87 Cab: Left Running Light Problem

[calvarenga]

Hi all:

I have an 87 cab and the left running/parking lights have started to blow the 5amp fuse. Left side break, signal and headlights all work. An 8amp fuse will also blow; but with a 16amp fuse in place the left front running light is fully lift, and rear left running light dimly. Only change since thy were working is I R&R the speedometer for recalibration (reinstalled with wiring correct). Any thoughts as I would hate to pay to have someone spend two days tracing down the problem, if I can avoid it.

Thanks, Carlos

[DRACO A5OG]

Check the tach where the signal light goes in, you may have dislodged it.

YOU MUST NOT use higher than spec fuses, it will cause the wires to melt and worse case a fire. You must find the fault.

Check the grounds for that side, it may be loose or shorting out ergo blown fuse.

[calvarenga]

I appreciate the info and the quick reply. I did not leave the 16 fuse in, thanks. One question I have is how to test any remedy without having to put in another 5amp fuse, just to have it blow if I'm wrong. Can I connect an amp meter across the fuse terminals and check the amp draw until it goes below 5? Also, do you happen to know which tach wire would impact the light circuit?

Thanks again.

[cabmandone]

Originally Posted by calvarenga

I appreciate the info and the quick reply. I did not leave the 16 fuse in, thanks. One question I have is how to test any remedy without having to put in another 5amp fuse, just to have it blow if I'm wrong. Can I connect an amp meter across the fuse terminals and check the amp draw until it goes below 5? Also, do you happen to know which tach wire would impact the light circuit?

Thanks again.

I'd start by checking my feed wires to ground. If you're blowing a fuse that fast it seems like you almost have to have a direct short to ground. All the 16 amp fuse did was allow wires to melt before the fuse wire. Don't do that, not even for a short period of time.

[kidrock]

check the wires to your engine lid lamp, they get burnt and dirty. The lamp could be bad also.

[calvarenga]

Got it. Will do. Thanks for all the advice.

[calvarenga]

Update: I pulled the tach today and checked all the wires, which were fine. I also opened the engine light lid and suddenly the lights were working again. But when I replaced the speedo (which I had pulled to push out the tach) turned the car on and, the fuse blew again. I pulled the speedo again and put in another 5amp fuse and no luck. It makes me think that maybe the engine light is corroded or something in the way the speedo went back in is the problem?

Thoughts?

[kidrock]

Concentrate on the engine lid lamp. Check and clean every single wire, and make sure that any of those wires don't have a bare contact with any part of the engine compartment. I can guarantee that the insulation on those wires will be dry and cracked. You may need to replace that lamp, they do burn up. I don't think your speedo has anything to do with your problem.

[calvarenga]

Agreed. The lamp looks pretty corroded in some parts so I just ordered a new one, since they're pretty cheap. Once I replace it, I will update with results. Thanks!

[calvarenga]

Quick update, all: The very good news is the short was definitely the lamp (thanks for that tip!). I got the new one today, and as soon as I removed the old one the lights worked fine. Bad news is that everything is fine until I screw the lamp back into the mounts on the underside of the hood. Nothing happens at first contact but when I screw it in, the short is back after one or two turns of the screw. I did not cross the wires when I installed the new one (and that should not matter anyway on a dome light circuit, right?) but it still shorts when mounted, although the lamp and mounting procedure looks identical to the old one. There was no insulation of any kind between the lamp and mounts (which are metal by the way).

Thoughts?

[targa80]

If your hood lamp circuit is like what I have on my 80SC then the way the lamp works is; one bulb contact has the 12volt supply and the other bulb contact goes to the internal switch. The other side of the switch goes to ground. when you open the lamp case cover you are closing the contacts to supply a ground to turn the light on. If you are just mounting the light fixture and you blow the fuse some how the 12volt supply is being shorted to chassis ground look for any burn marks on the contacts with in the light assembly and also look for a broken insulation on the 12 volt supply wire.

[calvarenga]

Got it, thank you. One additional question: the light should turn on/off with the sliding cover even when not mounted, correct? In other words, opening the lid closes the circuit to ground, whether or not the light is mounted. Which means that, as you say, either by mounting it I am pushing some exposed wire I can't see to touch the hood or somehow the wires coming into the lamp itself are grounding on/near the mounting bracket (which is hard since I wrapped both wires in electrical tape just to avoid that possibility)?

Thanks, Carlos

[targa80]

You could test the lamp fixture while it is not mounted to the car as long as you have the 12volt supply (Gray wire) and a ground (Brown Wire) connected. These are the colors on my 80SC, yours may be different. If you have a test lead with two alligator clips you can tap into the engine fuse panel for the 12volts supply (instead of using the Gray wire). Since your problem occurs on the old and new assembly I would suggest looking at the wire providing the 12 volts to the lamp. Somehow that is getting shorted to ground when you mount the unit.

[targa80]

Originally Posted by calvarenga

Got it, thank you. One additional question: the light should turn on/off with the sliding cover even when not mounted, correct? In other words, opening the lid closes the circuit to ground, whether or not the light is mounted. Thanks, Carlos

Yes as long as you have power to the circuit (Light switch on and you may have to have the ignition on)

[targa80]

if you have a multimeter (very cheap (~$5.00) at harbor freight) you can check for a continuity test using the resistance (ohms) scale. Disconnect the +12 wire from the lamp assembly. Mount the lamp to the chassis with the cover off you can access the contacts. Read the resistance from the contact you removed the gray wire from to the other terminal with the brown wire. With the switch open the resistance should be high. close the switch and the resistance should be very low. In this case your problem is in the gray wire. if you remove the bulb and do the test again, the resistance should be high in both cases.
if the resistance is low when the switch is open and closed in both cases above then there is a short from the 12 volt contact on the lamp assembly to ground.

[calvarenga]

Thanks for the additional info: the multimeter terminal test is a great idea. I'm down to one fuse so as soon as a few more arrive, I'll give it one more shot and update the thread.

Cheers

[calvarenga]

Final update (I hope): I pulled out the multimeter as you suggested Pat and found that there was a slit in the power wire that was not visible when it was stretched (which is why the lamp lit when not mounted but shorted when pushed back during mounting). I shrink-wrapped the power line from where it exits the wiring loop on the hood up to the lamp, remounted, and everything works fine.

Thanks to everyone for their advice. I have no doubt that I just saved two days of diagnostic expense!

Cheers and safe motoring,
Carlos

PS: Next task: figure out why the upshift light in the tach sometimes works and sometimes doesn't