Turn signal or brake light not working - not the blub or fuse - LCM

Thought I'd post this headache, as I couldn't see this topic on an earlier sort. Also have a question on LCM compatibility between Gen 1 & 2 Mark VIII's.

When it's not the bulb (I swapped with a working side), not the socket (I measured 12V @ the socket), and not the fuse(s) it may be the Lighting Control Module (LCM). In my case, the voltage was supplied, but the LCM couldn't provide the amperage to kick on the light. I was able to swap out the left to right outputs from the LCM to validate that it was the issue.

Now for the question: Is the LCM the same for Generation 1 & 2 Mark VIII's?

Regards,
Laurent
97 LSC
97
94
93

RE: Turn signal or brake light not working - not the blub or fuse - LCM

Doesn't sound entirely consistent to me.

Do you have good contact at the bulb?

Check the voltage with the bulb in place. If there's 12 volts, it's a contact problem.

Which bulb is it? where and how were you measuring voltage? Did you try a new bulb at all? Do you have anything "aftermarket" wired into the lighting system?

RE: Turn signal or brake light not working - not the blub or fuse - LCM

Check to see if it's the BOO switch (BRAKE ON - OFF). It's a very comman failure with FoMoCo products.

Oh--located at top of brake pedal.

Bud Pytko
'97 LSC Silver Frost
'05 F250 SD CC PSD 4 X 4 AZ Beige
'94 Prowler 5th Wheel
http://squarebirds.org/images/BudPytko97LSC.jpg

RE: Turn signal or brake light not working - not the blub or fuse - LCM

Both the left brake & left rear turn are not working. I have no aftermarket stuff, and measured voltage with a DVM (and measurements were made at the Mulex connector & bulb socket).

Yes - I have contact at bulb, and I had a 12V at the contact...as strange as it seems. This is what made my head hurt for awhile, and why I posted this note. The car actually validated that a contact was made when the computer would say that a brake light was out (I had something depressing the brake) only when I removed the bulb - but never complained when a bulb was in. I also tested the bulbs for illimination (just to make sure), and swapped with the right side (both bulbs seemed fine, and lit up on the right side). Fun, eh?

I don't know how the LCM is designed, but my guess is that my 'bad' LCM has it's left rear brake & turn signal output blown, but the sensor circuit(s) are still working. If this is correct, I can see how a sensing circuit could provide the minimal current draw that's needed to qualify the the circuit isn't open (bulb in't burned out or removed), but not enough current to illuminate the bulb. I have not cut the wires in order to measure the current flow that would help validate my theory, although I have thought about it.

What bothers me is the fact that both left brake & turn circuits are not working. I would expect only one of these to go...unless there is something shared inside the LCM between the left rear turn & left brake...

Per the other posting - if a brake switch was an issue, I would think it would exhibit a problem on the right side too, and not just the left brake. The (left only) turn signal's misoperation is also not explained as far as I can tell.

I don't have schematics or a wiring diagram, so I'm fighting a little blind & willing to take any & all suggestions.

Unless we have more ideas posted, I guess I'll have more news to follow when I replace the LCM...

Many thanks!
Laurent

RE: Turn signal or brake light not working - not the blub or fuse - LCM

First of all, 1st Gen Marks don't have any kind of lighting control module; they have a conventional turn signal switch/flasher/BOO setup.

Your diagnosis seems to be correct. LR signal and brake are the same output from the SCIL (Steering Column/Ignition/Lighting) module. It would appear that the output driver internal in the module has gone south.

There's a small chance that you've got high resistance in the lamp circuit wiring, but normally, if that were the case, any resistance high enough to prevent the bulb from lighting should also be high enough to set the "lamp out" warning.

The wiring in question changes colour partway through the vehicle- LR output is LG/O at the SCIL connector, pin #24, but at connector C431, in the L side of the trunk, it becomes O/LB to the socket. The P/W wire at the lamp is marker.

You could jerry-rig a bulb into the LG/O wire at the SCIL as a test to verify the module's inability to provide current before you run out and get a new module.

I'm sure you know where the module is, but in case not, it's right side of the dash, behind the glove box.