Heater woes. - PeachParts Mercedes-Benz Forum

tobyd · #1 12-29-2004, 06:23 PM

Hi all,

Can anyone help desipher what is wrong with my 500E's heater.

The heater only ever blows hot (not warm) air into the cabin until the temp selector is set to minimum and then we can get aircon cold air.

The other symptom is while the engine turns over at less than 1500rpm or while idling at lights, the warm air stops after a few seconds and then cool air until i get moving or rev the engine, then warm air arrives again.

This is a pain at this time of year so I'd be grateful to anyone who wishes to share their knowledge with me.

Many thanks

Toby.

Jackd · #2 12-29-2004, 10:07 PM

I hope you'll get an answer to your problem as I have the exact same problem with my 260e

BenzMatic · #3 12-29-2004, 11:08 PM

I am not familiar with your car but typically older MB has problem with Automatic Climate Control Unit. With age it would develop cold solder joints causing all kind of issues. What happens is that with cold joints the current do not flow as suppose to be and cause malfunction in the climate control system. One of the remedies is to take the ACCU out and resolder all the joints, very easy if you are electriconically inclind. The expensive route would be to replace the unit with a new one or rebuild one.
Another cause for your problem could be Monovalve, the diaphram on the valve become britle with age and finally rupture causing the heater issues. This is the valve which is responsible for the flow of HOT water into your heater core and if the diaphram is ruptures it can totally or partiall block the suplly of the Hot water. There is a repair kit for this, easy to fix. Do a search for your car, typically it could be found between the firewall in the engine compartment.
The last but not the least could be a vaccum leak causing the malfunction of the flow of the heat, when you accelerate the vaccum increases compansating the leak and heat start flowing in.

You can start looking into the above steps one at a time, once again I do not have your car so use these as a guidence and not absolute.

Hope it helps.
Thanks

Jackd · #4 12-30-2004, 10:05 AM

I doubt the mono valve is the cause of the problem
I have read several posts saying the default mono valve operation is "full hot". i.e.: if the monovalve is defective, it will be fully opened and only hot air will come out of the vents.
Any other suggestions?

BenzMatic · #5 12-31-2004, 12:38 AM

Quote:

Originally Posted by tobyd

Hi all,

Can anyone help desipher what is wrong with my 500E's heater.

The heater only ever blows hot (not warm) air into the cabin until the temp selector is set to minimum and then we can get aircon cold air.

Toby.

One of the toby discription does sound like monovalve is in default operation "full Hot".

What are your symptoms, could it be your ACCU?

Hope it help.

AdMaven · #6 12-31-2004, 01:51 PM

My '89 300E was only blowing heat when I would be at higher RPM's. It would blow cool air at idle or lower RPM's.

I replaced the circulating pump and it worked better, but no cigar. My indy unhooked the mono valve plug (don't know the name of it) and it defaulted to hot air--full blast!

He then unscrewed the mono valve and took out the metal part that looked like a piston and cleaned it and the heater has been working great at idle and all speeds.

There must have been some tiny particle inside the mono valve that was hanging up because it wasn't the diaphram.

BTW, I now have 204,000 on my car. Any idea how long these things run? I've read they will do over 300K if properly serviced. I bought it with 193K over a year ago and I just love this car!

ejsharp · #7 12-31-2004, 06:41 PM

I just love mine too. I'm hooked on Mercedes. Mainly because even the parts are fixable.

If you should continue to have heater problems then the trouble lies with the ACCU unit. Your symtoms seem to match the solder connections as described in above post.

You can do the repair yourself if so inclined.

J.HIDALGO · #8 01-11-2005, 05:34 PM

Did you fix your problem?
What was it? Monovalve, ACC unit, etc...

tobyd · #9 01-13-2005, 06:18 AM

This is still a 'work in progress' at the moment.

I did tap the monovalve and auxiliary pump with a spanner at the weekend and the heater seems to be staying hot while idling since!

I'm hoping to have a proper look at the weekend but the weather is pretty crap at the moment and my garage is full of 1956 Land Rover with no brakes (a job in very slow progress).

I'll update the forum when I get somewhere.

td