Reversing Light Switch

[Mensa]

Hi,

 

My C3 Pluriel (Sensomatic gearbox) reversing light and radar have stopped working. I have done the following tests:

 

With reversing bulb fitted, neither light nor radar work.

With new bulb the result is the same.

If I remove the rear reversing bulb, with gearbox in reverse, the radar works and there is 12v at light connector.

If I remove the rear reversing bulb, with gearbox in neutral, there is 4v at light connector.

If I remove the rear reversing bulb and remove the radar, with gearbox in reverse, there is 12v at light connector.

If I remove the rear reversing bulb and remove the radar, with gearbox in neutral, there is 11v at light connector.

 

I believe the gearbox sender cannot take the power of the bulb without shorting out, but can take the power of the radar on its own. If this is the case, I need to find the gearbox sender so I can test it.

 

Please could someone tell me EXACTLY where it is and what it looks like?

 

Many thanks,

 

Andy.

[stimulator]

The problem here may not lay with the sender/switch but in the BSI. On modern cars the switch is not connected directly to the bulb.

 

The switch sends a signal to the BSI, the BSI then does its computation and switches on the ouput stage to the required light.

 

Strange that the 12 V won't light the bulb evcen though its available, at the socket with the bulb missing. The output stage will usually go open circuit if too much current is drawn through it.

 

Are you sure that you do not have an earth fault at the return line of the light?

[Mensa]

The problem here may not lay with the sender/switch but in the BSI. On modern cars the switch is not connected directly to the bulb.

 

The switch sends a signal to the BSI, the BSI then does its computation and switches on the ouput stage to the required light.

 

Strange that the 12 V won't light the bulb evcen though its available, at the socket with the bulb missing. The output stage will usually go open circuit if too much current is drawn through it.

 

Are you sure that you do not have an earth fault at the return line of the light?

 

 

Many thanks for the quick reply.

I have attached a wire from the bulb to earth, but with the ignition on and the gearbox in reverse there is still no light or radar working. As soon as I remove the bulb the radar starts working, and the bulb holder has 12v through it. (I don't know what voltage is in the holder when the bulb is fitted, as there are no terminals visible.)

So the switch does work, although maybe when the bulb is fitted there is too much current for it.

Do you think it is a good idea to test the gearbox switch by shorting across its terminals - the reversing light and radar should be on permanently then. Providing I can find the switch, that is! Do you know where it is?

[stimulator]

Is the bulb you are using to test this the one that is normally in the holder OR is it another bulb?

 

the reason I ask is that the bulb itself may be the problem.

 

Also is it 21W or 5W,

 

If it's 21W it draws 1.75A try a 5W which will draw only 0.4A and see if that works, side ligh bulbs are 5W.

 

I'll look in my Haynes manual tonight to see if I can find the reverse switch on the gear box but your info really tells me that that is wroking OK.

[Mensa]

Is the bulb you are using to test this the one that is normally in the holder OR is it another bulb?

 

the reason I ask is that the bulb itself may be the problem.

 

Also is it 21W or 5W,

 

If it's 21W it draws 1.75A try a 5W which will draw only 0.4A and see if that works, side ligh bulbs are 5W.

 

I'll look in my Haynes manual tonight to see if I can find the reverse switch on the gear box but your info really tells me that that is wroking OK.

 

Hi,

 

That was really good advice. I was testing it with another new 21w bulb, unsuccessfully, but have now tried it using a 5w bulb and everything works. However, the reversing light is now very dim, as it should really be a 21w one.

 

This is a great short term repair, but do you have any idea of what is at the root of the problem?

 

Many thanks.

[paul.h]

This could still be a poor earth or other connection which is not letting enough power through. If you can find the earth connection for the light give it a clean. Also check the bulb holder and wiring connector to the light unit are making good contact and are not corroded. In the bulb holder check the middle contact is pushing up enough against the bulb.

 

Are the other lights on the same light unit bright or are these also affected - try the brake lights, indicators and fog lights which are also 21w bulbs. If it is an earth fault for that light unit they should also be showing the same problem. If these are all ok then also consider the reverse light fuse spade connectors (not the fusible bit which is obviously ok), could also be a poor relay if one is fitted - need to look in the car handbook to check.

[stimulator]

Well we seem to be getting somewhere now.

 

The answer to your question is that the reverse light switch is on top of the transmission case. However before you go and find that I'd disregard the switch as the problem as it is only acting as an input (trigger) to the BSI (built in systems interface)

 

In my earlier reply I explained how this works and my Haynes manual confirms this. On the C3 there are no direct connections between any switches and the lighst they appear to control.

 

the power to the switch is from the battery via Fuse MF7 20A to 80A switch. From the switch it goes to the Engine fuse box and out via F1 10A. This fuse is for the Diesel preheat, Air con and reverse lights. If all those work the others work the fuse is OK. The power is fed to the reverse swicth and when that is made, an input is sent to the BSI which is processed. The BSI switches on the output to feed BOTH of the reverse lights.

 

So does the other light work? If that is OK then the problem lays soley with the other one.

 

We now come to the earth path.

 

The radar msut be an add ON (is it?). If so it may have its own return path straight to the body. The rear light clusters have earth return points near the units. Look for a BROWN set of wires that go to single point on the body from the light fitting. Haynes says that this is for the left hand lights point E17 above the left hand wheel arch and for the right E10 above the right hand wheel arch.

 

For a further test try this. Run a piece of cable directly from the bulb holder to a clean metal point on the body and see what you have then. If you have a ohm metetr put one lead on the metal case of the holder and the other on the body it should measure near 0. If it's way above that fig then you ahve a bad earth.

[Mensa]

Well we seem to be getting somewhere now.

 

The answer to your question is that the reverse light switch is on top of the transmission case. However before you go and find that I'd disregard the switch as the problem as it is only acting as an input (trigger) to the BSI (built in systems interface)

 

In my earlier reply I explained how this works and my Haynes manual confirms this. On the C3 there are no direct connections between any switches and the lighst they appear to control.

 

the power to the switch is from the battery via Fuse MF7 20A to 80A switch. From the switch it goes to the Engine fuse box and out via F1 10A. This fuse is for the Diesel preheat, Air con and reverse lights. If all those work the others work the fuse is OK. The power is fed to the reverse swicth and when that is made, an input is sent to the BSI which is processed. The BSI switches on the output to feed BOTH of the reverse lights.

 

So does the other light work? If that is OK then the problem lays soley with the other one.

 

We now come to the earth path.

 

The radar msut be an add ON (is it?). If so it may have its own return path straight to the body. The rear light clusters have earth return points near the units. Look for a BROWN set of wires that go to single point on the body from the light fitting. Haynes says that this is for the left hand lights point E17 above the left hand wheel arch and for the right E10 above the right hand wheel arch.

 

For a further test try this. Run a piece of cable directly from the bulb holder to a clean metal point on the body and see what you have then. If you have a ohm metetr put one lead on the metal case of the holder and the other on the body it should measure near 0. If it's way above that fig then you ahve a bad earth.

 

Hi,

 

Thanks again for the reply. The radar was something I added about 3 years ago, and it has been working fine up until now.

 

I tested the earth with an ohm meter as you suggest, and it registered ZERO.

 

The reversing light earth wires on my car are green/yellow. They bolt to the body just inside the boot, by the inner wheel arch. They are tight and working properly.

 

There is only one reversing light on my C3 Pluriel. Fuse F1 only feeds this light, nothing else. I have removed it and it is fine.

 

However, the original problem still persists. With a bulb in the reversing light, neither light nor radar works. With bulb removed the radar works. With a 5W bulb instead of the 21W, both radar and light work, but the light is so dim that it cannot illuminate at all really - and then sometimes it doesn't light at all and the radar fails too.

 

The bulb holder terminals are perfect too, so I'm still struggling to understand what can be wrong.

[paul.h]

May be worth checking the new 21w bulb is ok by fitting it in one of the indicator positions.

Have you tried with the 21w bulb in place and the radar disconnected to see if the radar may have the problem.

Some of the odd voltages reminded me of when I fit the towbar to our C5, the voltages at a trailer connector were odd and totally confusing.