Can I Bypass BCM earthing of Rear Park/Stop Lights in VZ

[Oranger]

Just wondering if anyone has bypassed the X4-24 BCM earthing of the rear stop/park lights? Everything else that earths via the Black/Blue earth wire functions correctly. I’m only having issues with the stop/park lights that earth via the Brown/White wire back to the X4-24 pin on the BCM. They simply don’t work. It’s not throwing any dash light faults currently wired as it is. Just wondering if doing so will result in fuse and stop/tail light failures on the dash. Asking before I go and cut the Brown/White wire and connect to ground lol. Thanks.

Adam.

 

[lout]

try a temporary earth with a fuse for a few days

 

[07GTS]

cruise may not work as it uses the tail lights for earth and also when brakes used it cuts the earth to the cruise to stop it

 

[Fu Manchu]

It uses a switched earth by BCM to turn lights on and off. By changing the earth source, you will interfere with the lights not switching on or off automatically. That feature will be gone. It may also interfere with other circuit operation.

I would be going back through the body harness to find the break or bad connection.

 

[Fu Manchu]

Make use of these cheap bits of kit to find the break. Puts a signal through the wire and when the audible signal falters or weakens as you put the receiver along the harness, there’s your break.

 

[Fu Manchu]

It might not be working because it’s a switched earth via the BCM. It’s not a direct Earth to battery. When it’s off, it’s not going to work when you test it with a multimeter or test light (depending on your method).

Most likely place of the fault is the bulb itself.

 

[Oranger]

Thanks for the input there guys. Well I have spent the day playing around with it. The Calais has the brown/white wire which as you say, earths through the BCM after it measures the resistance. The donor vehicle had the same black/blue earth wires as the turn signal and reverse lights for its stop/park lights. Plus it has no wire connected to the X4-24 pin location at the BCM. I also have power at the rear light socket coming via the brown/white wire. So no earth. After backing out the brown/white wire from X4-24, it still has power. Looking at wiring diagrams, I don’t see any other connection for that wire and possible power source. It’s an earth wire for the stop/park lights, eye level brake light and the number plate lights. Now that I’m thinking about it, I have not pulled out the number plate globes and inspected them.

Im testing for power using a test light with the park lights switched on only.

What I did do though was back out the brown/white wire from the supply connector to the RH stop/park light, exchange that for the black/blue wire from the same position in the donor car, fit an eyelet terminal to that and screw it to the GP13 earth. I then have a park light that works and also a brake light that works off the pedal switch. Previously the brake light switch operated the park lights.

My plan was to back out the brown/white wire from X4-24 and turn that into a physical earth for that rear circuit. But discovered it still has power at that brown/white terminal at the rear light connector and the pin that I backed out.

Back to the drawing board and studying of wiring diagrams lol.

 

[Fu Manchu]

Thanks for the input there guys. Well I have spent the day playing around with it. The Calais has the brown/white wire which as you say, earths through the BCM after it measures the resistance. The donor vehicle had the same black/blue earth wires as the turn signal and reverse lights for its stop/park lights. Plus it has no wire connected to the X4-24 pin location at the BCM. I also have power at the rear light socket coming via the brown/white wire. So no earth. After backing out the brown/white wire from X4-24, it still has power. Looking at wiring diagrams, I don’t see any other connection for that wire and possible power source. It’s an earth wire for the stop/park lights, eye level brake light and the number plate lights. Now that I’m thinking about it, I have not pulled out the number plate globes and inspected them.

Im testing for power using a test light with the park lights switched on only.

What I did do though was back out the brown/white wire from the supply connector to the RH stop/park light, exchange that for the black/blue wire from the same position in the donor car, fit an eyelet terminal to that and screw it to the GP13 earth. I then have a park light that works and also a brake light that works off the pedal switch. Previously the brake light switch operated the park lights.

My plan was to back out the brown/white wire from X4-24 and turn that into a physical earth for that rear circuit. But discovered it still has power at that brown/white terminal at the rear light connector and the pin that I backed out.

Back to the drawing board and studying of wiring diagrams lol.

Have you got the factory wiring diagrams to go off?

 

[Oranger]

Yeah, lout gave me a link at one point which has been fantastic. Haynes manual was very limited in BCM info but at least it had the wiring diagrams. Lout gave me the VTVZ GHM link which has endless information.

 

[Oranger]

It uses a switched earth by BCM to turn lights on and off. By changing the earth source, you will interfere with the lights not switching on or off automatically. That feature will be gone. It may also interfere with other circuit operation.

I would be going back through the body harness to find the break or bad connection.

Just thought I would follow up on the question I originally asked and some of the responses. There actually wasn’t a break or bad connection as suggested. The Calais earthed via the brown/white wire back through the BCM so it can measure the voltage drop and then give you wonderful warnings on the dash about tail lights or stop light globes being blown. The Lumina earthed via the black/blue wire which went straight to the body of the vehicle. So all that was required was backing out the brown/white wire from X4-24 and turning that into an body earth wire which I attached to the body via the 10mm dash mounting bolt near the BCM. Problem solved.

The cruise control works fine too as it operates off its own switch at the brake pedal. I don’t think it uses the same earth as the tail lights, brake lights and number plate lights. That earth wire was the only thing I changed to match the original car the engine and trans came out of.

Not trying to step on any toes, just wanted to provide some follow up info so others aren’t put off trying this kind of thing. You don’t need to change the loom or anything full on like that. It was simply the engine and ECU, trans and TCM, BCM, PIM and key fob from the donor car. The only modification was that X4-24 wire which needed to be changed to a body earth and not a BCM earth. The Lumina had no wire fitted to the connector in that position.

The only thing I haven’t corrected as yet, is the ESP failure warning. Due to the fact it doesn’t run the original 5 speed with ESP or a 4 speed with ESP. My original donor car also didn’t have traction control. So after start up I get the ESP failure warning, I hit the mode button and this changes to ESP off and Trac off on the dash. It has no effect on the drivability of the car and I don’t even notice it. I would however like to look at changing the ABS module over from the donor car to see if this corrects the issue. Only difference between the two units is one has three lines out of it (donor car) and the other four lines out (Calais). But to be honest it might not be worth the hassle as there isn’t any real effect on the car besides the warning.