1985 Benz 300 Turbo diesel

Benzmzn was a GREAT help with block heater info. Who can lend a hand on getting the interior heater to work - please? It;s getting cold here in MD.

Post some details as to whats not working.
Is the blower motor working?
If so, is it blowing cold air only?

Blower is working and blowing cold air. However if engine is shot off for a short period and then restarted the heater blows warm air till the engine “recools” from the winter air blowing over the rad and that’s it for the warm air. The engine temp seldom ever gets over 80C in these cold Maryland winters. Thermostat or other grimlens?
Thanks in advance.

Those symptoms point towards a defective monovalve.

The monovalve is basically a pulse-width modulation system, that is basically either fully open or fully shut. Full open corresponds to full heat, full shut corresponds to no heat at all. When they fail, usually from corrosion,most of the time is in the shut position, meaning no heat.

You don’t need to change the valve, just the insides. Buy a rebuild kit.
Here’s one for about $65. http://www.performanceproducts4benz.com/product.asp?pf_id=P83-064&dept_id=5521

On your 300D the monovalve will be mounted high on the firewall on the passenger side with heater hoses lead to it on from both sides. it will have an electrical assembly that disconnects on top of it. The kit you buy has a piece of paper that describes how to change it. It takes more time to find all the right tools and drain some antifreeze than replacing the parts. A really simply DIY job.

Don’t forget to drain off some coolant before you start, as it makes things a lot cleaner when you’re working on the valve. Also, remember that the hood on your 123 chassis will go completely up to open 90 degrees by releasing the retaining bars on the hinges. This makes things much more accessable.

Again, this is a fairly easy do-it-yourself job. If you need more help feel free to ask. Good luck!

  • Wow !

Nice answer.

You’re great. I’ll keep you posted on my efforts and results. This was a San Siego 300 Benz and they probably never used the heater or the block heater,
The rear window defrost dosen’t work either. All fuses in tact. Any suggestions?

The fuses on a Mercedes may look intact, when in reality they may be defective.

Fuses on a vintage Mercedes should be replaced as a matter of maintenance every five years or so. Cost under $10.

It’s a simple procedure to replace all of the fuses, and you may be surprised how many electrical gremlins this fixes. You can buy a fuse kit for a 300D with the proper assortment of blue, red and white fuses. It wouldn’t surprise me if some of the fuses actually come apart on removal if they’ve never been replaced since 1985.

I managed to get a monovalve from my local Benz dealer cheaper ($52.00) than on line and without the $15.00 shipping fee.
Do you know where I can find an exploded view and detales of the replacement procedure. Will I need a new gasket or a sealer, etc?

This came from another site, but spells things out quite nicely.
The article is for a 126 chassis but the basics are the very same.
Your 300D’s monovalve is on the firewall and easier to get to than the 126 in the article.


Good luck!