Who Needs Fuses?

One recurring item on the Vette to-do list has been the headlight motors. Last spring I discovered how the delrin bushings in the drive mechanism tend to turn to dust over time, and replaced them with some chunks of 3/8″ rubber fuel line. They worked wonderfully until somewhere around the end of daylight savings when I drove the beast to work and needed the lights on the way home. Ugh!

After removing the left side headlight bucket the problem became quickly evident. In my haste to put the motor back on the frame I’d incorrectly routed the motor wires in a path where the hood support wore into the insulation. I immediately dug into all the fuse blocks to find both 30 amp maxi fuses intact. Ut-oh! So one boring Saturday I yank the passenger side bucket back out and find the motor windings looking and smelling like they’ve gone bye-bye. So $142 later I had two new motors on my workbench from RockAuto.

A week later on the hottest June day I can recall I get around to installing the new motors. I’m thrilled to find the manufacturer went to the trouble of using the correct weatherpack connectors so the harness would plug right into the factory connector. Unfortunately they were cheap with the length of wire, and I ended up having to “cut corners” to get them to work. Idiots.

Long story, getting longer. You guessed it, still no motor control. I think to myself there must be some control board that manages these things somewhere. A few google searches and I at least know what it looks like. Again, I waited for the hottest part of Father’s day morning to climb into the front right wheel well with a 10mm ratchet to remove it. I was not about to spring $150 for a replacement unless I absolutely had to, so I had mixed emotions when I didn’t immediately find scorched circuitry after prying the shell open. I do however take solace in the fact that there are no SMC components in this thing, and the circuit board is single-sided technology from the late 70s.

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After a little cleanup I noticed the ratings on the side of the Omron MY4-02-GM relay – “CONTACT: 5A 28VDC”. WTF? Immediately I recall pulling the fuses out of the fuse block and wondering why they had 30 amp fuses for something so small. I even vaguely recall seeing the color coding on the legend sticker indicating these were the original fuse ratings (and most likely the original fuses). In fact, after gaining a fair understanding of this portion of the electrical system I can’t see what purpose these fuses actually serve. This is one of those engineering blunders that generates more revenue for Delphi, because after examining the controller board I also notice there are soldered-in fusible links – see “F2” silk screened on board to the right of the 5 pin connector below. You may also notice the “fog of war” on the inside of the relay cover. As Mr. Franchini used to say in electronics engineering class, “you done went and let the magic smoke out”!

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Whatever these fusible links are rated at, it’s too much. I deduced this when I found the part of the circuit that burned, aka “fused” were the relay contact interconnects. What kind of moron does something like this? The only way the 30 amp fuses would ever protect anything would have been in the event the 5 feet of wiring between the fuse block and the headlight motor controller were shorted. Every other electrical short on the output side of the controller ends up eating the relay. I can see where GM thought they could get away with this – most relays are current rated extremely conservatively because the make and break surge (switching) is usually what causes the contacts to arc and fail. With the mosfets doing all the switching the engineers assumed they could get away with 300-400% of rated current. Unfortunately, the internals of this relay aren’t that beefy, and the 22 – 24 gauge interconnect wire like the ones used in this specimen will “go poof!” at about 8-12 amps.

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A little surgery on the relay to replace the interconnects was all I needed. I also spent some time reverse engineering the circuit board as I initially feared the n-channel mosfets that switch power to the motors were going to need replacement also. Thankfully they’re rated at well over 5 amps.

If you undertake a similar repair on your headlight motor control board, I would highly suggest “potting” the board in some clear acrylic or laquer when you’re done to prevent corrosion in the event the plastic case ever loses its seal. I would also suggest putting 20 amp maxis, or even better 10 amp auto-resetting breakers in the headlight motor sockets. If I can find a nice fuse holder I’m also going to splice an inline fuse into the line that’s powered when the headlight switch is in the off position. Interesting tidbit – this pin, the one that powers the motors when closing draws power from yet another circuit. When I finally took the time to hook a meter to this lead and pull fuses to trace it back, I was not terribly surprised to find the “down” power for the controller is provided by the “exterior lighting” circuit that is protected by a 60 amp maxi. The end result is that you have a system where you can blow one of three fuses (which probably never will because they’re so obscenely over-sized) and either have a right or left headlight that won’t go up, or both that won’t go down. Ingenious (sarcasm intended).

For the life of me I can’t understand how something this antiquated made it into a vehicle built in 1994. Well, that’s not absolutely true. The words “greedy idiots” comes to mind.

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