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Taillights do not work. Weird test results.


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This is on my 1992 Damon diesel pusher motorhome.

Taillights, front parking lights and all clearance lights stopped working.  I pulled the 20-amp fuse from the slot marked "taillights" and it tests good.  

Voltage test of the empty fuse slot shows exactly the same 12v on both sides of the fuse slot.  This does not seem like something that should happen.

Unplugged the multiple connector from the combination headlight/parking/instrument-dimmer switch.  

For those of you young folks who have never driven an older American car, this switch presents to the user as a k-n-o-b.  When the k-n-o-b is in, nothing should be on.  When the k-n-o-b is halfway out, the parking lights (amber front lights, red rear taillights and all the side lights and front and rear clearance lights) should be on and turning the k-n-o-b sets the brightness of the instruments on the dashboard.  When the k-n-o-b is fully out, all of the aforementioned should continue to work and the headlights should work.

When I unplugged the multiple connector from the combination headlight/parking/instrument-dimmer switch, the fuse slot appeared normal, with voltage on one side (the same as other circuits I checked on the same fuse block) and no voltage on the other side (the same as other circuits I checked on the same fuse block).

I bought a new switch (it's a Ford part) and tested again.  Same results except that there was a slight loss in voltage between the two sides of the fuse slot.  This test is repeatable.
I got out my ohm meter and analyzed both the new and old switches.  They match well.
Here is what I found out about each connection:
A.  The only one that is hot 12v on the harness.  Has an orange wire.
D1.  Has a grey wire.  No switch position connects it to any other.
D1.  Has a black wire.  No switch position connects it to any other.
P.  Moot because it has no wire or connector on the harness side.  Switch is configured to make this one hot only in the Park position.  In ancient times, cars were wired so the front parking lights went off when the headlights were on.  This would do that.
H.  Has red wire with orange stripe.  Switch is configured to make it hot only in the Headlight position.
B.  Has red wire.  Switch is configured to have this hot at all times.  It seems to have a direct connection to A. regardless of switch position.  Not sure what the purpose of that is.
R.  This is configured to be connected to A. in both the Park and the Headlight positions.  It is also visually connected to the instrument-dimmer rheostat.  The harness connector has a brown wire.  This brown wire has been cut off short and connected with a wire nut to two yellow wires which disappear separately up into the dashboard and one blue wire which disappears into a large roll of overlong wires the size of a tennis ball, bound together with a cable tie.  What could possibly go wrong?
I.  This is the other side of the rheostat.   Accordingly, it tests with A in the Park and the Headlight positions.   It tests with A. through k-n-o-b rotation from open to 3 ohms and gradually drops to zero ohms.
Obviously, the next step is to disconnect the three non-stock wires from that bundle and test the fuse slot, then test it with each wire hooked up solo,
I just know I am going to have to open up the tennis ball.  I can hardly wait.
Of course I had to type k-n-o-b because some forum software rejects a plain knob.

Edited by pethier
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More fun.  Disconnected the blue and the yellow and the yellow.  No difference to the fuse slot.

The brown stub tests live in Park and Headlight positions, as it ought.

The tennis ball to which the blue wire goes seems to be associated with two unlabeled fuse blocks attached to the bottom of the steering column.  All the fuses in these two blocks appear good.   




Edited by pethier
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It may make a difference what chassis you have as it does when you are running gasoline. On our Ford chassis motorhome there was a set of fuses that were labeled "body builder" and then something. Those supplied things like the marker lights and such which were different than those supplied by Ford. 

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The answer is probably that someone did some goofiness and that this factory-marked fuse slot now does nothing at all. 

I have disconnected all the tail-parking stuff at the switch.

There are four light housings on the front of the rig: Two combined park/turn set into the bumper and two double-sided truck units on the side at the front (these show both to back and front). 

I have identified the wires for turn/hazard and for park/tail. 

I have tapped into the park-tail wire with a long wire (still more leftovers from the 60-conductor submarine cable I took out of my Lotus Europa). When I touch the other end to battery plus, all four of these lights on the front light up and operate properly.

Next is to connect this test wire to the park wire coming from the combination switch. This is 99.99% sure to work, since there is only one power-in to the switch, and the headlights work fine, so this source of 12v surely has enough current capability. Once that is checks out, I can run new wire to all the tail/park lights on the motorhome. I should put an inline fuse on it, because the only fuses that are upstream of the combination switch would be sized to include headlights.

I may never find out what went wrong with whatever loony wiring the DPOs had for the tail/park circuit, and I don't care if I can get this to work.

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34 minutes ago, pethier said:

I may never find out what went wrong with whatever loony wiring the DPOs had for the tail/park circuit, and I don't care if I can get this to work.

Good attitude!! The only thing I suggest if you run new wires is to not just tap into the wiring. Cut the wires so that there is no back feeding. Who knows what potential issues you may find if you do.........

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  • 3 weeks later...

Did redo some of the existing wiring connections.  Who told one of the previous owners that wire nuts were a good idea for the exterior lighting on a motor vehicle?  Everything works now.

Also took apart the wiring for the dash AC/heat fan.  Could not find a cause of failure.  Put it all back together and it all works now.  I like that I fixed it, but I'd rather know what I fixed.

Just for good measure I replaced the 7-wire trailer connector.  The trailer hookup on the Suburban is happier with a new one, and the one on the pusher was looking a bit worn.


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