[corellian corvette]

1991 560 SEC.

Drove around last night and tried the heater - no luck. Not getting any heat in the cabin. It's "lukewarm" at best. Searched "no heat" and saw lots of posts on Mono Valve replacement. I also saw a post that suggested testing the power to the valve itself first.

So I tested the plug, and there is power to the valve no matter where I spin the wheel for the heater. So that's strange, because the tests I have seen suggest that would mean I'm always hot, not cold.

I'm not having any luck finding any troubleshooting beyond replacing the valve. Visually, it looks great with no signs of corrosion or leakage - it looks good.

So what makes the wiring to the monovalve work? And how can I go about troubleshooting why I have no heat?

Thanks!

 

[ianrandom]

If it were me I'd suspect the push button control unit. Cold solder problems are common on that version. I'm assuming you have the single temperature wheel version.

 

[MBL87560SEC]

Hi there,

In addition to what Ian has offered, here's quick monovalve test.

Ist thing I'd do is DISCONNECT the power feed to monovalve.

With the engine warmed, you should have full, uncontrolled [as to temperature] heat.

If this is the case, the monovalve insert has gone bad, a very common occurrance, and easily repairable.

If it proves bad, replace it with a Mercedes Original 0008350644
from your dealer.. $50.

If its easier for you: https://www.ecstuning.com/b-genuine-mercedes-benz-parts/mono-valve-repair-kit/0008350644/

The MTC branded ones do not work.

Cheers,

MBL

 

[John350]

Must be Autumn...

The monovalve always has power. The control switches its ground. When grounded, the valve closes. If either the control or the power fails the valve defaults to full open (heat). If you still get no heat when it's unplugged it's stuck shut or being forced shut by the water flow.

You can try this: With the key on and the engine off, put the temp wheel to maximum heat (it should "click".) Start the engine. If you get heat now, the valve is getting pushed shut by the water flow. The heat may stay on as long as it's in Max, or it may shut off if you rev the engine, and then stay off. It may then come back if you let the engine idle for a while.

 

[corellian corvette]

So I disconnected the plug and... HEAT.

So monovalve is bad, yes?

I may take the firewall shield off and put the fuse in the aux water pump while I'm at it.

 

[John350]

Most likely. Try starting with the heat in max. Also, see if the heat returns when the engine goes to idle.

 

[corellian corvette]

I drove the car to the exhaust shop this AM and heat seemed to be working really well. I was in light traffic and seemed to have heat just fine. Never really got above 50mph however.

 

[John350]

Makes sense. Only when the water pump is really pushing the water does it push the valve shut. It's a downstream valve. Once it shuts it doesn't have the strength to reopen until the water pump slows down. Sometimes the delicate balance allows it to stay open on the highway in max, but as soon as it comes off the full open position it gets pushed shut.

Do what MBL87560SEC said. If you can't find an MB part, BMW uses the same part. The BMW part number is posted in one of the other threads on monovalves. As far as I know, there are no aftermarket parts worth the postage to send them, unless you want to put together a hybrid valve, which I did 2 years ago. Works fine, but I don't drive the Benz in the dead of winter.

 

[MBL87560SEC]

MB of South Atlanta Lookup

It IS Autumn again... darn close anyways..

I just looked up the repair insert on the MB Parts World site... $33 & change!

https://www.mbpartsworld.com/oem-parts/mercedes-benz-lifting-magnet-0008350644

MBL