75ZT Community

Hi all. having an issue with the heating on my ztt ( air con and ATC (Automatic Temperature Control) version), the heat control motors are either going to hot or cold, I would presume they should stop at points inbetween to position the flaps and therefore control the heat? The system test using the heater control panel doesn't work either. I have TOAF, is there a way of testing the flap motors etc with this, could it be a BCU (Body Control Unit) issue? I've got some other motors on route, but as it is annoying me I thought I'd ask more informed people too.

With ATC, the BCU only controls the recirculation. So I doubt it's the BCU. It could be many things. There are some diagnostics in T4 (Testbook version 4.Computer Diagnostic System) for the ATC but I've never tried them, so I would assume TOAF could do the same things whatever they are.

First thing, there are three fuses, and you can get some oddities if one is open. So re-check them all.

After that, there are a few possibilities for your problem. The heat (L and R) and distribution motors have a motor, and a feedback line. The feedback works by using a potentiometer. One end of the potentiometer is fed from a reference, probably 5v that comes out of the ATC, orange / yellow wire on pin 10. The other end of the potentiometer is connected to a reference ground at 0v. Black / grey wire on pin 20. The feedback changes between 0v and 5v as the flap moves, so the ATC knows where the flap is by looking at the voltage coming back.

Now it seems both of your heat motors are doing the same and only running end to end. You haven't mentioned the distribution. So that points to either the reference, or the reference ground not being correct as a feedback wire fault would only affect one. Easy check at the ATC plug, and if OK check again at the motors.

The other possibility is some of the sensors. There are three coolant sensors, and a solar sensor. I can't see the solar sensor being the problem. I doubt a single coolant sensor would make it faulty in this way either. However, all three coolant sensors share the same reference ground as the feedback, pin 20 black / grey.

There's some ideas and tests. Give me some feedback and I should be able to point out some next steps. Historically the ATC units themselves have been very reliable.

Hi Duncan and many thanks for your reply. I have today replaced the interior temperature sensor and whilst checking the operation of the recirc motor (which appears to be fully opening and closing as it should), I noticed that the ATC Matrix heat sensor was almost falling out - I have pushed this fully back into place. Both of the heat motors now seem to be at least stopping at various intervals. To check the wiring, am I checking at the plug on the rear of the control panel or heater unit main connection ? I would presume that if all is good on pin 20, I should have no resistance at all? Also, being lazy here, which 3 fuses and I looking at?
Many thanks
Paul

OK. It's seems you have progress. If they are stopping at places other than the end stops, the control is at least partly working.

It's one of the two connectors on the control panel (the ATC). The circuit diagrams just refer to the connector number so the wire colours should guide you.

You don't need to measure resistance, look at the voltage. Put your negative probe on an earth point (away from the atc, maybe the door hinge strap bolts) and the positive on the pin at the back of the ATC. Pin 20 should be 0v approx, pin 10 I'm not sure but probably 5v and certainly not 0v.

I'm not sure which fuses it depends on the age of your car.

The sensor not been clipped to the matrix properly would for sure cause it to work incorrectly.

Sound mate, I'll check them tomorrow. The solar sensor - is that related - it's always thrown up an error code (21 / 22) even with a maglite on it. Someone said it was only to do with cooling and not heating, others say differently.

The solar sensor does only affect cooling on most systems. I can't say for sure on this one. But if it or its wiring have a problem they could affect the feedback from the other motors. It could be worth unplugging the other motors and confirming you then get an error code for them.

Duncan your breadth of knowledge about these cars will never fail to amaze me !

Absolutely amazing.

Trebor wrote:Duncan your breadth of knowledge about these cars will never fail to amaze me !

Thanks, but as you know this kind of thing is my day job. Most systems work the same way on all makes and models, so looking at the wiring diagrams in RAVE makes it easier to work out how it must work.

Hi all and thanks for all your replies, due to a heavy workload, I've only just got back into trying to sort my heater issue out.
So, today I hooked up a digital thermometer to the matrix and had the IPK (Instrument pack (IPK from the German)) in test mode, I also has an analogue thermometer in the face vent. Temp set to 25 c. IPK showing 82 - 84 rising to 87 when still, thermometer on matrix 62 max, analogue thermometer in face vent 20 c. The 'warm' air from the vent certainly doesn't feel 20 c. When the stat is closed, the heater seems fantastic - once it opens there is a dramatic fall in heater temp. I have changed all the motors (except directional on as it's a ~~## to get at, brand new matrix, I have also replaced the thumbwheel and cable ( and made sure the wheel fully closes and opens the flap). Do I; Vacuum fill the coolant (my farty pela type pump hasn't got the grunt), renew the matrix temp sensor or throw a match on it and buy the xj8 up the road p.s Internal ambient sensor also changed and cleaned.

The diesels are known for the thermostats opening way too early, but you would see this on the IPK as low readings. Did the IPK still say in the eighties once you start moving? You will only really notice a weak thermostat when the air is passing through the radiator.

Hi, well after lots of testing as per my previous post, another zt / 75 owner informed me that it is the way the heaters are designed - i.e heat to face and screen is lower than to your feet as a kind of safety thing, to stop drowsiness and keep you refreshed. The IPK never gets lower than 82 - I now put it down to my ignorance of this complicated heater system - though I love the car, I wish the heater was from my poor old 418 SLD. Obviously, if the info I have been given does not make sense - please do let me know.

I remember my 418 SLD heater as being a belter compared to the 75.