Shorted headlight bulbs?

[Seahawk]

I've been tracking down shorts in my buddy's 88 Ranger XLT 4x4 extended cab for 5 frustrating days now. The initial problem was that when the parking or headlights were on, the battery wasn't maintaining a charge (with the engine running). Left my friend stranded a few times. He replaced the alternator and battery. The starter was also recently replaced by him.

Now, I found shorts backfeeding into the parking/interior illumination circuit. I traced the problem down and both front parking/turn signal harness were shorted internally. With those removed, the short to ground in this circuit disappeared.

Thinking I had everything figured out, I began to put things back together one by one. However, with the master light switch (push/pull switch in dash, also dimmer switch for interior lights) connected and headlights turned on, a short appeared again. Tracing it down, I found the circuits to the low and high beams both shorted to ground.

Tracing the problem down, I isolated it to the front harness, somewhere between the connection in the left front area under the washer tank and the front lights. I did take part of this harness apart yesterday when I was tracing down the parking light short and saw no signs of damage.

Here's the thing - the short only shows up if either or both of the connectors to the headlights are connected, Are both of the headlights themselves shorted out? If I ground the headlight and check the low or high beam terminals they will also be grounded.

Also, with the headlights disconnected, there is no short to ground in the low or hi beam circuits (in the harness itself).

Thanks in advance for any help.

Ben

 

[4x4junkie]

Here's the thing - the short only shows up if either or both of the connectors to the headlights are connected, Are both of the headlights themselves shorted out? If I ground the headlight and check the low or high beam terminals they will also be grounded.

Also, with the headlights disconnected, there is no short to ground in the low or hi beam circuits (in the harness itself).

I hope I'm reading your post correct...
Are you testing all this stuff WITH the headlight bulbs connected to their sockets? If so, that would be why you're getting the readings you are...

Unplug the headlight bulb from the harness if you want readings of the harness itself (a typical incandescent/halogen headlamp (unlit, or cold filament) is about 0.2 ohms, or what would essentially appear as a short-circuit).

 

[Seahawk]

Thanks for the reply; I'm to the point now where I'm beginning to confuse and doubt myself. The headlights are not shorted out, I figured out yesterday. Realized my mistake as you mentioned.

However, there is still a short somewhere and I can't figure it out. I have a test light hooked up between the negative terminal and the negative post on the battery.

I have virtually every body harness connector unplugged and the pcm disconnected. The gauge cluster, glove box light, dome light, radio, all steering column switches, and main headlight switch are all disconnected.

If I connect the positive lead of my voltmeter to the battery and the negative to the orange/black on the fuse panel, the voltmeter reads battery voltage. This indicates a short to ground in a power circuit somewhere correct, or am I just losing it? With the voltmeter hooked up as mentioned, if I remove the fuses one by one, there is no change. Note that this is all being tested with the key off.

There are some connections inside the dash that have not been disconnected, like the blower motor.

What am I doing wrong here? Sorry to seem like a noob but I'm not, I've been wrenching and working on 12v electronics for years. Can't say I have much experience in chasing down a short like this though.

 

[Earl43P]

http://www.therangerstation.com/tech_library/EDiagrams/files/Diagram_Exteriorlights_1983to1989.JPG

http://www.therangerstation.com/tech_library/EDiagrams/files/Diagram_Headlights_1983to1989.JPG

Does it have a trailer connector?
They can be troublesome.
Look for evidence that it ever had one; on a truck that old, who knows?
I'll keep thinking about it.

 

[Seahawk]

Thanks Earl. I have been using those schematics from the Chiltons manual.

I believe the truck does have a trailer harness. I'll double check that tomorrow. I got the front harness reinstalled tonight and all of the exterior lights are working fine (didn't check brake or turn signals yet, separate circuits), although all the exterior lights worked fine before. The only problems the truck had were the cargo light didn't work and the heater/ac control illumination didn't work. Also, the blower motor worked intermittently. Also, the illumination for the 4x4 controls didn't work, although the map lights did. The HVAC panel does light up now, it's almost the only thing hooked back up inside.

 

[4x4junkie]

If I connect the positive lead of my voltmeter to the battery and the negative to the orange/black on the fuse panel, the voltmeter reads battery voltage. This indicates a short to ground in a power circuit somewhere correct, or am I just losing it? With the voltmeter hooked up as mentioned, if I remove the fuses one by one, there is no change. Note that this is all being tested with the key off.

You're using the Volts scale on your meter to check for shorts?
If so, you're probably getting the reading you are because of a tiny residual current flow somewhere within the vehicle's systems (one of which maybe you missed unplugging). These residual currents are entirely normal, and usually amount to somewhere between 10-50mA or so (your meter set to Volts however needs probably less than 10µA for it to read full voltage lol).

What it sounds like to me is you're confusing these residual currents with shorts. Instead what you should be checking for is excess current flow from the battery using the Amps scale on your meter. I would say more than 100mA with the key off should warrant further investigation. Anything less than that is likely normal, and does not drain the battery over a few weeks to a month's time.
If you haven't already, I'd say go back to square one and see if the truck still has the issue that prompted all this now that the alternator & battery have been replaced.

 

[Seahawk]

Thanks very much for the informative reply. I have the truck mostly back together now and will finish putting it back together tonight. I borrowed a charging system tester that will put a load on the alternator; perhaps the alternator itself is bad (voltage regulator maybe).

Thanks again, hopefully I can have my garage back after tonight.