accordivan

I've been having problems with the brake light fuse blowing so I'm checking to find a short in the system. I used a digital meter to check continuity to ground on the circuits to the tow hitch trailer socket. The left and right turn signal circuits and the back up light circuit all showed continuity to ground which, to me, seems wrong.

Can someone enlighten me please, should these circuits go to ground with no power to them? Also, is there a circuit that is common to all three which may have the short in it?

bent valves

To answer your question, yes you will see continuity from the left and right turn signal to ground. Both go to splice S200 and will show continuity through the Center High Mounted Stop Lamp. The back-up lamp is grounded and when not lit, will read continuity to ground.

Looking at the schematic shows a 20 amp fuse in the PDC controls the brake lights and must be the one that blows. If you put in a new fuse does it blow instantaneously? If so check the wiring from the PDC to the brake light switch. If it does not blow until the brake is applied, a white/tan/wire from the brake light switch goes to multi-function switch and Center High Mounted Stop Lamp and trailer connector. There are about a dozen connections to the multi-function switch.

I would try to trouble shoot the problem by using a 12-volt test lamp instead of the DVM. A test lamp will glow dimly with a connection in series with another bulb but be bright if it is shorted to ground.

Aren't these electrical problems fun?

accordivan

Dave, I don't know about "loving" these electrical problems but they sure do test your sense of humour

The fuse only blows when the brake is pressed so that suggests that the fault is after the brake switch (which I have replaced by the way) correct? I'm not very knowledgeable about things electrical so could you suggest a test system that I could use to find the fault? I've looked at the schematic but don't really understand it so any answer should use words of one syllable that can be understood by a child (of 62).

Thanks a bunch.

Ivan

bent valves

Fixing wiring problems over the Internet is like painting a portrait by using the telephone to describe your face. So I’ll try to narrow down the problem using a “Divide and conquer” approach.

The brake lights on our trucks consist of a power connection from the battery, wiring, the brake switch, the added brake controller, wiring to the taillights and trailer connector. You have replaced the brake light switch and the fuse does not blow until the brake pedal is pushed. This tells me the problem is after the brake switch. With no trailer connected, this tells me there should be no electrical load to the trailer connector.

My first guess is there is a problem with the Tekonsha Prodigy brake controller because this is an add-on that may have been wired incorrectly or may simply have been fried by a faulty trailer brake system. I found their link on troubleshooting here:

www.tekonsha.com/prodig.html

This is an easy test; simply disconnect the Prodigy wiring harness from the Dodge harness by unplugging the connector. This will completely remove all electrical load (other than the brake lights). Install a new 20-amp fuse in the PDC and push on the brake pedal. If the fuse still blows then more troubleshooting will follow. If the brake lights now work and fuse is good, the brake controller was the problem or the wiring to it is bad.

Let me know how it works out.

accordivan

Sorry to take so long getting back to you, it's been a busy few days. I disconnected the Prodigy, put in the new fuse, stepped on the pedal and "BANG" well "pop" anyway, the fuse blew so I guess it's not the trailer brake controller?

vzdude

Did you unplug the controller at the actual controller or at the brake light switch? It could be in the wiring.

accordivan

I unplugged the controller where it connects to the factory loom under the dash thereby removing all of the controller wiring from the circuit.

nkennedy

Check resistance from the blue wire to ground.

bent valves

Ok, so the brake controller should be eliminated from the problem. Looking through the shop manual at the brake circuit, it is possible to separate the 3 circuits coming off splice S200 by unplugging connectors. Leave the brake controller disconnected for this test.

Here is an ASCII schematic of the circuit; this comes from page 8W-10-17:

(Battery +) -> (Fuse C, 20 amp) -> (Stop lamp switch, pin 6) -> (Stop lamp switch, pin 5) -> (Splice S200) -> to A, B, C

A -> (Multi-function switch)

B -> (Connector C203) -> (Connector C308) -> (Center High Mounted Stop Lamps)

C -> (Connector C134) -> (Splice S114) -> (Electric Brake)

By disconnecting connectors C203 and C134, that leaves only the Multi-function switch connected. These connectors are located next to each other under the dash on the driver’s side. See page 8W-90-23 in the shop manual for the location. Now with the two connectors off, insert another fuse (I hope you have a good supply) and repeat the brake push test. If the fuse blows now, the problem is in the Multi-function switch. If the fuse does not blow reconnect one of the two connectors and press the brake pedal.

At the end of this exercise you will have found which of the three circuits has the short. My guess is circuit C, the electric brake.

Let me know how it goes.

kerryneb

the continuity to ground you are measuring could be the filament in the light bulbs. you can get a circuit breaker 'fuse' to help diagnose the problem...get one that's 5-10 amps smaller than the fuse. it will reset automatically a few seconds after the short is removed. look for bare wires at the trailer socket and each bulb socket.

accordivan

I finally found time to carry out the checks you suggested and proved your guess correct, the short is in the elctric brake circuit.

I could not get the other circuits to work with C134 disconnected so I cut the the white/tan wire to isolate that circuit. I figured I could re-splice it if necessary. I could not get the wire to detach from the plug so this was the only option. I now have brake lights but no electric brakes and no time at the moment to go further. I will have another go at the weekend to see if I can track this down further. Any further guidance would be much appreciated. I'm beginning to lose my fear of electrics but I'm a slow learner.

Thanks fo all the help so far you guys.

Ivan

accordivan

Final update, problem solved. The short was in the white/tan wire between connector 134 and the oem electric brake controller socket. There was no way to trace the wire without taking out half the dash so, I cut the wire at each end and spliced in a new one. Now everything works as it should. Thanks to all you guys that offered advice and help.