Ohms meter says good; Test light said bad

[Manifold]

I'm stumped and could use any advice which may help, please! :grin:

Car: 1968 Coronet 440. Problem: One day last week when I flipped on the headlight switch to the parking lights position, all dash backlighting went out. I figured it was just the fuse.

What I've found so far using the wiring diagram over on MyMopar for a '68 Dodge:
- the fuse is good as is the headlight switch (initially suspected this, but realized I misunderstood the wiring diagram after taking the switch out, opened it up, cleaned it up and continuity tested it)
- the pink wire at the headlight switch harness is NOT hot
-- when I put the ohms tester (how I check continuity) to the terminal at the fuse block where the pink wire connects (according to the wiring diagram), the needle jumps to zero which tells me that the connection between the two is not severed. So, I expect that when I hook the battery back up, I should have power to the pink wire at the switch if the terminal on the fuse block is hot. This is what is stumping me: the pink wire is not hot, but the terminal at the fuse block is hot. I sanity-checked the tester at the switch plug by checking the hot black wire and it is hot/lights up the tester.
- the pink wire that feeds the hazard/four way flasher relay/can also comes off the same point in the fuse block (per the diagram). I continuity test that wire to the fuse block and all checks out. When I put the test light on it, no heat at the wire for the relay. This is the location where the pink wire at my brake switch is connected - continuity checks fine, but the test light shows no heat. In case my test light is playing tricks on me, I simply stepped on the brake pedal to see if the brake lights light up - they don't. (Got a bit bothered by this since I drove it home from the drag strip at night - seems I had no lights in the back. I'm glad I didn't get rear ended.)

The wiring diagram says that the pink wire that feeds the headlight switch has nothing between the switch and the fuse block. The continuity tester says that there's no break in the wire. But juice is not flowing in spite of there being juice through the fuse to the terminal where these pinks wires connect. Similar situation with the feed for the brake switch - according to the diagram, there should only be the wire from the fuse block to the relay then to the brake switch.

I am hoping to avoid opening up the whole wiring harness to physically trace wires. In my mind, I shouldn't have to since the continuity test says everything that should be connected are connected. But I don't have juice, which is killing me.

Any insight on this? It there something simple I am overlooking? I've even started to think it's a grounding issue, but it doesn't fit - it's just a matter of me trying to make sense of all this.

Thanks for reading!

 

[gearheadbill]

dash lights

Just maybe you have your dimmer switch turned off,the one that controls the brightness of the dash lights? if possible
I know this would not affect the brakes,?

 

[68ChargerAG]

Maybe a grounding problem??? Are you using a good (or same) ground reference when checking all voltages? Maybe dash has poor grounding.

 

[beezee]

Had the same problem in my friends Duster.The headlight switch powers the fuse for the dash lights,changed the switch restored the dash lights.

 

[Manifold]

Just maybe you have your dimmer switch turned off,the one that controls the brightness of the dash lights? if possible
I know this would not affect the brakes,?

From what I can tell based on the diagram, the pink wire to the headlight switch is always hot. When the switch rocker is moved to the first position, the power goes from the pink wire to two other terminals on the switch, a single yellow with tracer and two black wires (confirmed with my continuity test). One of those black wires feeds the panel light switch (i.e. dimmer switch). With this problem I have, I don't have power to the panel light switch. When this problem first occurred, I was rolling that wheel up and down expecting it to be the problem. The dome light still works since it is fed by a different pink wire from a different terminal on the fuse block.

The panel light switch was working fine, but since I have it out and can easily test it to rule it out as a contributor, I'll do that. I'm glad you brought this up. I'd have likely tossed the switch panel back in without testing it.

 

[Manifold]

Maybe a grounding problem??? Are you using a good (or same) ground reference when checking all voltages? Maybe dash has poor grounding.

I was using the same grounding point. My next step was to continuity test from the negative terminal of the battery to the dash and my grounding point and, if needed, run some jumpers from the negative terminal of the battery to the dash to rule out a bad dash ground. I had to knock off my work before checking this. It'll be the first thing I do when I hit the garage in a bit. Thanks!

 

[Manifold]

Had the same problem in my friends Duster.The headlight switch powers the fuse for the dash lights,changed the switch restored the dash lights.

Interesting! I took the switch apart, cleaned it up, and checked continuity at each position of the switch. Maybe it is still misbehaving and I don't know it. I'll run through it again and see what I can figure out. Thanks for the reply!

 

[Manifold]

To add more information to the thread:

- I do have headlights and both beams work. The headlight switch is not mounted/grounded when performing this test.
- I have no running lights at all (makes sense since it appears I don't have power to the headlight light switch (and subsequently to the panel light switch)).
- Turns signals still work.
- Four way flashers don't work, but can be cohered to work if you switch on a turn signal first. Again, makes some sense.

I'll be stepping into the next phase by using jumpers to provide good grounds and power where I don't have it to start eliminating components. I don't like doing this since I have a small fear of setting the car on fire, but I'm not making any progress right now. I'll double check the charge on the fire extinguisher before I start.

Will report back if I figure out something!

 

[Manifold]

OK, I'm back to where I started.

When I use a jumper to power the pink wire's terminal at the headlight switch, EVERYTHING works (dash lights, side markers, parking lights) when I move the headlight switch to the first position. So, the switch is good and nothing caught on fire.

I double checked with my ohms tester between the pink wire at the headlight switch (no switch plugged in) to the terminal on the fuse box which powers this wire. The ohms tester show no resistance - everything is good. So, I would expect when I put power to the system, the pink wire would get hot since the terminal on the fuse block is hot at all times when the battery is connect - but, that pink wire (and the ones for the four way flasher and brakes) are not hot. It's the darn-est thing I've ever seen.

I'd prefer not to take the easy way out by running a new wire between the fuse block and the headlight switch. I'd much rather understand the problem and repair it. But, without a clue as to how this is even possible (ohms tester say A -> B is good, but the test light and the multimeter set to DC voltage says there's nothing getting to this pink wire), I'm at a loss.

Any other thoughts out there? Clearly I'm missing something, as I'm only a shadetree automotive electrician, and hope someone smarter than me on this stuff has some insight.

Thanks for the suggestions thus far!

 

[68ChargerAG]

I would say that the key to this problem has something to do with the fact that the pink wire for the brakes and the pink wire going to the light switch both go to the same terminal on the fuse box (at least that is how the wire diagram shows it). All the other pink power wires that you are not having problems with go to the next terminal over (can't tell from diagram if they have their own fuse, but am assuming not because you would have seen that). I would say pull those connections apart at the fuse box and ensure everything has clean contacts (clean them all up if not), and if there is a jumper for both terminals to use the same fuse make sure the jumper isn't rusted through.

 

[70WM21 W1 V6B]

Ok the wiring guide your using is it specific to 68's? I ask because the circuitry between 68 and 69 is different with 69's having the park lamps light with the headlights where 68's do not.

2nd usually when you have a problem with dash lights flickering or going off, besides a fuse, it is the thumb wheel dimmer switch, it wears out and gets corroded on the thin wound wire and contact and should be cleaned, you could use a scotch pad and file or replace the switch. Also one area many over look is the turn signal switch, they simply wear out and even sometimes get hot and melt enough that the contacts no longer connect. A Bad turn signal switch will cause havoc with any and all Lights and everything thing else on those circuits EXCEPT the headlights.

3rd as for tearing into the harness tracing wires as a last resort, unless you have done something recently and pinched a wire I would rule that out. If pinched on the hot side it would ground out and blow fuses.

I believe it is the orange wire that has the dash light circuitry, you may notice near & behind the cigar lighter/ash tray the wiring harness has, a female connector, 1 2 0r maybe even 3 ports and is where the optional accessory lamps usually get plugged into and when you turn on the headlamps it powers up that circuit also.

Just a few directions to point you in...

 

[Manifold]

Thanks guys for the input. I should have a few hours today to poke around some more.

I am using a '68 specific wiring diagram. I checked out the thumb wheel dimmer switch thoroughly, since I have it out, but power isn't making it to the dimmer switch from the headlight switch.

The turn signal switch! I have given out that advice in the past. That bugger is tied into just about everything - I have solved an odd electrical issue by replacing that switch. Maybe I'll unplug it and see what happens.

I am definitely interested in NOT tearing into the harness. This car is very unmolested and I'd like to keep it that way. If there was some odd electrical item in there causing this problem, then I wanted to isolate where it was and address it with the least amount of destruction. I'm motivated to drop the fuse box to take a peek - maybe someone made a change which nullifies the wiring harness.

I have been checking for any plugs or wires which don't go anywhere, but haven't found any yet. Only problem I found when I first bought the car was that the four way flasher relay/can was not grounded, so the lights weren't working. Maybe I'll check that again, while I'm in the area.

Thanks again everyone! I appreciate you all taking the time to respond.

 

[Manifold]

Mystery solved!

So I went right for the fuse block to start things today. Visually checked out the wires, etc. on the backside and all looked good. I hooked the battery up and started poking with the test light. The point where the pink wires connected to the block wasn't hot. But, when I flipped the block over and check the fuse, it's hot. See where this is going yet? ;) Then I used the tip of the tester to make a connection from the fuse to the terminal ear and got a small spark... the lights worked.

Turns out the connection between the fuse and the terminal was either corroded enough (cleaned it up with some emery paper) or not making contact at all times (one ear was bent too far open, so I bent it closer to the other ear) to give me the headache I was experiencing (or so it seems right now). After cleaning and bending, the connection appears constant and solid - everything is working.

So unless it poops out again, I'm going to say that was the problem.

Thanks again for all the great advice! It lead me to the source.