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This is very long and some of you will not like it!, but it was a fun project.
First let’s make sure it’s your HCM that’s bad.
• Check Fuses 3 and 4 in under hood fuse box
• Check Fuse 6 in fuse box under passenger side floorboard
• Raise passenger side headlight
• Remove 3 Phillips head screws from headlight bezel and remove bezel
• Remove 4 Torx screws from headlight lid (will have lower light some for rear screws)
• Remove nuts from both pivot screws (take care not to drop nuts)
• Support light fixture and remove the pivot screws. Gently lower the fixture out of the way.
• Remove nut from motor drive shaft and remove control arm from shaft. This will get the fixture further out of the way. You can disconnect the bulbs and completely remove the fixture.
• The HCM is mounted on the fender just behind the headlight.
• Remove the two connectors from the HCM
• With headlights off, Use a voltmeter to verify in 5 pin connector
• Neg lead of voltmeter to Pin D (Black Wire) Pos lead to Pin A (White wire) Reading should be zero volts
• Neg lead to Pin D Pos lead to Pin B (Orange wire) Reading should be 12volts
• Neg lead to Pin D Pos lead to Pin C (Dark Green wire) Reading should be 12 volts
• Neg lead to Pin D Pos lead to Pin E (Orange wire) Reading should be 12 volts
• Switch headlights on.
• Neg lead to Pin D Pos lead to Pin A Reading should be 12volts
• Neg lead to Pin D Pos lead to Pin C Reading should be zero volts
• If any of the above tests fail HCM is not the problem. It will probably be fuses, wiring or switch.
• If all of those tests are good, plug 5 pin connector back into HCM
• Connect a test light to HCM 4 pin output Pin A and Pin B
• Turn headlights off and on. Test light should come on momentarily each time you turn off or on.
• Connect the test light to HCM 4 pin output Pin C and Pin D
• Turn headlights off and on Test light should come on momentarily each time you turn off or on.
• If either one of last two tests fail HCM is bad.
• If both of those tests are good problem is either wiring or the motors.
If the HCM is bad, you can try to fix it. Remove the HCM from fender. Look at the back of the module. If the back is loose, water probably got into the circuit board. The back is just stuck on with some tar like stuff. You can easily pry it off with a flat blade screwdriver. I tried re-soldering some of the pads and IT WORKED for about 2 days.
You can get a used one off Ebay. You might even find one new old stock from a dealer.
I saw that one of the popular Corvette catalogs advertises new HCM for about $600.
If you are crazy, like me, you can build one!
Here’s the wiring diagram of the Headlight Door Circuit.
The motors run one way when the lights are turned on. They run the other way when lights are turned off. The direction is changed by switching the polarity of the wires. There is no switch that determines when light is fully up or fully down. The HCM has a timer circuit that shuts the motors off after a split second. HCM determines polarity based on signals on Pin A or Pin C of 5 pin connector.
So I need a Double Pole Double Throw switch to change polarity to the motors, a relay to sense on/off signals and a timer to shut the motors off. I know 555 IC is a timer circuit so that’s where I started my search. Found this little circuit with adjustable timer and relay.
Four of them wired like this.
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I didn’t even have to change the delay time. Factory default was perfect. There is a bit of difference from the way original HCM works. With original fuse 3 and 4 circuits control the motors separately. Home made HCM fuse 3 and 4 control Up or Down functions of both motors at once. I replaced the 10 amp fuses with 20 amp.
Here is the final product which I relocated between battery and fuse box.
It was a fun project and works perfectly. Total cost about $20. I'm sure some of you with more electronic building skill can make one that looks much more professional.
I am a firm believer in "where there's a will theres a way". Great job. Now I am waiting for the thread where someone conquers the EBCM. Mike
Mike48750, try absfixer.com . If your C5 is an '01 to '04 this company can fix it and I can speak from experience, they do a great job and are very fast. Pricing is very fair. I was lucky enough to live close to them and they fixed it while I had lunch.
What's the factory setting for the turn-off delay?
There was no statement of the factory setting. I'm guessing it's about a half second. I can hear the relays release immediately after the lights open or close.
There was no documentation included. If there had been I'm certain it would have been in Chinese. I figured out how they worked by experiment.
Last edited by UM Rebel; Mar 12, 2017 at 07:12 PM.
There was no statement of the factory setting. I'm guessing it's about a half second. I can hear the relays release immediately after the lights open or close.
There was no documentation included. If there had been I'm certain it would have been in Chinese. I figured out how they worked by experiment.
Good work!
The delay between the headlight motors turning on, when the "headlights on" or "headlights off" command is given, and the motors stopping later on, has to be enough to allow completely opening or closing the doors. It was a good thing the factory setting gave you just what was needed. I was just wondering if you had a time value for the turn-off delay setting
If you were more adventurous with an Arduino, you could set up something that closely resembles the stock control module. You would need to create an ammeter that's usable with the Arduino to detect the surge of current when the motor hits the stops, and have the Arduino shut off power to the motors when it does detect the surge.
As for me, I want to figure out how to make the headlights go down when the lights are turned off without going all the way to off.
As for me, I want to figure out how to make the headlights go down when the lights are turned off without going all the way to off.
I haven't thought it all the way through but how about tap DRL into OFF signal to HCM (pin C of hcm 5 pin connector)? Would need a diode between so that either DRL or Off would close doors.
I haven't thought it all the way through but how about tap DRL into OFF signal to HCM (pin C of hcm 5 pin connector)? Would need a diode between so that either DRL or Off would close doors.
I need to study the schematics more to see if that would work. I'm thinking a relay activated by the low beams may be necessary. It's low on my priority list, first is my FX-R 3.0 ACA retrofit project. Next is switchback sequential LED halo taillight turn signals.
Well, for anyone that wants to try the relay mod, it doesn't work. The white wire needs to be grounded for the headlights to close, so they just stay open when the relay is put in.
Homemade Headlight Control Module
