R129 Intermittant heater

[RichardF]

Hi My heater did not come on yesterday morning. After 45mins it was working okay for the rest of the journey.

I suspect a faulty valve somewhere any pointers?

 

[Sp!ke]

what year is the car?

Was it blowing cold or not blowing at all?

What temperature was your engine (thinking faulty thermostat).

Have a google for Duovalve for another possible culprit.

 

[RichardF]

Hi Thanks for the reply. Heater was blowing cold engine up to temp (80 degrees) after a couple of miles as normal. Heating didn't kick in for another 1/2 hr or so. It's a "94" SL:20

 

[Lxb3]

Was the control unit in the dashboard (guessing you've got the digital one, or is it manual control?) behaving itself... and when was your coolant last flushed?

 

[andy27168]

Hi,
It could be a dodgy/dirty heater valve, clogged/blocked heater matrix or a faulty dash control unit (though as it does work on hot sometimes I would look else where first).
If it were mine I would start by back flushing the coolant system including the heater matrix, and clean & inspect/replace the heater valve which is by the brakes servo under the bonnet.
If you don,t replace the heater valve only fill the system with fresh water till you have proven the problem is fixed, as you might have to drain it again.

 

[Sp!ke]

Andy is on the money here I suspect.

 

[RichardF]

Thanks, whats the best way to test/clean the heater valve?

 

[RichardF]

I've not been out in the car for a few weeks, seems fine now. So I will leave well alone (If it ain't broke don't fix it)

 

[RichardF]

The plot thickens. Drivers side works normally, passenger side can take 1/2 hr or so before warm air comes through. Mrs F wants this fixed.

Would this be a sticking valve? The blower motor makes a bit of a racket I suppose I should remove it and grease the bearings, is this fairly straightforward?

 

[Sp!ke]

if one side works and the other doesnt then it cant be a shared component like the jheater matrix at fault. It must be the thermostat (you have dual zones right?) or a sticky flap behind the dash or something?

I'm not 100% sure on the pre facelift models but I think there might be an inbuilt diagnostic utility on the heater panel.

 

[RichardF]

I think I'll start with a flush and duovalve.

 

[Mg330]

The plot thickens. Drivers side works normally, passenger side can take 1/2 hr or so before warm air comes through. Mrs F wants this fixed.

Would this be a sticking valve? The blower motor makes a bit of a racket I suppose I should remove it and grease the bearings, is this fairly straightforward?

Hello, I’m reading this almost 6 years after you wrote it...do you remember how did you solve this problem? I have exactly the same in my 92 300SL-24...tks

 

[RichardF]

I fitted a new duo valve, not much difference, a new waterpump solved it.

 

[Mg330]

Thank you! I’m gonna check that. Cheers.