A few weeks ago I rebuilt my dimming rear view mirror, using a new glass from MirrorJohn. Works a treat, BUT afterwards the auto lights function has failed. With switch in auto position the lights are always on, no matter how bright a day it is.

I've been searching online and found info on how it works, sensor in rear of mirror (faces forward), signal from it goes to the lighting stalk switch and then to a control unit presumably behind the dash. Info has it that the input to the control unit is high, 12V or similar and the sensing circuit in the mirror simply pulls the signal down to ground.

So it seems to me the easy way to prove if it's the mirror is simply to unplug it. Sure enough with the cable disconnected the lights go out. If it hadn't it would point to either a wiring type fault or a dodgy control unit.
If I'm missing something please holler!

Now, I know the light sensor on the rear of the mirror is working, it's also used to control the mirror dimming function, and that works perfectly, so my reasoning is that something is toast on the pcb inside the mirror.

So my question is,
A) does anyone happen to have a circuit diagram of said pcb to enable fault finding?
B) if not does anyone have a scrap mirror lying around that I could scrounge the pcb from? It shouldn't be too difficult to trace the circuit out and then suss out how it works (my background is 40 years in the electronics industry). I don't want to dismantle my mirror again until I have an understanding of how it all works, I could be Jagless for a few weeks

Once I have a circuit drawn out I will put it on here for all to use etc.

Terry

 

My headlights wouldn't turn on at dark sometimes. I found out quite by accident, that by moving the mirror at it's pivot, the lights came on. I went to a wreckers and bought another mirror. Problem fixed. Threw the old mirror away!

 

Thanks, thrre's is that option. Having spent 100 quid on a replacement glass for it though I'm a bit reluctant to bin. It should be repairable, only time will tell!

Terry

 

Terry, grounding the yellow blue wire in the connector up by the mirror will turn auto headlights on. From your description this wire is grounded from the mirror = always on. Ironically I always want mine always on. I've got a Titanium gray paint which blends in way too well with asphalt. Without the lights someone is going to miss me and pull out some day.

John

 

John

Thanks, thats really usefull, I'll confirm grounding that wire does so, I'm pretty sure already but it'll be good to be certain. Must find a junk mirror though, I want to work this one out if possible. Don't like throwing things away

 

Terry, Did you find the circuitry?
Thanks,
Bill

 

Bill

Sadly no, I haven't managed to find a scrap unit, or just a board.

Terry

 

Hi Terry. I missed original posts from 4months ago, then felt issue either resolved or abandoned. Frankly, with no car details in your sig, I moved on. Given your electronics background, I am surprised at some of your assumptions and explanations. Perhaps I err in telling grandmother how to . . . but others will be following, so I have 2 lines of advice.

Firstly, for reasons of thermal efficiency in semi-conductor switches (digital controls, modules etc), that switching is now arranged not in the power feeds which are connected to either B+ or IGN+ via fuse . . . but in the ground seeking return to the battery. Sensors are either single wire devices if the device can be tied to ground internally, or 2 wire where one must be tied externally to ground. Your "auto lights" circuit is just such an example.

Secondly, my belief, tested on both my XK8 & S-Type, is that a forward facing sensor is used for, and solely for, the ground switching of "auto lights" function. That for the mirror's "auto darkening" function is a separate rear facing sensor . . . ie it is the lights of a following vehicle that trigger that function, not oncoming traffic.

Caveat on all this, tested by flashlight by night and day, is that my Jaguars may be vastly different MY to yours . . . pre or post your mod. Even so, IMHO, using a single ground seeking sensor with no blocking diodes, for multiple circuits for different functions, is fraught with certain malfunction. Other threads here confirm the risks.

Best wishes,

Ken

 

Hi Ken

Well to be frank I hadn't a clue how it all worked till it dudn't. After repairing the mirror I thought all was well till I used the Jag a few weeks later in daytime and spotted the lights were on.

Testing the dimming mirror in daylight with a torch (flashlight), I found it only worked if I cover the forward facing light sensor, i.e. its dark out, with that done shining my torch at the rear facing sensor causes the mirror to dim.

As to the auto lights, if I disconnect the plug from the mirror the lights go out. Hence wanting to trace the circuit to suss out how it works.

Anywsy, for now in daylight I simply use the lighting stalk to turn the headlights off. One day maybe.

Regards

Terry

 

Thanks for the explanation Terry . . . my misunderstanding . . . I formed the impression you were using single forward sensor for both. Not so; in which case I am confident you will nail the cause of your "auto lights" misbehaving. I guess there are a few options - connectors, wiring, dud sensor - that could produce a false ground beyond the plug.

Best wishes,

Ken

 

Thanks.

Only thing I'm sure of is that if I unplug the mirror the lights go out, so the fault certainly appears to be there. I was given the connector pin no./ wire colour by someone and if I ground it myself the lights come on so the rest of the circuit looks spot on.

Cheers

Terry