Vintage Mustang Forums banner

Electrical question.

2307 Views 66 Replies 12 Participants Last post by  Snulma1
I have an electrical problem I’m trying to wrap my head around.
Dissasembled my dash to replace my gauge cluster, redo the radio, repaint, and then replace my heater core and wiring.
Swapped out the gauge cluster, and tested the lights with them all working. Then plugged back in the wiper switch and seated cluster as well as screwed in.
Went to check the cluster one last time and they lit up but then went dark.
Thought that may be the battery was low, so pulled it out and placed on a charger for two hours. Placed back in and the lights lit up and the wipers worked but then everything clicked off again.
I’m assuming there’s a short somewhere but not sure where to start. Does anyone have any suggestions? 
1 - 18 of 67 Posts
If the instrument cluster lights DON'T work but the horn DOES work then it's NOT the headlight switch.
So I pulled the cluster and made sure everything was seated and a good ground. So far everything is working.
But what’s weird is if I pull the light switch to the first stop, the dash lights and the marker/fog lights light up. But if I pull to the second stop, the dash goes off and the headlights come on.
Is that normal?
No. The instrument cluster lights should stay on in both "Park" and "Head" mode. Make sure you have the correct headlight switch as some are "pinned" differently. It sounds like the cluster lights are getting power from the "park" feed. Are the parking lamps on with the headlamps?
No they are not. And neither are the taillights.
Then I'd pull the headlight switch and test it with an Ohmmeter against the wiring diagram. If it appears correct then I'd check the "pinout" of the headlamp switch plug.
I’m sorry still sort of a newbie when it comes to electric. I have a multimeter (7 Function Digital Multimeter)
I can pull the headlight switch but not sure how to do the remaining.
Grab your multimeter, set to the 200 Ohms scale, put the RED test lead on the "B" (battery) terminal on the switch, move the switch through its positions and check the other pins with the BLACK lead as shown below.

Rectangle Font Line Parallel Slope
See less See more
  • Like
Reactions: 2
Is there a specific reading I’m looking for?
As close to zero as possible.
Sorry, I didn’t follow the directions to first time.
Everything has continuity in the proper positions, except for D1 and D2, which have no continuity at all.
Sine "P" did not change it sounds like you have a "bum" switch. "P" should be near zero with the switch in the first pulled position. D2 should be near zero with the knob turned all the way CCW into the detent.
I went back and retested, realizing I didn’t follow the directions properly. Only D1 and D2 don’t have continuity I’m assuming that still means bum switch.
Sorry, Look for continuity between D1 and D2, with the knob turned all the way.
I assume that means the switch is operating properly. So what would be my next step?
Exactly WHAT stops working properly when you have your issue? Just the instrument cluster illumination or more?
The marker lights and the cluster don’t work when the headlights are on.
When you say "marker" do you mean the front side marker lights or the front parking lights? The front parking lights do NOT work with the headlights, only in the "PARK" (1st) position. If the cluster lights work in the "PARK" position but shut off when you pull the switch to the "HEAD" position then either the switch is defective or the plug is wired improperly.
Sorry I meant the parking lights.
So since the cluster turns off when in the head position, and we tested the switch and it has the proper continuity it must be a wiring issue right? Any tips on figuring out where the issue lies?
Check that the BLU/RED wire is properly located in the headlight plug in the slot matching up with the "I" position. If it IS, examine the rheostat and its contacts that something isn't moving when the switch is pulled out all the way.
If I follow the discussion properly, you have an open in the wire somewhere in the highlighted area. Disconnect battery cable, with headlight switch on you should have continuity (almost 0) between A (or B) and R. Where are you located? depending on where you are I or someone I know may be able to help you. I'm in NC.
View attachment 865889
What does "R" have to do with things? That's the rear tail lights which, I believe, work fine. I though the issue was the instrument cluster lights, which is "I", and which the OP already tested, above and which stop working with the switch in the "HEAD" position even though a "bench test" shows continuity.
I’m still at a loss. Looking under the dash there are definitely a couple of wires that are unplugged, don’t know if that’s the issue, hard to trace many of them.
Cleaning the crud off the wires may help a little....
I’ll do my best but it’s not crud. It’s paint since the previous owner sprayed spray paint to cover things
"Goof Off Professional" works wonders. So does GoJo (without pumice).
I tried to use a Mr clean magic eraser and got this.

Yellow is 12v hot. Blue w/red tracer are instrument cluster illumination. Could be a console feed lead... yellow for the "cubby" lamp, others for shifter illumination and ?
View attachment 866237

This "triple tap" with Black/Yellow is a switched ignition "hot" for accessories.

View attachment 866239

Black w/blue tracer are courtesy lights. Probably for the under-dash courtesy lamp(s)?

View attachment 866238

This is the instrument cluster illumination accessory tap.... accessories that need illumination with dimming capability can be plugged in here.

View attachment 866240
See notes above each photo.
Great. So none of these should be the reason for my cluster not illuminating while the headlights are on.

Find that red tap with the BLU/RED wires and stick a test light into it. If you get a test light but no dash lights you can assume (I know, I know....) that the problem may be the cluster connector or "printed circuit".
So, using my tester, I was unable to get a light while tapping into the red tap with the blue/red wires. I tried with the key in the off position and light switch in the off, then park, then head position, as well as with the key in the on position, and the light switch to all positions
Okay... let my brain think for a minute.... Key doesn't need to be "on" for cluster lights to work.

With the headlight switch in the "park" or "head" positions is there power to both sides of the 2.5A fuse in the fuse panel?

One side only = fuse blown.
Both sides = okey dokey. Apply 12v to the red "double tap". Do the cluster lights come on? If so, power isn't getting from the fuse box out to circuit 19. Trace wires FROM the fuse box to the lighting circuit.
Neither = power not getting to fuse box from headlight switch. Trace BLU/RED wire (circuit 19) from fuse box to headlight switch. Make sure terminal in headlight switch plug is being properly retained by it's "tab" and not getting pushed out when plugged in. You already indicated power was getting to the terminal in the headlight switch so..... Also make sure something isn't getting stuck that is pushing the rheostat contacts away when the switch is installed..
See less See more
The 2.5A gets power on both sides.
how do I “apply 12V”?
Sorry probably a basic dumb question
Find something to use as a jumper wire, find a power source (you know the fuse has juice), shove some bare wire into the double "tap", touch the fuse with the other end and observe.
1 - 18 of 67 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.