10 Amp Gauge Fuse Blows

[mwilhelm]

1994 XLE Sedan. 10A Gauge fuse blows five seconds after car is put in reverse. I have ruled out the light failure module (disconnected it and same thing happened). The backup lights do come on (for 5 seconds before the fuse blows). The problem has persisted for about a year -- at first it was interrmittent (every week or so) and not correlated with putting the car into reverse. Now, the fuse blows after five seconds every time I put it in reverse. I ruled out the AC compresssor clutch, the AC/heater blower and the power window module. There are no obvious frayed wires. Because the fuse does not blow for five seconds, it does not look like a direct short but, instead, excessive current draw from somewhere. I put a 20 A fuse in and it did not blow for ten seconds, at which point I took it out because I did not want to fry the wiring. I'm stumped. The Haynes wiring diagrams are not particularly helpful because they do not show the backup lamp circuitry. Is there something else on this fuse other than the panel lights, power windows, fan motor and backup lights? Does anyone know the location of the switch that closes when the transmission is put into reverse?

Thanks
MJW

[karazy]

Wow, you picked a tough one for you first post. Like you said, the Haynes leaves you short on this one. With the delay thrown in, it may have something to do with the rear defog system. There is a timer in there, that through some strange way, may be causing this problem. You can try turning it on to see if it works properly, or try metering it first to try and find a short. Remember that if there is a delay in the fuse blowing, that there may also be a delay in the the short showing. That is just plain ugly.

If you have any info on earlier episodes, it has been going on for a year, it may help to narrow it down. Hot wires tend create more havoc as time passes, as they slowly melt through more and more wires.

[mwilhelm]

No further information on the prior failures except that they most often occurred in the first few minutes of the vehicle's use. I looked at Alldata's wiring diagrams that are more complete than Haynes and observed that there is a switch on or near the transmission that triggers the backup light and that there is another switch that senses what gear the transmission is in and supplies that information to the dashboard PRNDL indicator. If I can locate those switches in the engine compartment, I will disconnect them and see if one of them may be the culprit. And, I will look more closely at the bundle of wires that exits near the trunk hinge because I have read posts in other services that report instances of the 10A fuse blowing because of frayed wires there. (That, however, is inconsistent with the backup lights coming on before the fuse blows. The 5 sec delay between the time the car is put into reverse and the time the fuse blows has me stumped. Apparently there is a greater than 10A resistive fault - rather than a direct short to ground -- that is presented to the circuit when the transmission is put into reverse.)

[karazy]

There is a pinout of the park/neutral sw/ in the Haynes. Pin 2 & 8 for reverse. It is on the tranny.