Heater working on/off in '87 560SL - PeachParts Mercedes-Benz Forum

2quik6 · #1 12-17-2007, 12:49 AM

The heater has started working intermitently on the '87 560SL. I was told it may just be the diaphram and takes only a few screws to replace. Does anyone know exactly which part this may be and where its located?
The shop that did alot of work on this car earlier in the year told me it was a simple fix if I bought a rebuild kit of some sort and it should not take longer than a few minutes to swap out only the diaphram part or they could do it. I'd prefer to do it myself if I can figure out which part to get.

Thanks

hey_allen · #2 12-17-2007, 03:07 AM

In my 300SD, the diaphragm (monovalve) was indeed the problem.

If your car is setup like the w126 sedan models, the valve is near the firewall, slightly toward the driver's side from the battery.

If you look at that area, you should find a ~2 inch square cover with a connector at one edge, and a round bit sticking up in the middle, about 1/2" diameter. That little bit in the middle is the top of the diaphragm that you are looking for.

http://catalog.worldpac.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=2830II46329406L13C&year=1987&make=MB&model=560-SL-001&category=R&part=Mono+Valve+Repair+Kit

rocky raccoon · #3 12-17-2007, 08:22 AM

This is indeed a very easy fix. I have recently done it on my 88 560SL. See my post on the SL Forum of about a week ago. Title is "560SL Climate Control Repair Report". The part should cost about $40 to $45 from an on-line source. Much more from a dealer.
rocky raccoon

2quik6 · #4 12-17-2007, 12:07 PM

Thanks very much! My step-daughter should be bringing the car by tomorrow for me to take a look.

ctaylor738 · #5 12-17-2007, 05:39 PM

I'm all for repairing the monovalve, but it might be worthwhile to test it.

The valve works by closing to shut off the flow of hot water to the heater. The higher the temp wheel is set, the less frequently it will close. There are two wires attached to the valve. One is red/brown and is hot when the ignition is on. The other is brown/black and is a ground. The ground is switched by the control unit. When the valve gets ground it closes and shuts off the hot water.

So the test is to remove the connector from the top of the valve, and ground one pin. Then (carefully) touch 12V to the other pin. The valve should click as it closes. If it doesn't, replace it.

It is very possible that the valve is fine, but is getting a constant ground signal from the control unit. This can be caused by a bad control unit, or a bad sensor which makes the control unit think that the cabin has reached the desired temperature and thus tells the valve to stay closed (by sending ground signal).

TX76513 · #6 12-17-2007, 05:43 PM

....and sometimes the cylinder just needs to be cleaned off with steel wool. After a long summer run of no use, they will stick.

2quik6 · #7 12-17-2007, 06:51 PM

Thanks again for the tips!

rocky raccoon · #8 12-17-2007, 07:00 PM

By all means test your monovalve as suggested by CTaylor however if it tests good, the monovalve repair kit is your fix. Since your heat is intermittent however, I suspect a ruptured rubber diaphram on the monovalve piston. This seems to be the most common failure and is probable on your 20 year old car as it was on mine. That diaphram is part of the repair kit and to my knowledge is not available separately.

kchemers · #9 12-18-2007, 05:27 PM

I have a 560 Sl 87 changed the monovalve and my cabin temp still fluctuates greatly, will heat up perfectly then it will cool down unitll its almost cold in the cabin then it will heat up again. Is this the control unit inside the cabin or do I have a vacuum problem????????????

ctaylor738 · #10 12-19-2007, 08:01 AM

That sounds like the foam hose from the dash sensor to the heater box has deteriorated. You get at it by pulling the glove box. You can buy the expensive part from a dealer, or rig up something with pipe insulation from Home Despot. Several posts on how-to.

This hose causes air from the cabin to be pulled past the dash sensor, so the sensor can get a read on cabin temp.

2quik6 · #11 12-22-2007, 11:46 AM

Quote:

Originally Posted by ctaylor738

So the test is to remove the connector from the top of the valve, and ground one pin. Then (carefully) touch 12V to the other pin. The valve should click as it closes. If it doesn't, replace it.

Quote:

Originally Posted by rocky raccoon

By all means test your monovalve as suggested by CTaylor however if it tests good, the monovalve repair kit is your fix.

Ok, I did the test and with voltage applied, it doesn't really click, but makes a very faint thud sound. You can feel it move when the voltage is applied if your finger is on top of the valve solenoid. It is not opening or closing with "authority" you might say.

I'm not sure from the 2 quotes above if it should be doing nothing or opening/closing if its the valve. CT says if it doesn't click, its the valve. In RRs post it says if it tests good..does that mean if it clicks (good) the repair kit is the fix??

Are you 2 talking about different repairs..just the valve in this link posted earlier for the part for $45?
http://catalog.worldpac.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=2830II46329406L13C&year=1987&make=MB&model=560-SL-001&category=R&part=Mono+Valve+Repair+Kit
Or the entire electric valve assembly the wires plug into?

ctaylor738 · #12 12-22-2007, 01:22 PM

Kind of hard to judge the difference between a click and a thunk long distance, but it sounds to me like the valve is OK. I would call mine a soft thunk.

Look at the plumbing. You will see how the lower hose (1) runs to the auxiliary pump, then a short piece (2) to the monovalve body, then a piece (3) to the heater core. On the left side of the blower cover there is another hose (4) that returns the coolant to the engine.

Leave the wiring to the monovalve unhooked, run the engine for a while, turn the heat or defrost on, and see if you are getting hot water to the heater by feeling hose (3), and also check hose (4).

This will tell you if there is a problem with water not getting through the system and if not, where the blockage is.

2quik6 · #13 12-22-2007, 01:56 PM

Well currently with the temp wheel set to "max" its blowing heat, so by taking the wires off it should be simulating "max" by having an open circuit..no ground.
Running the car to get it warm, all 4 hoses became warm at about the same time.

So, I decided to hook up the voltmeter to the wires with them unplugged.
Putting the negative terminal on the ground wire and the positive on the red/black wire, it reads 12volts with the wheel set to "max" ?? Shouldn't "max" give you an open circuit therefore the meter should have read 0v ??

This is making no sense now since it got hot and blows heat with the wires unplugged, and it also blows heat with the wires plugged in and they are feeding 12v to the valve since it appears to be grounded when on "max".

With the temp wheel set to anything lower than "max", the voltage switches on and off every few seconds giving you 12v then 0 v then 12v then 0v again on the voltmeter, which I think is correct as it regulates how much hot water makes it to the heater core that way right?

ctaylor738 · #14 12-22-2007, 03:05 PM

Sounds like the heat control is working fine. You get no ground/valve open on max heat, and then it starts cycling on and off at lower temp settings.

This means the problem is somewhere else. Remember that two things have to happen to get heat - hot water needs to flow through the heater radiator and the flaps have to be set to "heat" to allow air to blow past the heater radiator. These flaps are controlled by vacuum and respond to signals from the push-button unit. When you go to max heat, you can feel the detent click. This "hard wires" the flaps to "heat." When you are at mid-position on the temp wheel, the control unit decides how to set the flaps and it sounds like it's not doing this correctly. This could happen because it's getting a bad reading from sensor, or because it's gone TU.

If you're still up to troubleshooting this, look at this post where a couple of us were trying to troubleshoot heating problems.

View of 107 ACC Modules

2quik6 · #15 12-22-2007, 04:39 PM

Quote:

Originally Posted by ctaylor738

Sounds like the heat control is working fine. You get no ground/valve open on max heat, and then it starts cycling on and off at lower temp settings.

No it appears that I am getting a ground when at the "max" position, which is letting heat flow..but heat continues to flow even when wires are unplugged....so maybe the valve is stuck open now.
But why is it getting ground when the wheel is at "max". Since the voltmeter inputs are connected only to the red/black and brown/white wire it should be reading 0v if the sensor is not connecting to ground at "max".

I'm going to pull the valve out just to inspect it at this point and see if anything is obvious, if not I'll let my mechanic fool with it at this point. Nothing is ever an easy fix on this car it seems.