P38A Door mirror auto dip

Gents,

I've been meaning to post about this for a while but never got around to it.
The mirror on the passenger side didn't work at all and the glass was ruined due to the liquid crystal leaking.
I've now replaced the glass (Non LC one but I can live without that)

Now, I've always noticed that when I select reverse gear, the drivers side mirror would tilt up to the sky and I assumed it was just wired up the wrong way. Now I've replaced the glass, the passenger side now works and that tilts down to the kerb when reverse is selected which I assume is correct.
The bit I don't understand is that when I use the button to adjust them, they both work the same way but the reverse gear sends them the opposite way.
Finally, the passenger side goes back when D is selected but the drivers side mostly stays looking at the moon and stars and needs to be put back to the correct position on the button.

Any ideas why this is happening?

P38 HSE 2000MY Thor

Thanks in advance.
Paul.

It's an issue with the mirrors themselves - there are potentiometers in the motors in the mirror unit. Over time these fail and give bad readings on the glass position back to the BECM - so when they move, and then go back to the original position, they go to the wrong place.

I had it on mine years ago, and tried to find just replacement potentiometer pieces (they're a ceramic disc built into the motor) as mine had cracked, but couldn't source them or the single motors anywhere. I was going to look into seeing if there are other memory mirror units from other vehicles which might be compatible as a replacement, but didn't get around to it! Possibly some Jaguar models of the same vintage might work as there were some with memory mirrors.

The easy way to stop it doing it is to disable the reverse dip - which is done by selecting reverse and then pressing and holding the 'Memory Store' button on the side of the seat - you should get a 'MIRROR DIP OFF' message on the dash. To re-enable it, press the button again when selecting reverse and you'll get 'MIRROR DIP ON' message, and they'll go back to moving to weird places...

Hope this helps..
Marty

martyuk said:

It's an issue with the mirrors themselves - there are potentiometers in the motors in the mirror unit. Over time these fail and give bad readings on the glass position back to the BECM - so when they move, and then go back to the original position, they go to the wrong place.

I had it on mine years ago, and tried to find just replacement potentiometer pieces (they're a ceramic disc built into the motor) as mine had cracked, but couldn't source them or the single motors anywhere. I was going to look into seeing if there are other memory mirror units from other vehicles which might be compatible as a replacement, but didn't get around to it! Possibly some Jaguar models of the same vintage might work as there were some with memory mirrors.

The easy way to stop it doing it is to disable the reverse dip - which is done by selecting reverse and then pressing and holding the 'Memory Store' button on the side of the seat - you should get a 'MIRROR DIP OFF' message on the dash. To re-enable it, press the button again when selecting reverse and you'll get 'MIRROR DIP ON' message, and they'll go back to moving to weird places...

Hope this helps..
Marty

Click to expand...

Thanks Marty, I guessed you might be the one with all the answers.
I still don't understand why the drivers mirror goes in the wrong direction though, surely the BECM will command down movement until it reaches its stored setpoint.
I could understand the pots meaning it doesn't go back to the right place but it's going in the wrong direction when you select reverse.
I don't really want to turn it off because it's quite nice to have on the passenger side.

I think it's more because of the voltage it returns... It will have a reference voltage (I'm guessing 5V) sent to it, and there's a feed back the BECM which is listed as an analogue input or something like that (I can't remember exactly - but when I looked it up in the BECM document I have, it said it was for mirror position). So my guess is that depending on the potentiometer reading, will return X voltage, and that's what it references. But if there's a break then it will return an incorrect reading, and that's what causes it to go the wrong direction etc.

The thing I haven't looked at is how it does the horizontal and vertical position on one wire - as there's only 2 inputs to the BECM - one for each mirror - rather than one for horizontal axis, and one for vertical. It's one of the mysteries I keep meaning to dig into, but I never seem to have the time!

I used to use it a lot on my '95 as it was handy when it dipped for reversing - could see the lines in car parks for example... but I must have been lucky as they both worked properly! That was back in NZ though, so the climate was a bit dryer over there - might have had something to do with it!

Cheers Marty, I'm guessing it doesn't need feedback for the horizontal movement as the auto dip is only on the vertical axis?

That was my thought too... but the actual position is programmable on vehicles with the memory mirrors/memory seats. The mirror position and a mirror dip position is stored in with the seat memory. So from that, I presume that it has to record horizontal and vertical position - given the fact that someone may set the mirror in both directions between 2 different seat positions..

In fact, I've just cracked out RAVE ETM to see if that indulges my curiosity, and it says that it DOES store both the horizontal and vertical positions.
The potentiometers for the H&V Axis (again from the ETM) feed straight back to the outstation - which to me says that memory data MAY transmitted via the serial link between the outstation and the BECM.

Which then does make me wonder what the other wire is actually for! The BECM document just says (for RH door): "Right Front Door Analog Output" and the description as "Voltage dependant on position of Mirror Potentiometers 0V-5V"

Still a mystery to me!