Electrical (lights) troubleshooting help (please)

I've been trying to isolate an electrical issue on a 1994 1500 with the small V8 for about a month. It's a family truck we use for everything from moving the boats around to scuba to letting the neighbors borrow it for runs to Home Depot, etc.

A couple years back a mouse had gotten under the hood and chewed up some of the wires around the fuse box and in the main harness running parallel to the driver's side fender rail. At the time my father fixed it with heat-shrink crimps. It wasn't the best job but it was mostly ground wires. After his fix everything worked fine for years.

A few months back the tail lights started acting up. I'll list what's happening but its something between the driver's side blinker and the reverse lights that I can't find for the life of me. When this happened I tore out his old repairs thinking that was the cause. No luck... I have now replaced or laid my hands on every inch of wires (outside the cab) on the truck. All soldered properly, replaced with proper color and gauge wire and I used heat shrink tubes so it's sealed properly.

So:

WITH THE RUNNING LIGHTS OFF:

The right blinker and brake light work fine
The right backup light works fine

When the LEFT blinker is used BOTH reverse light filaments are just barely orange and turn on and off WITH THE PATTERN OF THE BLINKER
The LEFT brake light causes the same thing, both reverse filaments are amber


WITH THE RUNNING LIGHTS ON:

The Right side of the truck seems to be fine with the lights on or off. (meaning brake light, running light and blinker work fine)

The LEFT blinker initiates an alternating flash pattern between the left blinker and BOTH reverse lights.

When the left brake light turns on it's dim and both reverse lights are very bright.


Any thoughts, suggestions or ideas? The only thing I really haven't checked is the reverse light switch (if that's on the transmission or on the column). I've also not gotten into the cab (behind the dash) of the truck at all. I have a hard time believing it's behind the dash since nothing has been touched under there - but sho knows. Can you guys point me to a place where I should look to find a place where the reverse lights, the left blinker and the left brake light would all come together?

It looks fine and I'm getting frustrated. Inspection isn't until November so we have time... but it's just a pain and only a matter of time until they stop issuing warnings and start giving fix-it tickets.

Thanks.

That is just weird man. No shorts in the fuse or relay boxes?

winstontj said:

So:

WITH THE RUNNING LIGHTS OFF:

The right blinker and brake light work fine
The right backup light works fine

When the LEFT blinker is used BOTH reverse light filaments are just barely orange and turn on and off WITH THE PATTERN OF THE BLINKER
The LEFT brake light causes the same thing, both reverse filaments are amber


WITH THE RUNNING LIGHTS ON:

The Right side of the truck seems to be fine with the lights on or off. (meaning brake light, running light and blinker work fine)

The LEFT blinker initiates an alternating flash pattern between the left blinker and BOTH reverse lights.

When the left brake light turns on it's dim and both reverse lights are very bright.

Thanks.

Click to expand...

This kind of symptom is usually caused by a bad ground. In your case, most likely at the back of the truck.

I will try to explain. If you looked at the way the wires are run, the ground wire from the brake light probably goes to the backup lights, then finally goes to the chassis. Now let's assume that the screw came loose from the chassis. You no longer have a ground connection, BUT you now have the brake light ground connected to the back-up light. the other hot side of the backup light goes to other connections. This really gives you a couple of bulbs (or electrical devices) in series. That is why these things light up dimly. They have some voltage across them,,,but not the full 12 volts.

Here is how you confirm.
Turn on a brake light.
Find the hot wire going into the brake light.
Check that you have a full 12V on the hot with respect to the chassis.
if it is not a full 12v, a ground strap somewhere is bad.
Then find the ground wire coming out of the brake light
Use a jumper and connect the ground wire to a clean spot on your chassis.

the brakelight should no be nice and bright.

NOTE: normally the Brake and Turn are the same filaments in the bulb.
the other filament is the running light. Unless you have amber rear turn signals.

Hope this helps.

Dan

dpinvidic said:

This kind of symptom is usually caused by a bad ground. In your case, most likely at the back of the truck.

I will try to explain. If you looked at the way the wires are run, the ground wire from the brake light probably goes to the backup lights, then finally goes to the chassis. Now let's assume that the screw came loose from the chassis. You no longer have a ground connection, BUT you now have the brake light ground connected to the back-up light. the other hot side of the backup light goes to other connections. This really gives you a couple of bulbs (or electrical devices) in series. That is why these things light up dimly. They have some voltage across them,,,but not the full 12 volts.

Here is how you confirm.
Turn on a brake light.
Find the hot wire going into the brake light.
Check that you have a full 12V on the hot with respect to the chassis.
if it is not a full 12v, a ground strap somewhere is bad.
Then find the ground wire coming out of the brake light
Use a jumper and connect the ground wire to a clean spot on your chassis.

the brakelight should no be nice and bright.

NOTE: normally the Brake and Turn are the same filaments in the bulb.
the other filament is the running light. Unless you have amber rear turn signals.

Hope this helps.

Dan

Click to expand...

This is what I was thinking as well. Did a little more digging yesterday and found some new information that hopefully sheds some light on this disaster.

The left side (driver's side) brake light filament is not being used at all. We pulled the bulbs out of the tail light assembly and saw that only the parking/running light filament (top small filament) is being used for all three conditions - brake, turn signal and parking/running lights.

Somewhere, somehow, the power is being fed into the parking/running light circuit instead of the proper brake light circuit.

Could this be a bad headlight switch at the dash? I swear I've gone over the wiring on this truck 1000 times and have found nothing - not only that but I've got a lot of British (Triumphs & Land Rover) and Lucas electrical experience which means I'M REALLY GOOD AT ELECTRICAL STUFF...

I would bet my life that the electrical/wiring harnesses from the cab bulkhead (rear of the driver's seat) is 100% perfect and fine. I've laid eyes or hands on every inch of the wires. Also, under the hood I've done the same. The only part of the truck I haven't really messed with is inside the cab (under the dash).

Could it be that the headlight switch is bad? Where else on the truck would the running lights and turn signal and brake light all come together (from a hot/positive perspective, not a ground)???

I believe that the power is being pushed through the wrong circuit and therefore burning through the other lights/filaments on it's way to search for ground.

Any ideas?? I'm a bit desperate here... Truck has until November to get re-inspected however that won't stop a fix-it ticket from shutting it down.

Thanks in advance!

It sounds like you have not brought out the volt meter.
You symptoms still sound like what I was explaining.

A bad ground can end up with two bulbs in series when you apply the brakes.
So that is 6volts on each bulb.
And then for the dual filament bulbs, the brighter filament actually has lower resistance.
This mean if you put a parking light and a brake light in series, they are not equal resistances.
So if you put 12 volts across them, the parking light will see more voltage and light up more.

You should get a sharp probe and a jumper wire with a alligator clip.
Clip the probe to a known good ground and stick it into the insulation of the ground wire on the bad light.
That will make it work if there is a good 12v on the hot side (you need to measure 12v with a meter).

Keep me posted.

Dan

dpinvidic said:

It sounds like you have not brought out the volt meter.
You symptoms still sound like what I was explaining.

A bad ground can end up with two bulbs in series when you apply the brakes.
So that is 6volts on each bulb.
And then for the dual filament bulbs, the brighter filament actually has lower resistance.
This mean if you put a parking light and a brake light in series, they are not equal resistances.
So if you put 12 volts across them, the parking light will see more voltage and light up more.

You should get a sharp probe and a jumper wire with a alligator clip.
Clip the probe to a known good ground and stick it into the insulation of the ground wire on the bad light.
That will make it work if there is a good 12v on the hot side (you need to measure 12v with a meter).

Keep me posted.

Dan

Click to expand...

Dan and others,
Thank you for your continued responses - very much appreciated!

The brake and turn signal always go through the wrong filament when the running lights are on.

We did break out the volt/ohm meter this afternoon. Grounds all zero'd out to the bumper and chassis just fine.

Without the running lights on there was always about -0.08V at the brake light/filament circuit. When you press the brake pedal it goes to 10.7V and the running light circuit got about 1.67V. Just enough to make the filament amber when you look at it in the dark.

WITH the running lights on it's similar, there is about -0.08V to the brake light circuit (always) and then when you press the brake pedal it goes to ZERO (exactly 0.00V). That's when the current feeds over to the running light circuit and the reverse lights.

The problem is the current is going to the wrong wire when the running lights are on. When you hit the brakes the TOP (small, parking/running light) filament goes on.

The grounds are all fine as far as I can tell. Everything zero's out on the Ohm Meter and we used a ton of alligator clips & probes and forced all the grounds to properly ground out to the chassis - no change.

Also, we swapped the two light harnesses - from the chassis plug out to the tail lights. I swapped the driver's side and the passenger side. No difference. No change.

Still just as puzzled as ever...

Dont' give up

I reread your post, so the truck is on the old side.
You did not say what your battery voltage is, so I will assume 12v.
The points that measured 10.7v are a little low, but we can assume they are coming from your 12V source. You might want to measure on the truck side of the fuse block terminals. You might be getting some voltage drop by the time it runs through the fuse contacts. But this would not cause the wrong light to illuminate.

There is no way for two bulbs to light unless there is voltage across both of them. AND if the voltage is less than 12V, they will be dim.
That is a fact.

The easiest way to get the voltage split like this is with a bad ground.
IF this does turn out to be a bad ground, you owe me a six pack.

This is what I would do. I would start at the back of the truck and make the lights work by bringing my own 12V line from the front of the truck.
You should be able to unplug a connector at the back somewhere to disconnect the rear light fixtures.

Find where the ground comes from. It is probably a pin in the connector, but the rear light assemblies could be grounded directly.

Make sure the ground on the back light assemblies is good.
Then use your 12v wire and connect it to each pin in the connector to be sure each filament works right. IF they don't, you know the problem is in the disconnected rear harness.

I am sure you know that there will be one pin each for the backup and running. Then one pin for left, one pin for right brake/turn filaments.
So there are only 4 pins with each having a different function.

By doing this, you have cut the wiring in half so you can confirm which half has the problem.

Just be careful you don't short your 12v line to ground by accident.

Dan

Just noticed this

When you press the brake pedal it goes to 10.7V and the running light circuit got about 1.67V. Just enough to make the filament amber when you look at it in the dark.

Did you notice that 10.7 + 1.67 = 12.37. This is about the right voltage for a battery a short time after the engine is shut down.
This tells me you have a voltage divider...this is two filaments in series.

TRY THIS FIRST

We can assume the ground is good to the bed because the right side works fine. You should turn on your left signal and running lights. Then remove the bulb from the left side socket. Probe the two center contacts.
One should be constant 12v and the other should be flashing 12v. make sure you ground your meter on a "known good ground" not the side of the socket. My bet is that you are getting good voltage to these two contacts, which shows it is a ground problem.

Do the backup lights work correctly - are they as bright as they should be?
If they glow with the brake lights, that means their ground is bad also.


Dan

Good afternoon!
I wanted to follow up in the even that someone else ran across this thread in the future...

Dan and everyone who helped - I can't thank you enough. All I can say is it's fixed. I knew it felt like a ground issue from the start and you guys were 110% exactly correct with your diagnosis... My only issue was I couldn't find the bad ground ANYWHERE... So I did the one thing I hate and never wanted to do - spliced into the existing ground right at the rear driver's side tail light and fastened it to the chassis. It works perfectly but still ****** me off every day knowing I was never able to find the actual break in the ground!!

Thanks again for everyone's help. I really appreciate it!