Backup Light Issue

Has anyone had their jeep just blow the fuse to the backup lights after cranking the jeep? I can replace that SFE 9A fuse and the lights work fine with just the key on, but after I crank the jeep, it blows the fuse. Any insight is appreciated. Thanks

 

Trans in neutral, crank,start, put in reverse and pop?
Or trans in reverse while cranking and pop?

 

Cranked while in reverse and pop.

 

First thing to do is remove the backup light bulbs and measure the resistance from the fuse connection to the body. This will tell you if there is a short somewhere that's putting an extra load on the fuse.

 

I'll try to check it tonight. The reverse lights never worked for the past 30 years until I replaced the switch and light assemblies. So I don't have a before-restore situation to compare to. Since the reverse light circuit is easy to follow, I can't picture where a short might be. Could I have gotten a bad switch?

 

Problems like that is why I have a 5 and a 10 and a 15 amp circuit breakers for testing. The audible clicking sound lets you know when you have found the short.

 

sounds like maybe there is a grounding issue. The starter/solenoid may be finding ground through the backup light circuit.

do you have a fresh engine to frame ground cable and solenoid case to battery negative wire with clean connections?

Another possibility is that during the engine rocking on the mounts during cranking/starting, the reverse switch terminals are hitting the tub and shorting.

 

Tracing the wires is the only way to eliminate a grounding issue as the cause. I once had a tail light wire that got pinched by a body bolt and shorted out. Drove me crazy trying to track it down.

 

I'll check those cables. I know the engine to frame cable is original.

 

I didn't get around doing it after work because I had to deliver a load of firewood into town. So, am I looking for a number greater than 9 amps on the tester if I test the circuit from the light socket to the body with just the key on? Or, have someone test it while I crank?

 

I assume that if you turn the key on (engine off), and put the car in reverse, that the lamps light and the fuse does not blow. If not, then the problem is not really related to starting.

Do you have the floor covers off? That would help rule out that the switch terminals are shorting out to the body.

If you measure the current in the circuit, you should just have the load due to a bulbs. Wattage of the bulb / volts = amps. Two bulbs. I'd guess it's an 1156, 27W / 12V * 2 bulbs = 4.5 amps. There will be a momentary turn-on surge as the lamp lights, but that should not pop the fuse.

You can also measure the resistance of the circuit from the switch through the bulbs to ground. R = V/I, 12/4.5 = 2.7 ohms... but low resistances are hard to measure, and the bulb's resistance increases at the filament heats up (see the turn-on surge above). So I'd first take the bulbs out and make sure that the fuse does not blow when you start the car with the bulbs out. This will rule out (or rule in) that the load from the bulbs contributes.

 

I checked using the 10A max setting on my Craftsman tester, and the reading went to 18 and then the fuse popped. ?? I'm voltage and amp challenged and confused, so this doesn't make any sense to me. This was with just the key on and engine not running. I checked and saw there is no engine to frame ground strap.

 

Install a ground to the frame and the problem might go away!,

 

I found a couple of old battery cables in the barn, so I'll put bolt through terminals on both ends and install one between the engine and frame and one between the engine and body. Thanks for the input.

 

To close this thread out, I believe my problem was the ignition switch. I added the ground cables from engine to frame and body to frame before changing the switch with no cure for popping the 9A reverse light fuse. I have since replaced the ignition switch, and so far the lights stay lit while cranking in gear in reverse.