Tracking down a short??

Hi, Valid idea, make sure you have good sight and nose(smoke, smell of burning sensing). Did it quit all of sudden? Of is there a history of an intermittent problem? Tony

Jeff Wisnia wrote in rec.autos.tech

A place to start is to consider what you were doing when it blew. What was the last door you opened or closed. Did it blow when you were rolling a window up or down? Things like that. However, with that type of circuit, one place that is prone to failures is at the doors where the wire harness goes through the boot between the body and door. I would try looking at the driver's door, since that is the door that gets the most use. Try to see if there is a connector separating the door from the rest of the circuit.

It wasn't intermittant, I've owned this car for 10 years, and this is the first time any fuse blew.

The short ocurred "during" a half hour trip to a restaurant last week. There were four of us traveling, I unlocked the driver's door with my key and pushed the door lock switch to unlock the other three doors for the passengers. When we go to the restaurant parking lot, the passengers got out and I pressed the door lock switch to lock all the doors before I pushed my door closed. That's when the door locks didn't work and I noticed the courtesy lights were out too.

Good suggestion on the driver's door being most used, thanks.

From the diagrams in the shop manual, all the connectors for stuff are inside the doors, and that's my recollection too from the couple of times I've had to get inside a door to lube or fix something there.

If I find something interesting to fix I'll report it here.

Jeff

Get a 10 amp circuit breaker and connect it across the fuse terminals, Use a compass (or actual short detector meter OR a slip on style amp meter (type that you just hold over the wire to read the current flow)). With the breaker tripping on /off follow the wiring harness around the vehicle from the breaker back. Watch the needle on the indicator, you will see it swinging back and forth in time to the breaker. As you follow the wires along you will find a spot that the needle will stop moving. That is where the short is.

shows one and how it works.

I would first check any bulbs that might be on the circuit. Bulbs can fail to a short, especially the 1157 or 1034 types. The glass falls out when they get old and the wires in behind the glass can short to ground.

Second I would be looking at moving parts. Places where the cables run through door frames or trunk lids.

I use a meter across the fuse to test it, I don't think a bulb will handle a dead short's power. You are blowing 20A fuses, any hotter and fires start.

Good luck!

Mike

86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00 88 Cherokee 235 BFG AT's

Jeff Wisnia wrote:

Huh? "a dead short's power", what are you talking about?

Why doesn't an 1157 bulb connected directly across a car battery pop like a flashbulb, then?

I eagerly await your answer.

A test lamp across the fuse is a perfectly safe and good idea for tracing out a short and it will not cause a fire.

Methinks I needed another coffee, have no clue what I was thinking about...

Mike

Found it in less than half an hour this morning!

I connected my trusty old Simpson 260 ohmmeter between the output side of the fuse holder and ground. It read less than half an ohm.

I got the teen aged son to go around whacking on things and when he slapped the inside of the RR door the ohmmeter would wiggle upward. I pulled the inner door panel and tracked the short down to a crimped splice in the harness which split the body bus into three leads, to two door nounted lights and the power door lock switch.

The tag end of one of the wires was sticking out at almost 90 degrees from the crimped sleeve and in 15 years had worked it's way right through the tape insulation of that splice and into a similar splice adjacent to it which was spliting the two return leads to the door lamps.

All's well that ends well.

Jeff

Jeff Wisnia wrote in rec.autos.tech

Great. Glad to hear you found it. And a nice use for a teen ager. My wife always told me that they had their uses, other than emptying the refrigerator and filling the floor with dirty clothes.

Every once in a while ya get lucky. Finding such a problem, especially in a caddy, can be frustrating. You definitely took the right approach by reading the schematic FIRST then deciding your diagnostic route. I/ve seen people scratch their heads for days over this type of situation, mostly because of stupidity.

Thanks,

One of the many one liners attributed to Yogi Berra was,

"I'd rather be lucky than good."

(Spending 45 years designing electronics for a living didn't hurt my chances either.)

Jeff