98 F150 4wheel drive, a/t, 5.4 L
Everytime I turn key to start, Fuse #20 (starter interrupt relay) blows. Sometimes will start, other times blows fuse. Had the starter tested and checked out OK and replaced starter selonoid/relay on fire wall--still have problem. Any suggestions?

 

Are you sure its fuse 20? Im showing that for manual transmission only and fuse 21 is a 15 amp for starter interupt. Also makes a difference if it has RAP module for vehicle security.

 

Sorry--it is 21 not 20. It's a 15amp--my mistake. I don't know if it has RAP. Bought it used and don't have any keyless entry. What is the RAP and where would it be??

 

RAP module is Remote Antitheft Personality module, a beige colored module mounted to the inside firewall all the way to the left side of the dash. Looks like that circuit also loops back through the clutch switch bypass and back through fuse 20 and on to the PCM. Kinda hard to tell where the problem might be here. May possibly be a wiring harness burn through on the exhaust pipe on the passenger side. The harness has a bad habit of falling off its retainer and landing down on the exhaust manifold or egr tube and getting burned through and causing a short. The intermittent fault may be leaning to such a problem. The circuit also is tied through the GEM module, so be also mindfull that the F150 is prone to water leaks from the windhsield and often causes shorts in the interior fuse panel and Gem module as a result. Being a recent purchase there is no telling what you may find.

 

I pulled the wiring harness from the starter all the way up to the starter selonoid on the firewall--no burn through spots. Retaped, resealed it, reinstalled--no bad spots on that. Went up under dash--no burned wires. Thanks for the help--I'll keep looking.

 

There is an off chance that relay is shorting internally itself. Wouldnt hurt to swap it out with a known good one.

 

I'll give that a try--Thanks!!

 

WHich relay--I already replaced the one on the firewall that goes directly to the starter?

 

I was referring to the starter interupt relay. But I have another view of this circuit from another section of the EVTM. Looks like it actually goes thru the Trans range sensor. You may have a short in it. Pull the connector and look to see if its full of water, oil, or mud. The sensor itself could be shorting or any of the wiring up to it.

Here is the diagram for without rap module.

 

And this is with RAP module

 

I'll try that tomorrow morning--thanks for all your help. I'll let you know how it turns out.

 

can't find anything wrong with wireing to trans sensor. any other suggestions?

 

Sometimes you just have to reproduce the short and trace the circuit while it is shorted.

 

Thanks for the help, i'll try to trace the circut from start to finish.

 

I found the problem--it was the wire harness that runs down the passenger side firewall and goes up over the transmission where the exhaust and transmission come together--the wire harness touched the exhause pipe and melted a spot. I found it by tracing the transmission sensor wire from the transmission wire back up toward the engine. I appreciate all the help from Homerwinzlow--hope I can help you sometime. Thanks!!