Grrr

[culky]

Hi folks. Hoping someone can point me in the right direction here. I've got a Mk1 (1996) sport that i use at weekends. Last weekend on a trip to Birmingham, the wife and I notice cold air coming out of the vents. A/C was not switched on and the temperature control was set to a constant 21 (which is where it normally sits).

Twiddling of the temp knob provides no more heat - its constant cool air. The only reaction we get is by cranking the temp to maximum (30), then we get hot air. Anything below 30 results in cold.

Throughout our testing, the fan speed worked okay and the aircon would switch on and off it's just the temperature control. The car was on a motorway run and the engine temp was rock solid in the middle of the temperature gauge like normal.

What things can I check? From browsing the american site and trying to study the limited heater section on my MODS cd, It *looks* as though the rotary temp knob connects to an ECU which then controls a motor, which activates a rod which connects to a heater control valve. Does anyone know if this sounds right? a few folks with soarers and LS's in the states seem to have had similar-sounding 'HCV' problems but some seem to have physical connections and some dont.

The problem's liveable with but I'd like to try and diagnose it a bit further in an evening than try and get it in to Lexus for a diagnostic. Unfortunately it looks as though most of the heater system is buried within the bowels of the dash.

The fact that i can actually GET both hot and cold is encouraging and I'm hoping it's just a sticking valve or control mechanism of some sort...

Thanks in advance for any help

Darren

[culky]

Hmm... A bit of prodding around under the bonnet when I got home this evening and I've found the heater control valve on a bracket on the firewall, just to the passenger side of centre. It has a control cable going to it which pushes an arm towards the passenger side to shut it off or towards the drivers side to open. It has two water pipes - one running downwards and one running off towards the driver

With the engine running, I twiddled the heat knob and it operates the valve perfectly - from fully open to fully shut.

So I seem to have:

hot water flowing through the heater (because i'm capable of getting hot air)

a heater control valve that opens and closes

a working fan that's pushing the air into the cabin

I can only imagine the problem must be a blocked heater core.. does that sounds reasonable. Sounds like an expensive trip to Lexus....

[chips229]

Hi,

I think as a general rule,these climate control systems have sensors which tell the heater brain how hot or cold it is inside the cabin,and adjusts the heat to suit.

Id try and find out where they are,id say that could be the problem.

When you choose max hot or cold air then i think that overrides the sensors.

Sorry i cant be much more help.

Good luck with getting it sorted out.

Cheers chips.

[Brettster]

As chips said

perhaps the the cabin was hotter than you thought and the car was trying to make the temp 21 by blowing in cold air

[culky]

As chips said

perhaps the the cabin was hotter than you thought and the car was trying to make the temp 21 by blowing in cold air

←

Thanks Chips and Brettster. That sounds much more probable - I hadn't even thought of that. I'll go down that route and see what i can find - a bad connection or some other kind of open circuit might mean it thinks i'm asking for cold air, unless its on max.

Cheers guys!