Electrical When Headlights Are On, Dash Gauges And Lights Go Crazy

[FoxMustangLvr]

I have a 90' LX 5.0 5spd. This problem has been happening ever since I bought the car so NONE of my mods created this. My car has an aftermarkey Pioneer CD player which works well and I have upgraded to a 3G alternator and contour fans and my ground is 4ga as well.

Problem: when I turn on my headlights my gauges all read incorrectly (except speed of course) and my hazard lights light up on the dash. I've searched about this and everybody goes straight to grounds but nobody has reported a fix. Some think it's a bad voltage regulator that's in the dash itself. Most of my driving is when the sun is up but now the Fall is coming and my morning drive to work is getting darker every day. From time to time the instrument cluster will go back to reading normal while driving with my lights on and then go haywire again and back and forth.

Does anyone know if the problem really is the dash cluster or something else?
@jrichker @liljoe07

 

[jrichker]

The secondary power ground is between the back of the intake manifold and the driver's side firewall. It is often missing or loose. It supplies ground for the alternator, A/C compressor clutch and other electrical accessories such as the gauges. The clue to a bad ground here is that the temp gauge goes up as you add electrical load such as heater, lights and A/C.

Any car that has a 3G or high output current alternator needs a 4 gauge ground wire running from the block to the chassis ground where the battery pigtail ground connects. The 3G has a 130 amp capacity, so you wire the power side with 4 gauge wire. It stands to reason that the ground side handles just as much current, so it needs to be 4 gauge too.


See http://assets.fluke.com/appnotes/automotive/beatbook.pdf for help for help troubleshooting voltage drops across connections and components. Be sure to have the maximum load on a circuit when testing voltage drops across connections. As current across a defective or weak connection, increases so does the voltage drop. A circuit or connection may check out good with no load or minimal load, but show up bad under maximum load conditions. .

Voltage drops should not exceed the following:
200 mV Wire or cable
300 mV Switch
100 mV Ground
0 mV to <50 mV Sensor Connections
0.0V bolt together connections

 

[90sickfox]

I had the same issue and it ended up being the connector going into my multifunction switch had overheated and caused a short to my hazard lights. The turn signals would stay on and the high beam light would glow real dim. I also had an issue with the headlight switch on my gt that caused similar problems.

I'd advise you to check the headlight switch connector and the connectors at the turn signal switch.....and the ignition switch.

Over the years the pins in our connectors loosen....causing corrosion and heat. That causes heat and melted connectors. Especially, our headlight and turn signal circuits.

 

[FoxMustangLvr]

I had the same issue and it ended up being the connector going into my multifunction switch had overheated and caused a short to my hazard lights. The turn signals would stay on and the high beam light would glow real dim. I also had an issue with the headlight switch on my gt that caused similar problems.

I'd advise you to check the headlight switch connector and the connectors at the turn signal switch.....and the ignition switch.

Over the years the pins in our connectors loosen....causing corrosion and heat. That causes heat and melted connectors. Especially, our headlight and turn signal circuits.

I think you are in the ball park. I did check my grounds and all are present and in fair condition. Yesterday i drove my car as the sun was coming up so I used my lights even though I didn't really have to. When I turned my lights on the gauges did their usual dance and hazards on the dash light up (but not outside of the car, only the dash). When I adjust the dimmer switch to dim the dash lights the gauges go slowly back to where they should be and if I completely turn off the dash lights and turn on the interior lights everything goes back to normal. I don't want to drive with my interior lights on so I played around with the dimmer some more as I was driving and found a sweet spot where my dash cluster wouldn't go crazy and my interior lights remain off. I was able to drive just fine this way for nearly 45 minutes total. Maybe I have a faulty dimmer switch? I'm not ruling out the head light switch or a loose connection.

 

[Knowbody]

Sounds weird, but I just had a similar issue after messing with some wires under the hood of my '91. Never had an issue before, but after I installed my 2-step and fan relay kit, when I cycled the key to "on" the fuel pump would only run for a split second, all of my aftermarket gauge lights would come on, and the turn signal indicators would stay on solid. When I hit the headlight switch, thinking that was the culprit, the OEM oil pressure gauge would spike ( car not running). Jrichker posted a diagram, which led me the recheck the grounds and fusible links. With the key "on" I wiggled the harness under the starter solenoid and I could hear relays clicking and the fuel pump would come on and off. Turns out the yellow 18ga fusible link was previously repaired and the crimped butt connector was faulty. I replaced the fusible link, soldered and heat shrinked, and everything is back to normal now. No weird lights, no weird gauge movement, car starts right up like it did before. I'm not saying this is your issue, but it might be worth looking at the fusible links coming off the solenoid. There is a thick yellow wire connected to the solenoid that feeds power to 4 fusible links. Good luck.