simple". Ignition switch > F21 > TRS > Starter relay. Not much can go wrong unless some ham-handed mechanic screwed it up along the line.

 

Thanks. I solved the problem. You are right, there is no starter interrupt relay. It is the original circuit. Ignition switch > F21 > TRS > Starter relay.

Here is how I troubleshoot the problem. I disconnected the TRS. I measured there was no short between F21 and TRS, and there is no short between TRS and starter relay. Therefore it is a faulty TRS.

I replaced the TRS. It starts like a charm

I suspected the original owner may know the problem before sold the car to me. The original owner put a new radio system in the car. I discovered many non factory wiring under the steering wheel to install the radio. I guess they may overloaded the wire and the fuse just burned once in a while. The TRS may be constantly overloaded and after a while it is burned to be falsely grounded.

Anyway, thanks a lot guys. Cheers.