theiceman

not sure what the nominal resistance is but it sure will not be zero . Sounds like if your fuse is blowing it is close to zero .

current is voltage divided by resistance. if you assume 12volts, and factor in the size of the fuse for current ( not sure what fuse rating it is ) you should be able to determine how low the resistance has to go before blowing the fuse. Use this as a guide, lower resistance means the fuse will blow, higher will be safe.

Sounds like a crossed wire or something though doesn't it !

As a test when you take the load off the switch your resisance should go to infinity as your current in your meter ( set on ohms ) should go down the load side of the fuse wiring, through the switch and then to air ( as you have the wire off ). As you put the wire on the resistance will drop. Just continue down the line until you find the problem. Once you get the hang of it. it will be easy.

85Cab

Admittedly, I am a complete novice when it comes to electrical, I have always avoided it like the plague. This is the first time that I have used the multimeter; however, I believe that I have this right. I pulled the blown fuse and hooked up the black to the negative side, bottom, of the #8 fuse, (turn signal & reverse light - 16 amp) and the red to the positive, top, of the #8 fuse. With key switched on, resistence as you previously suggested went to infinity. When I threw the car into reverse, the resistence dropped close to zero. I sequentially pulled and checked the connection on every bulb in the turn signal housing and the resistence always remand at zero. What do I do from here. I believe that you previously suggested checking the resistence in the reverse switch. What is the procedure for this. I believe that I read somewhere that a flasher switch could be to blame, any thoughts or comments?

theiceman

I would start by pulling the wire off the reverse switch that goes to the lights etc. If it goes back to infinity you know that switch is okay. then just start mving down the line. By the way what was your reading for resistance and what is the fuse rating ? I can't remember the details without rereading and I am already in edit mode. Did it only blow with the signal on in reverse or any time in reverse. ? If it was only with signal on , turn the signal off and check the resistance and provide the readings.

85Cab

You mean the wire off the switch and not the fuse box right? Can you tell me which wire off the switch goes to the lights? After the wire is pulled off the switch, if it doesn't go to infinity then it's a problem with the switch? When you say go down the line, you mean check each wire at the light housing? I may have previously posted that the fuse did blow when in reverse and w/ turn signal; however, it is blowing any time it is put in reverse. The resistence was a 2 or 3 and fuse (off the top of my head) is the red 16 amp.

theiceman

Okay some good info. With the thing in revers you want to measure the resistance ( which is currently very low. ) with a resistance of 2 ohm assuming a 12 v system that would be about 6 amps, should not blow the fuse if it is 16 amp, not sure what the red is. By the way you should have your meter between the cold side of the fuse ( going to the switch ) and a good ground. What you are checking is the resistance seen on the circuit in question. the currrent is the voltage divided by the resistance you measure ( I = E/R ) if you assume 12volts ( E ) and you know the fuse rating ( I ) it is pretty easy to see how low the resistance has to be before the fuse blows ( R= E/I ) . Now with the meter on and measuring the resistance of the circuit ( the ignition should be off with the fuse removed ) you should be able to start unlugging things and watch the resistance ( lets asume the red is 16 amps ) hopefully some one will correct if this is wrong, any resistnce below .75 ohms will blow the fuse, you should be showing less ( assuming red is 16 amps ) with your shifter in reverse. The next thing I would do is cu the problem in half.
Unplug both wires from the reverse switch if you are not sure. If the resistance goes up there is no short before the switch ( which should be the case. ) Plug the wire back in that comes from the fuse box ( not sure which one it is ) if the resistance goes below your .75 it is probably shorted to ground inside the switch. If not and the resistance goes up the short is further down the line. Connect both wires back to the reverse switch ,then go to the next stop and disconnect the light fixtures. If the resistance goes up the problem is in the fixtures, if not there is a pinched wire between the switch and the light sockets.
Anyway i hope you get the idea of how to aproach this. This method a=can be used to identify every short as long as you have a schematic , a methodical aproach and patience.

Good luck

85Cab

Iceman, your post was incredibly helpful. I went back to retest with much better success. Although, I was not able to resolve the problem, I believe that I have narrowed it down significantly.

I was able to isolate the wire (green/yellow) to the back-up switch from the #8 terminal (fuse is a red 16 amp) on the fuse box block #1. I found a solid ground, removed the fuse, removed the (green/yellow) wire from the fuse block and attached the meter to the wires end. With the ignition off, I received a reading on the meter of .3 ohms. Per your formula, this clearly indicates a short. I removed from the reverse switch only the wire (brown/white) that continues on to the taillight housing and received a reading of 12.88 k ohms. I removed from the reverse light switch only the wire (green/yellow) that comes from the #8 terminal a reading of 12.88 k ohms. I removed both wires from the switch and received a reading of 12.88 k ohms. Based on your prior post, this should indicate (I am uncertain why) that there is no short in the switch. I again removed each reverse light bulbs one at a time and checked housing without change in resistance. This leads me to then conclude that the short is after the switch, possibly in the taillight housing itself and before the ground. Bentley’s wiring diagram for the reverse switch circuit states that the ground is at locations G414 and G413. Bentleys does have a diagram for ground locations; however, this one is not included in the diagram.

How do I further isolate or diagnose this problem. Can you help tell me where the ultimate ground location is?

theiceman

Sounds like you are doing great, you are almost there , the location of the ground at this point really isn't important, but you are definitely making progress. You have found exactly as we had suspected. You should now be able to put the wire back on the fuse block ( green / yellow ) and just hang your meter lead off the wire coming from the reverse light switch ( brown / white ) and a ground ( should be real low, back to your .3 ohms ) . I do not have your year of car so can't tell you where your ground is. It should tell you in your bently though.
I would now if I were you, pull the light housings, you should see that same brown/white wire going to each housing, remove one at a time and one of them when unplugged should cause your resistance to go back up. ( unless there is a wire pinched ) . But since these are the only 2 bulb housings in the circuit ( that I know of ) you are getting VERY close.

good luck.

ice

85Cab

O.K. that was easy. I could not have found and fixed the short without your help. The problem was in the left turn signal housing. Apparently, the copper tab that connects the wire to socket was bent up and shorting out on the back of the turn signal housing. Simple needle nose pliers straightened it out and tighten all the connection then a piece of electrical tape for good measure. I can only assume that at some point, someone took the housing out and tried to get the housing as far from the car as possible without detaching the wires. This in turn bent the tab. Thanks again, I learned a lot.

theiceman

Glad to see she is finally fixed up . What you learned here you can carry with you for other electrcal problems. once you understand how to troubleshoot this type of problem you can do a lot with it ..

Congrats on the fix.