Voltage drop in Reverse

[Bowlerwell]

I'm seeing about 0.7v drop in battery voltage on the gauge when I engage Reverse. Is this normal?
The voltage drop when I put on my headlights, or Rear Screen Heater is much smaller.
Is there something else that will take current in reverse apart from the lights?

Does this indicate a problem with the reverse state, or the power supply (alternator, battery, regulator)?

[Ian Simmonds]

Yarn time, sorry... But I took my rather modified old Coupe GT to Germany many moons ago and after a good ol' thrash down the Autobahns, she'd blow the fuse every time I selected reverse. Turned out the wiring for the switch had melted to the gearbox bell-housing with the heat... Might just be something similar with the wiring going on.

[Bowlerwell]

Thanks Ian.
I'll take a look at the schamatics...
Just want to know if this drop is normal...

[mcandmar]

Now you mention it both my cars do that too..

[UrS4boy]

1. Check your reverse switch. CBD and CBL 01E's are as shown below.

2. Do you have an autodipping passenger (curb/kerb) side mirror? Have you got the adjustment button for it pushed in? If so, does the mirror dip? (If not, you found your problem). If the button is pushed in, push the other button gently until the passenger side button pops up and then check if the current draw is the same in reverse (if it is less, it is the mirror).

[Dan]

autodipping passenger (curb/kerb) side mirror?

Holy crap, such things exist?!

Where do I get one? That actually sounds damn useful.

(sorry for the off-topic)

[S2central.net]

was just thinking the same... never new such a thing existed on older cars... every day a school day

[Nuvo]

If you are seeing a voltage drop like this I would expect to see a either a poor or dirty connection at the connector or a partial break in the wiring.
That's what I'd be looking to find at any rate.

[Bowlerwell]

Also didn't think such a thing existed...

But when I take out both reversing bulbs, the voltage drop is non existent. So it can't be the switch.
Perhaps dodgy wiring or a dodgy bulb, but not quite enough current to blow the fuse?

15A is an awful lot of current for only 2 bulbs isn't it?
7.5A per side is 97.5W at 13v!!
Maybe I should replace the bulbs and see what happens...

[Ian Simmonds]

Main beam reversing lamps!

Excellente!

[UrS4boy]

was just thinking the same... never new such a thing existed on older cars... every day a school day

AFAIK, all the C4 UrS4/S6s got the autodipping curb/kerb side mirror. 100% for the NAmerican cars. Not so sure about Europe and ROW.

When you have the mirror adjustment button for the curb/kerb side mirror pushed in AND you put the car in reverse, the mirror adjustment motors dip the mirror down so you can see how close you are to the curb/kerb when parallel parking. When you take the transmission out of reverse, the mirror returns to its previous (adjusted) position. If you don't have the button pushed in, the mirror does not move when you go into reverse.

[Mikes2]

reverse lamps are on fuse 12, which covers a load of other circuits which is why the fuse is 15A

[Bowlerwell]

AFAIK, all the C4 UrS4/S6s got the autodipping curb/kerb side mirror. 100% for the NAmerican cars. Not so sure about Europe and ROW.

When you have the mirror adjustment button for the curb/kerb side mirror pushed in AND you put the car in reverse, the mirror adjustment motors dip the mirror down so you can see how close you are to the curb/kerb when parallel parking. When you take the transmission out of reverse, the mirror returns to its previous (adjusted) position. If you don't have the button pushed in, the mirror does not move when you go into reverse.

Very interesting... Has anyone ever retrofitted this?
How does this work, is the dip/recover timed, or is there feedback from the mirror showing position?


@Mike, what else is on Fuse 12? I might just stick an ammeter on and measure the currnet.

[UrS4boy]

Very interesting... Has anyone ever retrofitted this?
How does this work, is the dip/recover timed, or is there feedback from the mirror showing position? .

It will dip until you take the trans out of reverse. There is some kind of memory because it returned to the previously adjusted position.

[s2driveruk]

thats a bloody good find..always wondered why cars (i've had at least) dont have a mirror dip function of reverse parking - more often than not i wind mine down if the kerb is big.

Oh, and my reverse gives the same kind of drop, i.e. noticeable when engaging reverse...