1976 XJ12C rear window mechanisms designed by satan

Thank you Robin.

Phillip

Update. Installed the two window motors. Right works, with help up, and absolutely no reply on the left. Checked voltage and got 12 on left and 12 on right. Left motor was “rebuilt” so I believed to be ok. Tried another used motor in left and same, nope, reply. Is there a source for rebuilt window motors or, even better, a rebuilder of these old motors.
Thank you,
Phillip

Parts from wiper motors may fit

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Have you tried applying a 12V jump to the motor, Phillip
?

Two possible scenarios; you read 12V at the motor, but there is no power in the voltage. A bad connection may give 12v


but no/little current.

Also, check ground connections - without motor ground the voltage will do no good


Frank
xj6 85 Sov Europe (UK/NZ)
**

Frank,

but there is no “power” in the voltage. This, I do not understand. I will do some reading up on voltage and amps. Never too old to learn.
On the grounding issue, this is provided by the earth grounding of the motor to the steel car body. I cleaned and sanded the areas of contact on both surfaces very well, so I thought.
Per your instructions, I will attempt to jump the battery directly to the motor. What could possibly go wrong? :thinking:
Thank you again for the advice.
Pillip

From your thread title choice I presume you already knew to look in the details. :slight_smile:

What Frank means, I think, is that there is some high resistance in the path from battery to the motor. High resistance in this context means an R significantly higher than that of the motor, but significantly lower than that of your voltmeter. Say the motor’s R1 is 1 ohm. That means the current (V/R) it draws at 12V is 12 amps. And say the meter’s R2 is 1 million ohms. that means it draws 12 microamps at 12V. Now say you have a bad connector or wire in series with either the motor or the voltmeter reading voltage at the motor, its R3 is 100 ohms. If you do the math (Ohms Law for a 2 resistor series circuit) you will find that the meter will measure close to 12V with the motor off (virtually no voltage lost across the bad connection) but will measure close to zero volts with the motor on (virtually all of the 12V lost across the bad connection).

So the voltage will “power” the low current draw of a meter, but won’t “power” a low resistance motor. Same holds for the battery itself, which has internal resistance. It might read 12V with a meter, but much less with a load tester–a meter and a low R resistor in parallel.

What he was trying to say is that you could be measuring 12V with your voltmeter but if the cables or connections are bad, i.e. very high resistance, as soon as you putt a load on those 12V there will be a substantial voltage drop across this resistance, and therefore no “power”. (P=I*V and I=V/R)

And this is the purpose of adding relays and big cables, to diminish, as much as possible, the circuit’s resistance.

Aristides

Phillip. Those motors are made by Delco. I expect that if you visit a wrecking yard you will be able to find the same motor in almost any gm product of a similar era and probably up into the 80’s or 90’s. I have no personal experience but I do have a vague recollection of this being mentioned in other threads in the past and it makes sense as Jaguar was good at raiding the gm parts bins.

**
A burnt out motor - unless you fit a fuse to the jump, Phillip
:slight_smile:


or better power the motor only briefly. The problem may just be too high friction in the window guides or lift mechanism. If the switch is the problem, the jump should at least cause some motor reaction - even with high window friction


And ground is provided through the switch, reversing polarity to the motor. Motor ground plays no part - all current goes through the switch


And further; resistance expresses the ability of current to flow - like the faucet on a water pipe. Except that ‘water’ is the original voltage; always present, except if the faucet is completely closed. Like current turned off or the electric connection is ‘break’
:slight_smile:

Frank
xj6 85 Sov Europe (UK/NZ)
**

Frank,
I was able to get back on the windows. I was not able to jump the motor so I removed both rear motors and had them gone through and bench tested by a “professional”. They both checked out fine. Re-installed and same scenario. Right worked, left did not. Nothing from the left motor but still getting 12 V’s.
The window guides are clean and well lubed so that is not problem.
I have attached a diagram that shows the left and right windows are wired each in series from the relay to the thermal breakers to the switch. Am I correct, then, to assume that if the front left window works and the rear left window does not, the relay, master switch and thermal breaker are not the problem?
The only window wiring that I have not been able to inspect is what runs through a “sealed” tunnel located on the rear floor under the front of the rear seat. Is there a way to access this area?
As always, Thank you,
Phillip
xj12 wiring diagram.pdf (681.2 KB)

If it worked on the bench and is getting 12V, possibly it isn’t grounded?

The motor is earth grounded at the mounting to the body points. Body piece was well sanded and cleaned before motor was installed. As Frank says above, the ground comes from the switch. It’s a mystery and I am quite fed up with it.
Thanks for the reply.
Phillip

Don’t give up. From your thread title, the problem is obviously in the details. :slight_smile:

Sorry, I had assumed S2 was like S1–select either of two windings for up and down.

But if ground comes from the switch it is like S3–one winding, reverse polarity for up down. When you say you have 12V, I assume that’s when you activate the switch? If so, is it read on both motor terminals? One should be ground, the other 12V. Activate in the other direction and the two should reverse. If you see 12V on both with the switch in either position, you are getting 12V to one motor terminal and nothing to the other–but reading the 12V through the motor winding on one of the terminals.

Does this make any sense; maybe I still don’t get it.

Phillip,

sorry I haven’t dug into your original wiring scheme. Instead I remembered a wonderful wiring scheme made by Dave Collishaw (Sparks) - hope he’s still doing fine! - in an album “Series 2 XJ6/12 electric window wiring diagram” that I keep in my hard copy folder. The power windows are switched in line, so that while the driver’s window is operated the others are dead. The connection is RHF, LHF, RHR, LHR, RHRR, LHRR with RH right hand, LH left hand, F front, R rear, RR rear, switch in the rear. - I suppose that Dave’s scheme is based on RHD cars, so that LHD cars probably have a different hierarchy (change left and right), but are constructed the same way.

Maybe your motors are good, but your switches are bad? That might interrupt the connection to the rear left motor. Again, the 12v on a DMM are not necessarily conclusive. If you can’t jump the motor maybe you take a serious consumer like a head light bulb and check whether you can power it with the juice you get at the motor. To get to the rear left motor the power line has to pass through four switches and eight spade connectors (if I have counted correctly). Did you pull all of the switches to polish the spade connectors and put some grease on?

Good luck

Jochen

75 XJ6L 4.2 auto (UK spec)

ALL window winder mechanisms are the Devil’s handiwork
 all have 666 in their part numbers.

:smiling_imp:

1 Like

I’m not sure if this is a possibility but can you swap the wires on the switch so that the l/h powers the right and viceverca?

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Indeed, Phillip - if any window works, the items mentioned are working


However, your diagram shows the S3 set-up - the S2 set-up is different. And I’m fairly sure that the S2 V12 pre-empted the use of the S3 electric layout.

One question; are the other windows all working as they should - the rear windows also operating with their ‘local/slave’ switches.

In the S2; power is connected through the switches in series, as Jochen says. ‘RH front’, ‘RH rear’, ‘LH front’, ‘LH rear’, ‘LH rear slave’, ‘RH rear slave’. This means that only one motor can operate at a time - using one switch unpowers all ‘downstream’ switches. Basically, if the drivers panel is used it overrides the slave switches - but the driver can still only operate one window at a time.

(This is a theoretical assessment of the diagrams, which are somewhat simplified. And should be taken with a bit of salt)
:slight_smile:

The complications of the switches is why I suggested checking power at the motors. Not using a voltmeter, but a test lamp - to ensure that actual power is reaching the motor.

However, you may also use the switch terminals to check power (with a test lamp) - verifying that there is power to all switches. In this case specifically to ‘LH rear’. And indeed that the ground, connected in series the same way


It is very unlikely that there is anything wrong with the wire between the switches - the fault is almost certainly at, or in, the switche(s). If the switch/connection ‘upstream’ has failed there is no power, or no ground, downstream. Both power and ground must work for the motor to operate - the switch changes the polarity to the motors for ‘up’ and ‘down’


The wires to the ‘RH left’ motor is green/orange and red/orange (I think). Power to the switch is red/black (according to ‘my’ diagram) powered with ign ‘on’.The (reasonably) safe test is to connect a (substantial) test lamp between these to wires - and operate the switch. The lamp should be lit in both ‘up’ and ‘down’ position - providing the switch has power.

If no power on red/black; check *red/black on the ‘previous’ switch - and red/white on that switch, which has power from the one ‘previous’ to that.

If power is OK but test lamp fails to light in one position; either power or ground is lost in the switch - switch fault. Be aware, that lit in one position; the ‘up’ position won’t work - as the motor is already fully up!

If the test lamp is lit in both positions; it gets ‘interesting’ - the motor should actually run
:slight_smile:

Also, with jumping, the motor wires must(!) be disconnected from the switch. Power and ground can then be applied to the wires switching polarity to reverse motor rotation


All this appears complicated. But once diagrams are fully understood and the switches’ wiring complies with diagrams - testing takes much less time than reading
:slight_smile:

Salvo errores et omissions in this intricate matter
:slight_smile:

Frank
xj6 85 Sov Europe (UK/NZ)
**

1 Like

Yes, the other windows operate fine. The rears do not operate with their slave switches.

I will do the tests with the lamp as suggested and report back. Thank you for hanging in there with me. Using the S3 diagram was symptomatic of my ignorance. This should change in time. Thank you.
Phillip

Robin,
Tried that and did not work. Something with the wiring in series I suppose. Good suggestion though. Thank you,
Phillip