Hello
my blower motor runs constantly even when the ignition switch is off .
I read the a few topic and decided to change the darlington as suggested.
it did not fix the problem .does anybody have decent idea .
the car is 93 xjs
the circuit board looks pretty simple , a couple of capacitors and a couple diodes and a resistor and a darlington
DSC02498|690x337
I replaced the transistor on mine. Do you have the schematic handy? If so please post it here and I can try to help troubleshoot.
Do you have a photo of the top of the circuit board too? If that relay is stuck shut, the blower will always run. You could test that theory by temporarily unplugging the connection at the lower left there next to “B”.
Mike
Here is a picture of the top of the board , I replaced the relay with a good known relay , problem still exist.
The motor will run if:
-
Assuming the darlington is functional:
There is current of maybe 5mA or so thru the K wire into the base of the darlington.
That will give a voltage of about +1V on the K wire.
That turns the darlington fully on and the motor at max RPM.
Presumably there is a feedback control system that modulates that base current/voltage so the motor speed varies to suit. -
The relay contact is closed.
-
The diode connected to wire KB is short circuited, and the KB wire is connected to ground.
-
The darlington is faulty and short circuits the motor to ground.
The diode was bad (shorted) on mine, so I would change that. (the larger black one)
Mike
I will replace the diode and report back
The big diode is most likely the one across the motor with the resistor in series.
That is to absorb the voltage spike you would get as the motor switches off.
If it short circuits the motor might not run, or it will run very slow depending on the resistor value.
If it open circuits you have no protection for the darlington.
There is about 100 times more chance the darlington is faulty as opposed to the diode/resistor being faulty.
Maybe the feedback or control circuit is the culprit and keeping the darlington fully on.
Richard
this may sound a stupid question to you, however when you say feedback system you mean the a/c control module ? my electronic knowledge is basic.
Richard
your second fault possibility on the list which is relay being closed is not the case because it was still running without the relay in there. yes exactly was running without no relay.
I do not have your A/C system in any of my cars, so not an expert.
I expect there is a control or feeedback system that compares a set temperature with the actual temperature in the car. It will be configured to apply a varying current to the base of the darlington and that varies the speed of the motor.
At a guess that small diode is connected to the KB wire and that is a feedback to the control system to give info on what the voltage is across the motor. Coiuld be if that is not functioning the system thinks the motor is turned off, so it will turn the darlington fully on to try and increase motor speed.
You can check the diode. The KB wire should be at a voltage about 0.5V higher than the voltage at the low end of the motor. In your photo the left side of the small diode should be about 0.5V higher than the right side.
I know nothing of the XJS system, but in S2 XJ climate control, there is a in-cabin thermistor that is part of the “feedback” circuit.
I followed the fault finding chart in the FSM, this was the last item on the checklist, and was faulty
Have you got insulators on the screws that hold the MJ to the PCB, from the pic it looks like the case is shorted to ground, the case is the collector and is connected to the blower motor, so without insulators the blower is turned on all the time.
Warjon
yes I did have the two round plastic insulators , however i used the old used insulation on the surface of the bottom of darlington .maybe the insulator has become conductive , it looks like a gasket , it may have some weak spots .
I will check and report back
What happens if you disconnect the K wire? It would help narrow things down some. Also worth noting that these capacitors in your photo are not on the schematic. I really don’t remember if mine had them… 9 years ago I did this…
Mike
pre 1992 xjs’s do not have 2 capacitors , they only have one orange capacitor.
I do not know what happens if I disconnect the k wire.
I will let you know after I try the insulator pad under the darlington
Take a multimeter on Ohms and meter from the case of the Transistor to ground, I would bet there is a short and you need to find it.
The capacitors are there for noise and to prevent oscillation of the Trans.
Warjon
I did put a new insulator pad under darlington , that did not fix the problem , the short must be somewhere else.