On my 86, I was experiencing the issue of my headlights only raising when I held the headlight switch in the halfway point. If I pressed the switch fully, the lights would turn on, but the headlights would only raise for a split second. Replaced the headlight switch as it seemed loose and worn out, but the new one did the same thing. Googling told me to blame the isolation relay, but it seems they're hard to find new ones of now. I wanted to retain the factory look, and avoid cutting and splicing my harness.

So I took apart the one I had. Disassembling is easy, there are just a few tabs on the relay by the pins, if you bend the relay case outwards, the pins and relay can be yanked out, leaving the empty case.


It still clicked fine, so I knew the coil worked. Digging through the schematics and testing the diode with a multimeter told me the diode was bad.

Soldering in a new one took a few moments. You'll need a gun-type soldering iron to make a good connection; the relay frame really sucks the heat. It's the diode in the schematic above, which supplies current to the actuator relay coils. Ensure the orientation of the diode is the same as shown on the relay here. If you install the diode with the stripe on the wrong side, it will not allow current to pass, and your lights will still not raise.

Any garden-variety diode should work, I believe I used the 1N4001. There are probably better choices, but the current and voltage here are very low, so it doesn't much matter. Anything from the 1N400x series is cheap and readily available.
So when you switch your headlights on, if your isolation relay clicks but the actuator relays don't or briefly click on then off, this might be the repair for you. If the contacts are very rusty, or the actuator relay doesn't click when you switch on your headlights, this may not be the fix for you. The contacts on mine looked like so.
As well, this fix doesn't apply to the newer headlight motor circuit, as there's no isolation relay to fail.

[This message has been edited by DaveM (edited 11-17-2015).]

Welcome to the Forum and what a terrific first post! What year is your car?

Nice job and very creative repair. The only thing that I would impress upon people trying this repair is to make sure that the cathode stripe on the diode is put on the correct solder terminal and to hold the diode leads with a miniature needle nose pliers to prevent excessive heat from traveling up the lead and killing the diode (or you can use a mini clamp on heat sink).
If the repair works that's great if not , a standard automotive 10A DPDT 12 VDC relay can be wired in to work . You can buy them on eBay for about $5.00 but it won't use the Fiero connector.

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quote Originally posted by DaveM: Any garden-variety high-current diode should work, I believe I used the 1N4001.

High Current? 1N400x are only 1a.
x = 1 to 7 and change reverse DC Volts... 1 = 50v, 7 = 1000v
Here doesn't matter what 400x is used.

http://www.diodes.com/_file...tasheets/ds28002.pdf

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Thanks for the positive responses guys, edited my post to reflect your comments. My car is an 86, this fix obviously doesn't apply to the newer electronically-controlled system (87-88). Ogre, I suppose high current means different things in different contexts, I learned electronics from guitar pedals, where 1A is a monster amount of current. As far as automotive wiring goes, 1A is pretty minimal indeed.

quote Originally posted by DaveM: Thanks for the positive responses guys, edited my post to reflect your comments. My car is an 86, this fix obviously doesn't apply to the newer electronically-controlled system (87-88). Ogre, I suppose high current means different things in different contexts, I learned electronics from guitar pedals, where 1A is a monster amount of current. As far as automotive wiring goes, 1A is pretty minimal indeed.

Diodes aren't not all the same. Relays and Many other things mostly use "Rectifiers." Most Relay use them as surge suppression. If you hook the coils wrong way you blow the diode and coil likely works but w/o surge protection. (Protect ECM etc from power surge when coil is De-egerize.)
Guitar pedals use Rectifiers and likely other types, like Switching or Zener diodes, depend where in the circuit they are.

I used "simple" Rectifiers, SCR, etc, that handles 20 - 30 amp in stud cases mounted to a big heat sink. Now they make diodes handle 2100 A like http://www.vishay.com/diodes/list/product-93539/ For fun Read the app note how to mount them.
More see www.vishay.com/diodes/med-high-diodes/