Hi There,

My 99 F150 blew the #24 PCM-TRANS-CMS fuse (what's cms?) about a month ago (OD light blinking, check engine light on, trans felt like it was slipping). Replaced fuse and range selector switch on the trans(had starting problems and thought it could be related), cleared the codes and has been great for the last 900 miles. Until this weekend when I towed my boat 3 hours from home. I pulled off at my exit and the same scenario happened again! I immediatly checked #24 and it was blown. I replaced it but it quickly blew three more times. I limped it to a parking lot and called my buddy with a trailler.

Got it home checked some wires on the trans, installed another 24 fuse and it didn't blow. I was going to take it for a short drive and started to go then The engine died. #24 fuse didn't blow but #2 PCM fuse did!!!???? Replaced #2 and engine started but blew 24 again.

Anybody got any ideas???

P.S. I borrowed a code reader the last time this happened and it spit out about 5 codes. I'm going to buy one tonight so I can scan it tomorrow. Thanks

 

Check the wiring to your Front O² sensors for chafing or shorts to ground. Pay close attention to the Violet with white stripe wire. Check the connector at the tranny too. Same colour wire. Also check the PCM power relay. If it is shorting out it will blow the #2 fuse. What if any mods do you have done to the truck? What work, repairs have been done?

JMC

 

The #24 fuse feeds a relay that feeds power to many locations for the PCM and the outside hardware, Trans etc.
It is going to be a process of discovery to find the intermittant ground or fault.
I have had luck subbing a good sized tail lamp (takes a lot of current to light) and pig tail, in place of the fuse.
Doing this, the lamp won't light or light dimly on normal current but light or go brighter when the fault is in or hit by moving wires, if a short if found. This won't normally hurt anything and still tell you when you found the source of the problem.
Sorry I can't be of more specific help.
Good luck.

 

97-8 schematics show fuse #24 feeding the PCM power relay. 99's show the #2 fuse feeding the PCM power relay. #24 fuse feeds the solenoids in the tranny as well as the Front O² sensors.

JMC

 

Actually, F24 only feeds the DOWNSTREAM sensors and the transmission. F23 supplies the upstreams.


Do check for melted/chafed wiring for those downstreams....

Steve

 

I need glasses.......


JMC

 

Thanks for the replys. I will check O2 sensor wires and trans plug. When #24 blows it seems to affect transmission only(tries to take off in higher gear). I may be in over my head but I have to try to find it myself first. If the water pump went bad, I would have no problem replacing it, but this stuff drives me crazy! There has been no mods or repairs other than trans service about 10,000 miles ago.

Thanks again for the help.

 

You're right about the tranny when F24 blows. F24 also supplies the power to the shift solenoids, which, of course, are lost when the fuse blows.

Focus on the exhaust pipes and wherever the harness goes around a sharp corner. Both have been the sites of this exact failure.

Steve

 

Think I may have found it? Took the heat shield off the right side to access better and the wires to the plug on the right side of the trans were really bent over and dirty. Couldn't see any insulation that was worn through but unplugged and found it wet with trans fluid. I wiped the plug dry, plugged it back in and so far have not blown fuse again and shifts normal. Have only driven about 5 miles but I'm optimistic. I'm going to unplug it again and dry real good with break cleaner and see what happens. Thanks again for the help...