Hello,

I have an 89 XJS and I am trying to override the heater controls so that it only blows hot air. The temperature knob is clearly broken, as it spins freely and is not stopped by the indicated temperature indication. The blower control on the other hand has some functionality, the off works and every other position seems to set it to full defrost mode.

The air comes out the defrost vents quite strongly, yet it is always cold. I am essentially trying to override the system in order to have hot air, in order to avoid pulling the Mk III system with the faulty control dials right out of the dash. I disconnected the vacuum to the heater control valve and ensured that the heater core is not blocked. The heater blows cold air still, though due to these tests I am fairly sure the heater core has flow.

I am therefore led to believe it is the flaps that direct air through the heater core. I have read several threads but I have not found one that has specific details as to where I can find these flaps. If anyone has a solution or more information, it would be greatly appreciated.

Thanks,


Adison

 

The system uses vacuum servo operated flaps to control where airflow is delivered and two internal electric servo operated blending flaps. It is the internal flaps which control temp of air from the system. The servos for blend flaps are on the RH side of aircon unit, one high up and almost impossible to remove, and one lower down and more accessible. It may be possible to butcher these servos and freeze the flaps in a certain position to give you the constant heat but it may be easier to simply fix the system to operate correctly.

 

Look at my blog I have pics there when I repaired mine.

Here is a repair manual that will help (courtesy of Doug).

WARNING be very careful when probing the connector if you short pins you could damage the transistor drivers in the AC controller. I use a jumper as pictured, these are used in computers to connect to the motherboard. Put a paperclip in one end to connect your DMM to and the plug end will fit onto the pin on the AC controller.

 

I need to replace the lights behind the gauges, so when I pull the instruments I will also pull the dash and glovebox and see if there are any loose/dirty connections, I may just get lucky. Worse to worse, I will just shim the flap somehow until I can fully fix it. Thanks for the information, I will hopefully post back when I have some results.

 

Have you checked that there is hot water flowing through the heater matrix? The heater valve is on the bulkhead and just could be jammed shut, although they normally jam open.
Greg

 

Thanks for the help everyone, the delanair booklet was helpful. I pulled the stepper motor off the lower blend flap and manually set it to direct air through the heater core. Now there is hot air to the legs but still cold air to the defrost vents. So slight better. I am going to try to pull the other blend flap motor off and do the same thing as now I know which position it is in.

On another note, I went to replace dim/burnt out bulbs in my control cluster and I got super weird results. I initially put an LED in the place of one of the bulbs, but when running that I found that when I turned the headlights on, they were somehow linked to both the oil pressure and temperature gauges. Meaning they both immediately went to middle when car was one and maxed when headlight switch was turned on. I then replaced the led one with a new regular bulb and now when I turn on my headlights my tachometer immediately zeroes. Now I really have zero idea what is going on in my control cluster.

I will pull it again tomorrow and do some more experimenting but I am just utterly flabbergasted as of right now.

 

Classis symptoms of a bad ground. Common on these cars.

Easy to fix

Clean all contacts and connections, if you haven't done so already, and add a redundant ground wire to the cluster.

Cheers
DD

 

I have a '93 and replaced all bulbs with T10 LED's in the dash without a problem. In fact, I soldered all my warning lights to the printed circuit. I think you might have made a connection between your lighting system and the instrument system
You might try removing bulbs individually and see if one is causing the problem an if so, troubleshoot from there. You might also look at the main cluster plug's contacts and see if they have not shorted. lastly, look at your LED,s to insure you have the right polarity.

Doug's response is very appropriate a defective or intermittent ground can do weird things. Your instruments qualify!

 

That upper blend servo will be a problem. At least one of the screws is very hard to get at and the servo itself is captive between the a/c unit and a body gusset, you may need to partly destroy the servo or the gusset to get it out.

 

I haven't had a chance to get replacement bulbs, so the instrument cluster is still sitting on my table. However, I feel like I have found a solution to the upper servo problem. If I can run the appropriate signal through a function generator, I should be able to move the servo to wherever I want. The problem then remains, is what exactly is the type of signal that the ECU gives the servo? I have looked for datasheets about the servo, but I cannot find much about it. I don't have an oscilloscope to read the output of the board to see what it is outputting. Does anyone know what kind of signal controls the blend flap servos? Also, what voltage are the lights on in the instrument cluster? So I can test them.

 

The bulbs will be 12V and l believe the servo operation can be tested using a 9V battery
but you need to be careful when operating the servo as it can jam in the extreme positions making removal the only option.

 

It's not a servo motor as such. the motor is a geared DC unit with a feedback (DC Voltage) pot on the LH end of the blend flap. You can drive the motor back and forward by swapping the polarity on the motor. That is all the AC controller does and keeps track of where the flap is by the feedback voltage.

A good place to start is by cleaning ALL of the connectors associated with the AC. They will be full of gunk and possibly even old coolant.

Bax is right the upper motor is a bugger to get to. You can just see here in the pic behind the blue hose.