[modulus]

Hi all,

My '88 560SL provides hot air only when stationary. As soon as I start driving the air temp drops to cold. The engine temp gauge stays steady.

What I've done so far: Flushed out heater core, replaced cabin temp sensor and hose with foam end, rebuilt heater valve and swapped in 3 different control panels, all of which changed nothing.

Any ideas / advice would be appreciated.

 

[tdoriot]

How about vacuum issues? On occasion a leaking system can barely keep up, but under power the vacuum drops and the pods drift open or closed and boom, no heat. If you are on a long down hill coast, does the heat come back? That's a high vacuum state. How about your circ pump?

 

[Aussiemerc]

Maybe the scuttle vents (below the fluted pieces) are not closing, allowing entry of cool outside air.

 

[modulus]

Thanks for the ideas guys.

 

[Ears]

Modulus,
Try this... Unplug the aux water pump and go for a drive and report back... be careful and do not short the plug on the aux pump.

 

[modulus]

Thanks, I'll give that a shot if it ever stops snowing in NY.

 

[tdoriot]

When I saw your location, I figured heat in a SL was the last thing you needed! :grin

 

[Ears]

How about vacuum issues? On occasion a leaking system can barely keep up, but under power the vacuum drops and the pods drift open or closed and boom, no heat. If you are on a long down hill coast, does the heat come back? That's a high vacuum state. How about your circ pump?

on the 88, the monovalve controls the hot water (heat). vacuum does not affect its operation. Vacuum does, of course, control from where the (hot) air emanates.

 

[tdoriot]

That's why I recommended checking for vacuum issues. I figured since the car has heat while sitting, it has water circ. but why does it go away? My supposition is that perhaps vacuum loss might allow some vents to close that should be open.

 

[Ears]

That's why I recommended checking for vacuum issues. I figured since the car has heat while sitting, it has water circ. but why does it go away? My supposition is that perhaps vacuum loss might allow some vents to close that should be open.

If the monovalve is open, there will always be hot air from the outboard dash vents (unless manually closed)...There is no flap or diverter that closes off airflow across the heater core and into the cabin. The diverter valves determine if floor, dash or def only, and whether the recirc vents are open or closed.

replacement aftermarket monovalve inserts have been reported to close when underway due to water pressure. As the OP mentioned he has rebuilt the monovalve, i suspect it first. Apparently the springs in the replacement insert are too weak and relax to close the valve. Several 126 owners have dealt with the issue and reported such. I should have also suggested the original poster unplug the monovalve and take a drive as well, separately, I might add (do one thing at a time).

I'll try and look up the thread in the 126 forum where this issue is fully discussed.

Here ya go... the whole thread is worthy reading, and I encourage the OP to scan thru it...
http://www.benzworld.org/forums/w12...60-sel-heater-problem-not-5.html#post10786673

whole thread.... http://www.benzworld.org/forums/w126-s-se-sec-sel-sd/2162201-1990-560-sel-heater-problem-not.html

 

[tdoriot]

There ya go ! The definitive word by Ears ! I suggest you follow his advice. If that frickin iceberg you live in ever thaws out ! :grin

 

[Ears]

There ya go ! The definitive word by Ears ! I suggest you follow his advice. If that frickin iceberg you live in ever thaws out ! :grin

Well, I don't know about Definitive , but the climate system on the 560 is a fickle beast!
best of luck to the OP, as the refrigerator he lives in appears to be plugged in for a while yet!

 

[modulus]

Taking advantage of blue sky in NY I tried a few suggestions. Disconnected the aux water, closed off the scuttle vents and removed power from heater valve, no change. Noticed the floor vents aren't working. I think because of time considerations I'm after handing this off to a pro at this point. Appreciate the advice and I'll post what is discovered. Thanks.

 

[modulus]

Update: Progress is slow but happening. Found control panel was the problem with the heat going off; replaced with rebuilt one. Also discovered a few vacuum leaks and a broken heater box which had a previous unsuccessful repair attempt. Now back together it goes.

 

[modulus]

Patched heater box crack but discovered a hole in the bottom, probably from someone trying to lever it out over the bar. Now the box needs to come out for repair. Onward.