New to the C5 platform. Chasing numerous neglected electrical items at once. Car is a 2000 Auto First up..

Reverse lights don't come on. Fuse 21 looks good in the foot-well, Fuse 2 under-hood is good, and I pulled the 38 relay- I checked for power on the middle pin where the set of 3 pins are on the relay, oddly that pin has power when the car is off in Park, but when the key turns to On wether P or R there is no longer power at that pin?... Not sure if I should just throw a shifter switch at it? What else should I be looking at? Sadly it didn't come with any FOB so I can even see if they blink with approach click on the FOB but the bulbs and sockets seem good. Where should I go from here?

 

Looks good, and IS good. You need to know that both fuse ARE good. Visual coupled with measured voltage on the load side.

Ok. You should be measuring voltage (or checking with a test light) at the relay socket, with the relay removed. The relay contact numbers are on the bottom of the relay as pictured below:

Check at the relay socket with the ignition off and car in park....you should have:
at 30 -0v
at 85 - 0v
at 86 - Battery voltage
at 87 - Battery voltage
at 87a - 0v

Check at the relay socket with the ignition ON (not started) and car in reverse....you should have:
at 30 -0v
at 85 - 0v, No Ground
at 86 - Battery voltage
at 87 - Battery voltage
at 87a - Battery voltage

You can get a quick win by jumpering the socket 87 to 30 and the reverse lights will come on provided no issues with wiring or bulbs.

 

Great reply I appreciate it. You're right I'll verify those fuses.

I'll re-test but Im pretty much certain I have battery voltage at 87a -Key off in Park, and then 0V Key on in Park or in Rev. Not sure how thats possible as it seems 87a should only have key on voltage? Im actually losing the voltage when key on no matter the gear.

 

This is where using a test light or some sort of load (turn signal bulb, etc) helps confirm if you are actually reading a voltage or ghost voltage from adjacent wiring.

 

Is that really possible with me pinned directly into that socket? Seems like it would be unlikely to get 12V directly on the probe in the socket if it wasn't actual no? I'll take a closer look and see what I can verify.

 

Ok partner. Good luck with your issue.

 

Ok so I went back and everything checks out as you described in your post AND when I jump the pins I do get the reverse lights to light. What are you thoughts? That mini relay? Reverse switch in console? A way to test that relay?

 

Ok, so that's good news. That means all the wiring is working to the lights, ground is good, etc.

Relay is possible. Try this. Install the relay, pull mini fuse #2 in the underhood electrical center, key to ON, and shifter to reverse. Are the reverse lights on?

 

I’ll give this a shot when I get out to the shop. Appreciate your help

 

OK, so I did that and the reverse lights do not light in that condition. What does that tell us?

 

If all the checks were good from post #2, the relay is no good.

 

I appreciate you helping me through those steps!!
I have some replacement front turn signals coming cause those were shot when i got the car too so ill check back if I get hung up lol

 

Well now I’m even more confused because all of those items checked out, I replaced the relay and still.. no Rev lights. The only time they turned on is when I jumped those two connections you listed in the post.

What prompts the relay to close? The shifter switch? I’m at a loss where to go from here.

 

This is why it is important to load test as I suggested previously. You did reinstall minifuse #2, correct?

 

I did yes. I mean I verified the fuses showed same voltage on both sides of them, and they had continuity when out. Is there a different process I need to follow for them?

is it possible the selector at the trans is causing a problem or was that effectively isolated when we jumped terminals under hood?

 

There are two modes of operation for the reverse lights:
1) Relay is not energized and lights backup lights are illumninated via the PNP switch (Minifuse #21 in IP Electrical Center)
2) Relay is energized by the BCM ...using key FOB to unlock, etc. (Minifuse #2 in Underhood electrical center)

In a previous post, you confirmed that jumpering 87 to 30 at the relay socket made the back up lights illuminate.
You also confirmed you check all the contacts as described in post#2, but if everything measured as you indicated, a relay would have fixed the issue.
I would revist all the voltage checks at the relay socket using a load i.e. turn signal bulb, etc.

You can further check the PNP switch by jumpering contact 87a and 30 at the relay socket, Key to "ON", shifter in reverse. The reverse lights should illuminate.

 

I understand, and I have done each of those so I'm not sure what I'm missing. Ok so here's a more deliberate detailed look at what we got. I went back and double checked the steps.

I DO still get Reverse lights by jumping 30 & 87...

I DO have power at 86 and 87 with key OFF (small mv reading only on the rest)

I DO have power at 86, 87 and 87a with key ON, However, the power I have on 87a differs at 11.6V where I have 11.87V on 86 and 87. ALSO I have the 11.6V on 87a with key ON regardless of Park or Reverse position on shifter. Should 87a feed any power while in Park?

In cases using test light, the light will illuminate on 86 and 87 but will NOT illuminate on 87a at 11.6V yet it will on 86/87 with 11.87?. Likewise, jumping 87a and 30 will NOT illuminate my Rev lights as 87 and 30 WILL do so.

Here's a complete breakdown.

Battery Power 12.16 (weak I realize)

Key OFF
30-7.8mv
85-32mv
87a-5.8mv
86-12.16V
87-12.16V

Key ON (P or Rev nets same results overall)
30-14.4mv
85-60mv
87a-11.6V
86-11.87V
87-11.87V

Obviously the stand out to me is 87a and 30 will NOT get me illumination but 87 and 30 will. I also didn't know if 87a should be showing feed voltage while in Park. The test light illuminating on 86/87 with 11.87V but failing to light at all on 87a with 11.6V seem normal? What does all this say to you?

 

Ok, I'll take one more shot at this. You are reading voltage at a fusebox. Given the close proximity of all the wires together in the fusebox, you can read ghost voltages .....like low mV readings instead of 0v. So moving forward, just use the test light to check everything.

The test light illuminating on 86 and 87 makes sense, this confirms Minifuse #2 is good, and you can make the back up lights illuminate by jumpering 87 to 30.

The test light not illuminating on 87a is important....but....did you have the key "ON" and shifter in reverse when you checked this?

 

Also be aware in order to test the back up lights to illuminate when using the FOB to unlock the car, you must have it enabled in the DIC. This is in your owners manual.

 

Yep,

Test light not illuminated included Key on.. and Key on in Rev. in both of those cases the 87a went from no appreciable Voltage to having the 11.X.. but that 11.X didn’t light the test light no.

I probably probed them 15 times to make sure I was accurate

should 87a be having a signal in Park with the key on or only in Rev? I though that would be the signal to engage the relay and should only happen in Rev but it shows the same wether in Rev or Park with key on.