Frieghtliner xc chassis Brake Light Problem

Wow. I didn't realize that. THANKS for the help. I will definitely follow up with Freightliner.

The Freightliner XC Chassis is subject of a recall of the Power Distribution Module. One of the indications is problems like you are talking about. Not sure if it covers your year model but work pursuing. I bought my rig used in October and it had three outstanding recalls. Good luck.

thanks for the input....the mission continues.

You have a very good resource at the Freightliner Customer Support Center.

CUSTOMER SUPPORT CENTER
1-800-FTL-HELP (1-800-385-4357)

Have your VIN number handy when you call. Ask for their help and also ask for a wiring diagram for your chassis. This is a free service for owners of Freightliner chassis.

Fred

Don't assume a Freightliner shop has an knowledge about your RV, especially electrical. They should be familiar with OTR trucks but not necessarily a RV which has various modifications including chassis. Try another shop or mobile tech. Get wiring diagrams and equipment location information from your RV dealer or mfg. Talk to the shop mgr and make sure he understands your problem and conviences you they have someone that can locate the problem. Tell him what shop you went to that failed to find the problem and what they did.

BTW Replacing pressure switches sounds like parts swapping without any knowledge of the problem which is a common and sometimes expensive technique. Are you sure those switches were any good? What is the voltage on each side before during and after the brake petal is pushed? Did they leave the new switches in place since they didn't fix the probem?

Are the brake lights the correct ones (oem spec)? Do the lights go off when you pull the brake light fuse - could be a short somewhere? Ditto when you disconnect either pressure switch wire? Also wondering why there would be 2 pressure switches? What harness did you disconnect, location, any other wires? Bad brake petal switch is possible, have you disconnected it's wiring only? Bad ground is a possibility.

Anothebr possibility is a blown fuse that supplies power to whatever controls the lights, relays or electronics.

What is the voltage at the chassis battery terminals? Ditto lights and any where else you can measure.

When looking for this type of problem ALWAYS connect the voltmeter to a good clean frame ground as a open or partially open ground wire can be misleading. You want to measure 12V+ (close to the battery voltage) on the positive wire and 0V on the negative. Measure a common + and - wire to the lights or each and every one. A DC clamp on ammeter to measure amps can be helpful and every DIY RVer should have one anyway.

Sounds like a bad ground somewhere. I'd start by checking the ground at the light itself. If that doesn't do it I'd check every other ground in the system, including the taillights.

no change in brightness, any idea what/where to look for relay?

Look real close at the taillights. Any change in brightness brake on/off? Both lights same brightness?
I suspect a bad ground causing a backfeed, which engages whatever they use for relay nowadays.