Dash Lights, Fuses, and Ignition

[zsn0w]

Got the car a few months ago now, and the dash lights never worked. I put in a new cluster the other day that was given to me when I bought the car because it was in better shape than the cluster in the car, and I started working on the dash light problem. I found that the fuse had blown. So, I replaced the fuse, and the lights started to work. Later, I started the car and the fuse blew again. I figured maybe I'll get lucky and it's just drawing 6-7 amps so the 5 amp fuse is blowing, so I installed an 8 amp fuse. This blew the taillight fuse instead when I started the car. So, I replaced the 8 amp with another 5 amp and put another fuse in for the taillights. Turns out that if I start the car with the lights on, the fuse blows. If, however, I start the car with the lights off and THEN turn the lights on, the lights all work. Pretty confused by this. Not sure how the cranking of the starter is effecting anything on the accessory circuit. Could the drop in voltage as the starter is engaged spike the amperage to the rest of the car and blow the fuses? Tomorrow, I'm going to get some more fuses and do a couple more start-up tests to make sure that the fuse blowing on ignition is completely repeatable. Any ideas?

 

[Ironbuilt]

Sounds like a short in the headlight switch.

 

[Shorthorse]

Is the harness original and is it in decent shape? Do you see any signs that someone has messed with the harness (splices, things like that). Year, make and model please.

 

[zsn0w]

Sounds like a short in the headlight switch.

I tested the switch with a multimeter to see if all the outputs switched correctly when I had it out of the car. As long as that was all how it should be, there shouldn't be any problems with the switch, right? Could there still be even though I checked it over and it all checked out?

Is the harness original and is it in decent shape? Do you see any signs that someone has messed with the harness (splices, things like that). Year, make and model please.

It's a 1974 Roadrunner. It is the original harness, but I haven't noticed much spliced into it or changed on it. There has been a tach added, but it's plugged into the extra spot on the fuse block for power, so the only place something has been spliced that I can tell is for the tach, but nothing into any of the orange wiring.The wiring all seems in okay shape. There is a problem with the wiring for the oil pressure gauge through the firewall, but that's the only other thing I've noticed. I just ran my own wire from the sending unit to the gauge for that and it worked fine.

 

[Shorthorse]

Your car is a little newer than I am used to. I would normally suggest MyMopar.com which has wiring diagrams including your 74 Road Runner but after looking at the diagrams I believe the one for you car has some errors in the starter circuit. Does the fuse blow exactly at the time the starter is engaged?
In general terms, the process for starting most older Chrysler products is that battery is delivered from the ignition switch via a yellow wire to the starter relay. The yellow wire passes through one of the three plug/connectors in the bulkhead terminals. The problem is that the ignition switch delivers battery and if the lights were shorted to the yellow wire, the lights would come on during cranking, not blow a fuse (short to ground).
At this point it could be anything. A factory service manual is your best bet if you can get your hands on one. A good volt meter and a handful of fuses. Just remember, if you use a volt meter to check continuity to ground on any circuit that has lamps, you must remove all the lamps first or you will read continuity through the elements which gives you a false reading.
Good luck and keep us informed.

 

[Nacho-RT74]

74 diagram at myMopar is not correct. Get the FSM

 

[zsn0w]

I've looked at the wiring diagram a lot, and I'm having trouble wrapping my head around how the ignition could possibly have any effect on the circuit the lights are on... Just strange to me that those two are interacting at all. An ignition wire would almost have to be shorting to one of the wires into the light switch, right? Thinking about putting a time delay fuse in and seeing if that fixes it. That should be safe, right? I drove with the lights on last night for about 45 minutes with no problem at all since I waited until after I started the car to turn the lights on.

 

[Shorthorse]

Where did you get your diagram?
They aren't related so you've got some sort of short, somewhere. If there is a problem somewhere in the dash or engine harness it's best to find it now. You could continue your routine but it won't fix itself. The alternative is not pretty so you'd better get a fire extinguisher.
Try removing the yellow wire from the starter relay for your tests. That way you won't keep starting the engine when you turn the key. A delay fuse will only allow the short to stay engaged for a very short time but it only takes a few seconds to melt wire insulation or start a fire.

 

[zsn0w]

I've been using a combination of a couple diagrams. One I got from MyMopar's 71 Plymouth service manual. The other I found here http://www.dodgecharger.com/forum/index.php?topic=23315.0 and it seems to be more accurate, but I don't have the accompanying engine bay diagram, so I have to use the 71 diagram occasionally. Even though some things are a little different, I've found it to be accurate enough for my purposes. I'll do what you said with the starter relay wire and test it some more to see what I can find.