Help me troubleshoot my dash light problem

[jcode68]

The dash lights in my 68 haven't been working for a while, but I put it on the back burner since I don't usually drive it at night. Well, my punchlist is getting smaller and it's time to resolve this. Here's what I know:

• The headlight switch was replaced new a couple of years ago
• The fuse for the instrument lights will blow as soon as I turn on the parking lights.
• I have power to one side of the fuse, but not the other. I think this is the case, will have to doublecheck to be sure.
• If I attach a lead to both sides of the fuse, it lights the bulb in my tester (basically a light with 2 leads) with the switch on (will need to doublecheck this as well to be sure - did a lot of crawling around under the dash today and a bit fuzzy at the moment).

So I have the guage cluster out and can/will pull the headlight switch for any testing/isolating that I need to do. Electrical gremlins has never been my strong point and could use some help to isolate the problem. The wiring harness is original and in excellent condition. Only things aftermarket is the stereo (not wired to this circuit) and the CD ignition (again, not using this circuit). What's the logical approach to isolating this?

 

[downwardspiral]

If the fuse is blowing, but the test light is lighting, the amperage is exceeding the fuses limits. It seems like the parking lights are shorting out to me, or improperly wired at that.I suck with this stuff too, but I would check the wiring of your parking lights, make sure the fuse is the right amperage.

 

[jcode68]

The fuse is an AGA 2.5 amp fuse per the spec in the manual.

 

[Tim65GT]

With the test light hooked up to the fuse block and the light switch at the parking light position, unplug the Gage cluster, If the test light goes out, the short circuit is in the gage cluster, if not the short is in the blue red wire from the fuse box to the plug.

If the short is in the cluster, pull out all the gage illumination bulbs and check to make sure its not the cluster. Insert the bulbs one at a time. If the test lamp comes on, then a bulb was shorted. (rare but possible)

 

[jcode68]

Tim65GT,
Thanks, this is the simple approach troubleshooting I was looking for! Ok, so I did as you suggest and when I disconnected the guage cluster, ths light stays on, suggesting a problem in the blue/red wire from the switch to the fuse block. I traced the harness and it is completely sealed in the original wrapping, no cut marks or abrasions all the way to the fuse block. I wiggled the harness is all locations to see if the light went out and it did not, it stayed on solid, which is not what I would expect if there were a wire with the insulation compromised.

See if you can follow me with this - with the switch on in the parking light position and my test light on, I turn the dimmer from all the way left (dimmest position) and the light is on, if I rotate the dimmer switch clockwise, the light stays on until it turns all the way clockwise. I removed the switch and looked at the contacts of the switch to see if there was anything obvious. What I see is that with the dimmer rotated full clockwise, there is no longer any contact with the metal spring (not sure of the technical term, hopefully you know what I mean), at the end of it's travel, the contact is on the ceramic part of the switch. So with the cermic piece making contact, my test light goes out. Does this indicate a bad switch?

 

[jcode68]

One other thing, I unbolted the fuse block to check the contact points of the wires, specifically the blue/red wire and they look perfect, no corrosion and not loose. So based on what I have read, power comes through the switch and travels to the fuse block when the parking lights are switched on, travels back up to the switch and then passed along to the instrument cluster. So as best I can tell, the power is making it back to the switch, but not to the cluster. Would this indicate there is a short in the switch?

 

[Tim65GT]

jcode68:
So based on what I have read, power comes through the switch and travels to the fuse block when the parking lights are switched on, travels back up to the switch and then passed along to the instrument cluster. So as best I can tell, the power is making it back to the switch, but not to the cluster. Would this indicate there is a short in the switch?

First, your welcome. I hope we can figure this thing out.

I should have included a basic description of the current path earlier. You are correct about some of it. When you turn the park lamps on the switch power goes through the internal dimmer (the spring would technically be called a rheostat). It leaves the switch on the blue-red wire (ckt #19 in the schematics) it goes to the fuse block through the 2.5 amp fuse and then on to the gage cluster to feed the gage lamps. As the light switch knob is rotated, the dimmer varies the amount of resistance, thereby dropping the voltage across it sending less voltage to the fuse and gage lamps.

Now, from what you say about the test lamp still indicating a short when the gage cluster is unplugged, I would have to say the blue-red wire going to the plug must be pinched somewhere to ground. It's very possible that there is a tie point along that wire taking the power to another path, such as an ash tray lamp, or the shift indicating lamp. So look around for a blue-red wire coming out of the harness that may have an unknown destination.

I can't remember if my shift indicator lamp was tied to the dimming circiut, but I know the wires ran under the carpet were they could be unknowingly damaged.

 

[jcode68]

So based on the results I shared, you do not think it's a problem with the switch? It's pretty tough to trace the wires because it's wrapped in the main harness. The only exposed parts of the wire is where it goes into the connector for the headlight switch. I will try some more and see if I can get better access to it by removing my seat to get a better position under there (my back will be thanking me later...). As for another blue/red wire, there is one coming right off the fuse block, it's a short wire with a pigtail connector and it has nothing plugged into it. My car is a manual trans car, so I'm thinking this is probably the lead used to run to the shift indicator light if the car were an auto trans. Thanks again for your help.

 

[jcode68]

Update - found and fixed it!

Tim65GT, I did as you suggested and traced the blue/red wire(s) and found the problem. First, i didn't expect to see multiple wires that were blue/red stripe, but there are. What I found was one of the blue/red wires has a factpry terminator with 2 female bullet connectors on the end. For some stupid reason, a ground wire from my radio was plugged into one of the connectors, which of course creates a short. I found a new (proper) home for the ground wire, put in a new fuse and my night driving is back in business . Thanks a lot for your help!

One last issue - while I was under the dash looking at the fuse block, I noticed one of them was missing. It was the 20 amp fuse for the cig lighter and flasher circuit. The manual specs an SFE 20 amp fuse, however I have 3 different packs of SFE 20 amp fuses and all of them are too long to fit in the fuse holder. The length of the fuses I have is 1.25". I also had a SFE 14 amp fuse which is just a bit shorter and fits perfectly, so that is what I have in it for now. I know this has a high possibility of blowing since its under rated, but is there a special fuse that I need to get for this?

 

[Tim65GT]

Glad to hear you got it fixed.

My 65 also calls for a SFE-14, but states an SFE-20 with flashers.

Hmmm, strange because all the references I searched and found showed the SFE-14 at 1 1/16" and the SFE-20 at 1 1/4".

trying to put a square peg in a round hole deal.

Do you have flashers? If they blow the fuse, I would get an auxilliary inline fuse holder and tap off the battery wire that feeds the Ign switch or Light switch to feed it separately.

 

[jcode68]

Glad to hear you got it fixed.

My 65 also calls for a SFE-14, but states an SFE-20 with flashers.

Hmmm, strange because all the references I searched and found showed the SFE-14 at 1 1/16" and the SFE-20 at 1 1/4".

trying to put a square peg in a round hole deal.

Do you have flashers? If they blow the fuse, I would get an auxilliary inline fuse holder and tap off the battery wire that feeds the Ign switch or Light switch to feed it separately.

I'll test the flashers today to see if the fuse holds. Just doesn't make sense to me that the spec calls for SFE-20 amp, but it isn't possible to fit one in the fuse block. Anyone else with a 68 (probably 67 is the same) have this issue that can comment?

 

[mod68cpe]

aux fuse blows at least once a day

i also have an electrical issue on my 68 cpe with a 351w /auto transm. i put an alluminum radiator with electric fan and fan is wired to the ignition so that when u turn the key one spot the fan comes on auto with the tachometer but i've noticed that the 14amp fuse on the aux spot of the fuse box is the fuse that controls the fan and i guess ignition and tachometer blows once in a while. im trying to figure out why that fuse is blowing?

 

[Rusty67]

i also have an electrical issue on my 68 cpe with a 351w /auto transm. i put an alluminum radiator with electric fan and fan is wired to the ignition so that when u turn the key one spot the fan comes on auto with the tachometer but i've noticed that the 14amp fuse on the aux spot of the fuse box is the fuse that controls the fan and i guess ignition and tachometer blows once in a while. im trying to figure out why that fuse is blowing?

You should start your own thread for this.

Please tell me you used a relay and a separate fuse for this. If you are running the fan straight off the of the ignition you are asking for trouble. You can use the ignition source to activate a relay to power the fan but do not power the fan directly.

 

[mod68cpe]

the fan has its own fuse before its connected to the ignition....but that fuse doesnt fry......do i still need a relay? how do i know its the fan doing it and not something else?