Volkswagen Passat Forum banner
1 - 6 of 6 Posts

·
Registered
Joined
·
467 Posts
Discussion Starter · #1 · (Edited)
I'm in the process of diagnosing back up / reverse lights that don't work. I believe at least one problem is the MFS * / neutral safety switch so I temporarily swapped it out for an old one that I kept for emergencies and now the backup lights are working and a new MFS is on the way.

I saw an unusual voltage measurement in my testing regardless of MFS. Would anyone be willing to share the voltage measurement they get with ignition on and engine off, car in park, on fuse 231 receptacle (ie driver's side dash fuse 31) compared to their battery voltage? Across the receptacle I get voltage 1.4v lower than battery voltage. From the left side to ground I get battery voltage. From the right side to ground I get 1.4v which is what concerns me.

Fuse 231 powers the N110 solenoid (shift selector lock) through the TCM which completes a circuit to ground when the solenoid is supposed to be on, and the backup lights through the MFS which completes a circuit to ground when the shifter is at R. I've checked the grounds in the transmission wiring harness and the ground terminal itself (lower a pillar) and there's no resistance to battery negative. In addition the hot wires to the transmission wiring harness are battery voltage. Also the N110 with ignition off is at expected resistance (22 ohms). Therefore, as far as I can tell, the 1.4v is coming from the TCM on what should be going to ground? I don't know if I have a bad TCM or that's normal so if anyone else can contribute their measurements I'd like to compare.


* Note on automatics the back up light switch is integrated in the MFS, unlike manuals which have a separate switch. You can have an otherwise working MFS (no fault in computer) with a bad back up light switch. For reference purposes in case anyone else has this issue you can test the reverse lights with ignition on engine off by putting the shifter at R. If the lights don't come on then check the voltage of the blue/red wire to ground wire on both tail light connectors. If there's battery voltage then check bulbs. If there's not battery voltage then disconnect the brown connector in the lower right a pillar and test for resistance across the two pins in the pic below:

Motor vehicle Electrical wiring Circuit component Computer hardware Cable


If you get open or high resistance then next you'd have to check where those pins go to, which is the MFS connector under the car, pins 7 and 8.
ox_oo
oxooo
the x's are the male pins to check, the o's are the other male pins, the underscore is where there is no pin. Unfortunately I didn't take a picture.
 

·
Registered
Joined
·
467 Posts
Discussion Starter · #2 ·
I saw an unusual voltage measurement in my testing regardless of MFS. Would anyone be willing to share the voltage measurement they get with ignition on and engine off, car in park, on fuse 231 receptacle (ie driver's side dash fuse 31) compared to their battery voltage? Across the receptacle I get voltage 1.4v lower than battery voltage. From the left side to ground I get battery voltage. From the right side to ground I get 1.4v which is what concerns me.
Anyone?
 

·
Registered
Joined
·
5,786 Posts
You didn't mention the year, but early B5s with vacuum cruise control tie the brake light switch to the cruise control module, which is how the module disengages the throttle when the brake lights come on. The module is above the glove box. If you disconnect it, that stray voltage might disappear. A later B5 may make similar use of the brake light circuit.
 

·
Registered
Joined
·
10,337 Posts
You didn't mention the year, but early B5s with vacuum cruise control tie the brake light switch to the cruise control module, which is how the module disengages the throttle when the brake lights come on. The module is above the glove box. If you disconnect it, that stray voltage might disappear. A later B5 may make similar use of the brake light circuit.
It is a 2003 Passat Wagon 1.8T AWM Auto.
The OPs car would have contacts in the brake pedal switch for the cruise control.

The OP doesn't have a problem with his cruise control, his problem is with the reversing lights.
 
1 - 6 of 6 Posts
Top