Heater Not Working - Please Help

[tim_k]

I'll be really grateful if someone could point me in the right direction to fix the heater in my 1990 mk3 Supra

The blower fan always works fine, but the air is sometimes freezing cold, even after an hour of driving. It is an intermittent problem, the heater mainly works fine in the summer and doesn't work in the winter (typical!), which makes me think it's an electrical problem which is worse in damp conditions. It's been getting worse over time, and now more often doesn't work on cold days.

I've checked the following:

Heater control unit works fine (all the right continuity between switches and resistor is ok).

Circuit breaker is fine.

Relay works fine.

I've found a problem with the vacuum switch under the bonnet: when the problem exists, it isn't getting the 12V supply. When it is working it does get the 12V supply. This makes me think it's an electrical problem. I believe this vacuum switch lets the hot air into the cabin, but even when I hard-wire this on, the air is still cold, probably because the air-mix-servo (the next stage?) is still stuck on cold setting.

The engine does warm up very quickly, as indicated by the temperature gauge, and hoses, so the thermostat is fine.

Looking at the circuit diagram in the Toyota Repair Manual, maybe the 'amplifier' is faulty but where is this located? Also just from reading the diagram, one of the following parts could be faulty, but I don't know where any of them are, other than the top three which I've checked.

Heater Relay (measured ok)

Blower Motor (must be ok - fan works)

Blower Resistor (measured ok, and controls fan speed ok)

Recirc/Fresh Control Servo Motor (?)

Water Temperature Switch (?)

Auto Relay (?)

Program Switch (?)

Air Mix Control Servo Motor (?)

Water valve VSV (?)

Air Vent Mode Control Servo Motor (works ok I think, as air gets directed ok)

Amplifier (?)

Dodgy wires in-between (?)

Has anyone had similar problems - or can anyone help, it's freezing!

[Karma Supra]

Very common problem. I can't beleive no one replied in two days!!!!

Suprasonic Heater fix

[tim_k]

Thanks very much for your reply, a useful link.

However, I don't think there's a problem with the actuator, as I have already made the mistake of having this part replaced with a brand new one and it didn't fix the problem. It is the supply to this part that is not working - hardwiring the supply (so it always on) or bypassing it (as your link suggests) does not fix the problem, probably because the air mix servo is stuck on cold (the next part).

I have done some more research and have realised that the car actually has climate control. On further reading, the parts which may have failed are:

The in-car temperature sensor (on the dash)

The ambient temperature sensor (in front of the radiator)

The water temperature switch (behind passenger side kick panel)

The water valve (which provides the SUPPLY to the actuator - could well be this)

Or maybe faulty ignition switch terminals.

So my next step is to investigate all of these, wish me luck! If I find the solution I plan to post it on the web with a full description of how the system works.

[Karma Supra]

Take the heater control panel out, take it apart and run over the solder joints... "dry joints" is the next big killer... ;) its usually the cause of the no.1 speed fan failure......

[tim_k]

Thanks for the tip, will try this tomorrow.

I took the Supra out for a long ride today, and the air was freezing cold for 45 mins, then suddenly the heater and all heater controls started working perfectly. So it must be a damp problem, or a dry joint problem, or a dodgy earth somewhere.

Interestingly, I thought the car had automatic climate control because of the following:

It has temp numbers on the temperature control slider.

The blower resistor is wired up to be 'automatic' as per the manual. (I took this out today to fix the low speed resistor which has also broken).

However, I can't find either the ambient sensor (it's not behind the radiator as shown in the diagram) or the in-car sensor (it's not above the passenger kick panel). So I'm thinking maybe the car doesn't have auto climate control.

I'm confused - is there a sure way of telling?

[Karma Supra]

You do have auto climate control.

The internal sensor is above the glove box, the "fake vent" type thing, on the drivers side of it there is a concealed vent... that is the internal air temp sensor...

You will also fine a small block on the inside of the windscreen, this is for the air con.

Like I say dry joints are common... you car is showing classic symptoms... same thigns my girlfriends mkiii is doing!!!!!

[tim_k]

Thanks for that, I've found the internal sensor.

Ok, Ok, this morning I repaired the blower resistor, but a couple of loops broke off , so when I rejoined it, it was a little shorter. I tested it and the lowest fan setting does work, but this resistor glows bright red (due to the the lower resistance I guess). :(

So I need a new blower resistor block, where can I get one off the internet???

[Karma Supra]

I'm sure there were a cpouple of people here breaking cars!! if not you might be suprised at how cheap some toyota parts are!!!

[tim_k]

The internal sensor is fine, measured 3kohm at 10degC as per the manual.

Can anyone tell me where the ambient sensor is? - so I can check it. I've had a look at the manual - as per the link below:

Ambient Sensor Position - Manual Page AC_42

It's showed on the bracket in front of the radiator, that also holds the horns, but I can't find it anywhere near this position? Anyone know?

Also - what is the solar sensor for? What's the point of it???

PS ordered a new resistor, brand new from toyota dealer at £17 - could be worse.

[Martin F]

Also - what is the solar sensor for? What's the point of it???

The solar sensor increases AC on sunny days. It works on the principle that if it's very sunny then you will need the AC a bit cooler to compensate for the heating effect from the sun.

[tim_k]

Getting there. I don't think the ambient sensor is the problem anyway, because even when I open circuit the in-car sensor it doesn't heat. And it should always heat when the temp slider is in the hot position.

The actuator is getting 12V okay, but it's not getting the ground. The ground comes from the airmix servo, then from ground point A. It could be the servo (hope not) or possibly control panel needing reflow (waiting for flux to arrive to reflow this), but:

The easiest thing to check is ground point A.

But where is ground point A? The manual suggests it is "loacted on left front fender". Does fender mean bumper?? Can anyone tell me exactly how I would find ground point A please? Thanks in advance. Oh and merry xmas.

PS Thanks Martin for the info about the solar sensor, at least I know this is definitely not the problem for my heater, (another part ticked off the list).

[tim_k]

Take the heater control panel out, take it apart and run over the solder joints... "dry joints" is the next big killer... ;) its usually the cause of the no.1 speed fan failure......

←

YEHHAAAAAA! Many thanks for your advice:

Just now I inspected the heater control panel that I removed yesterday, I measured the resistance between pins 1 and 9 (1 & 9 in my manual are the two right hand pins on the larger off-white connector, which I think are labelled 1 and 10 in later models as this Wiring Link (pic AC1855) suggests), and sure enough there was no continuity on any of the temp slider positions - except when I pushed really hard on the pins to bend them, then there was continuity.

On inspecting these two solder joints on the back of the circuit board, both these two solder blobs had cracked rings around the centre pin (a dry/broken joint). Interestingly there were quite a few other solder blobs with this problem, in fact most of them.

I've never been so delighted to find a problem. So I'm going to reflow them and try it again tomorrow - and I'm 99% sure that this'll fix it. My wife will be very pleased (and warm) if this works! Expect a full write up with some nice photos if this works!

Merry xmas everyone.

[Karma Supra]

If you want a place to uplaod them, its the kinda thing I am compiling on my site ;)

[tim_k]

Yehhaaaa x2! :D Today I reflowed every joint on the heater control panel and then installed it back in the car - it worked beautifully. All temp settings and functions now work fine. I also fixed the blower resistor, and all fan speeds work as well!

I am over the moon, so much so that I have written a website devoted to the problem solving that I did to fix the heater and fan speeds. It has lots of photos, have a look and let me know what you think:

Supra MkIII Heater Problem Solving

Karma - As you use the forums a lot, firstly feel free to recommend this website to others that are posting heating problems - at your discretion. And also feel free to cut and paste any of the images and information if you wish - the more people that see this the better. Likewise I've got a links page and if you want me to link to your site, then just let me know.

Merry (and now warm) xmas.