MB-Dude

Strange problem developed on my 2005 S55. With the headlight switch ON, whenever the engine hood is closed, the LH headlight turns off (but RH headlight remains on). Open the hood and the LH headlight turns on. This is very predictable and very repeatable. I discovered the ‘Hood Open’ switch (inside the RH hood latch mechanism) is what activates the problem. With the hood open, I can depress the switch and make the LH headlight turn on and off at will; just the LH headlight. The RH headlight remains on under both conditions.

Thinking the switch is bad, I disconnected the switch. This caused the LH headlight to remain off. If I jumper the switch with a short piece of wire, the LH headlight turns on but I get a red ‘Hood Open’ warning whenever the car is in motion. Obviously, there is a cross-communication somewhere between the hood open switch and the headlight activation circuitry.

Other symptoms… When the hood switch is properly in place and the hood is closed (normal situation), I get 2 additional errors on the dash display – ‘Display Defective’ and ‘Tire Pres. Monitor Not Operational’. These errors ONLY occur when the system detects the hood properly closed; same condition to cause the LH headlight to turn off. These errors are very repeatable. If the hood switch is jumpered, I do not get these errors, but get the red ‘Hood Open’ warning.

The hood open switch is not the problem as jumpering it out does not fix the errors. I have replaced the battery with a genuine MB part. That made no difference. I have disconnected the battery overnight. That made no difference. And the dash display was replaced under recall warranty long before this problem began.

What I’m hoping to find is someone who can provide a wiring diagram of the harness so that I can trace the harness from the hood open switch to the CAN Bus. I suspect some corrosion, grit, stuff, dead bug, something that is providing a cross-communication path for the hood open switch signal. I believe the problem simple, once I know where to trace. Any help in wiring diagrams or moral support is greatly appreciated!

Cheers,
Jeff

drewk88

Are you able to get the car connected to a star diagnostic system?
If not a simple problem as you suspect, it could be something like a logic circuitry malfunction inside one of the SAM's. If so, the star diagnostic may help to narrow down, if not pinpoint the problem.

MB-Dude

Thanks Drew for the response. I do not have a SDS and had been resisting getting one due to what seems like 50-50 odds of getting one to work/connect properly. Nevertheless, I'm starting another thread specific to that question.

To date, I tracked down a copy of StarFinder which gave me some of the information I need about wiring and interconnectivity. Short of an SDS, I pulled the 2 front SAM's, cleaned with contact cleaner, inspected connectors, etc. No change. I suspect a problem in one of the SAM's where it's sending an erroneous signal down the CAN Bus, since what seems like unrelated things are happening. Only other possibility might be the controller for the factory Anti-Theft Alarm (ATA). But I need to find that controller on the car. (The hood switch is actually a part of the ATA logic - open the hood and the alarm goes off, etc.)

More to come as I continue to t/s this perplexing problem. Any and all information would be greatly appreciated – even moral support!

Cheers,
Jeff

drewk88

Jeff,

Yes, I had lots of problems getting my Star Daignostic system to work at all, due to extremely poor instructions and support from the seller in China. I do have the WIS and DAS working, but no Xentry or EPC.

I have my Star Diag in SoCal and I would be willing to let others come to my location, connect their car and give them the diagnosis. It would be nice to have on this site, a list of other users who have a Star Diagnostic system, by city or region, who'd be willing to do the same, either gratis or for a nominal fee.

The only other thing I've read is that a bad battery can cause strange behavior in various electrical systems. But also disconnecting the battery requires lots of things to be reset or re-initialized. I'd check the connections at the battery terminals for tightness and cleanliness, use a digital voltmeter to check battery voltage, and maybe give it a top-up charge without disconnecting it.

Regards,

Drew

MB-Dude

Yup... my first inclination was a marginal battery as the original factory battery was in the car. After 7 years, it was probably getting weak. So I replaced it with an offcial MB unit from the dealer. No change. If there is any other resets other than the windows up/down, sunroof through the motions, and steering wheel fully left then right, I'd love to hear what they are. I did try disconnecting the battery and leaving it overnight; no change. Strange problem.
Cheers,
Jeff

MB-Dude

Bad right front SAM. Picked up a Star DAS unit but it didn't provide any absolutes. StarFinder however, strongly suggested the SAM as the culprit. Replaced it this afternoon and all problems cleared up. Can't explain how the SAM was bad, but replacing the module was all I did.

cheers,
Jeff