No heat while driving 86300sdl - PeachParts Mercedes-Benz Forum

Base1375 · #1 10-22-2014, 01:42 PM

Yesterday I changed the glow plugs and the fuse on the OVP. couple things got fixed and now I only have heat while the car is parked or driving under 15mph. As soon as I speed up I only get cool air. Anyone have any idea why this is happening? I also changed all the fuses in the box since I got the kit to change them all, maybe that had something to do with this? Another coincidence is now the AC works.

SD Blue · #2 10-23-2014, 11:58 AM

I would suspect the monovalve. Here is a diagram if your ACC circuit:

http://www.startekinfo.com/StarTek/outside/12253/program/ETM/ACCfrom86.pdf

There are rebuild kits available to repair the monovalve.

Base1375 · #3 10-24-2014, 08:05 AM

Is the mono valve the 40$ part that i can just drop right in or is it the whole assembly? Also had the thermostat and water pump changed less then a month ago, when i try to put the heat on at times the temp on the dash drops under 80, if that has anything to do with it....

SD Blue · #4 10-24-2014, 09:01 AM

The mono valve has a replaceable insert. Many sites/dealers may list it at ~$40. However, I've seen it priced today at ~$15. The entire valve assembly is probably near $200, if it is even still available.

Your last post gives me some hesitance. It almost appears that, with the drop in temp, that coolant is entering the heater. Are you getting air from the knee vents? Where are you checking for heat?

Shooter · #5 10-24-2014, 09:14 AM

I have a 1988 500sl and a 1988 420sl that does the same thing. I have replaced torn monovalve inserts on both but hasn't fixed this issue. They had heat full time to start with and the monovalve inserts have fixed that. The system checks ok everywhere else and I suspect plugged heater cores. Tried to flush both and maybe a little improvement. I have unplugged the aux water pump on the sl to slow the water flow and it helps but its not fixed. Both customers have declined repairs so I cant say 100% if that is the problem. Curious to know if anybody has had a heater core replaced to fix this problem.

Base1375 · #6 10-24-2014, 10:46 AM

Quote:

Originally Posted by SD Blue

The mono valve has a replaceable insert. Many sites/dealers may list it at ~$40. However, I've seen it priced today at ~$15. The entire valve assembly is probably near $200, if it is even still available.

Your last post gives me some hesitance. It almost appears that, with the drop in temp, that coolant is entering the heater. Are you getting air from the knee vents? Where are you checking for heat?

I have never gotten any air flow from the feet/knee area, and when i turn the heat on and its blowing hot air when the car is stopped its coming from the top at the windshield and the two side vents by the windows if that helps at all, going to pull the mono valve out soon to check it out...

pawoSD · #7 10-24-2014, 11:44 AM

If you get heat while sitting its probably a torn monovalve seal, sometimes the actual electromagnet will get weak and won't open the valve properly too, for that you usually have to find a good used one...not sure if they are available new anymore. Or you could hand rebuild it with copper wire....

SD Blue · #8 10-24-2014, 01:33 PM

Here is the info from the FSM:
Mercedes-Benz 126 Service Manual Library

Maybe it will help to sort things out. It sounds like you may have at least one vacuum pod leaking as well.

Base1375 · #9 10-27-2014, 10:57 AM

Well i unplugged the switch on the top of the valve and now i get heat all the time but a small amount, working well for me now since its cold out...i didn't realize i had to drain all the antifreeze when i was taking off the 4 flathead screws, going to need to drain all the fluid before i take those 4 out to check the mono valve I'm guessing now...

SD Blue · #10 10-27-2014, 11:22 AM

It isn't necessary to drain the coolant. When I did mine, it lost maybe a cupfull. Since the monovalve is so high in the coolant system, there is minimal loss. Just be sure to do the replacement when the engine is cool.

PaulH · #11 10-28-2014, 01:54 AM

Anyone know where to get a GOOD monovalve?

fm900 · #12 10-28-2014, 02:47 AM

I just fooled with mine, and rewound the coil, not for the faint of heart, BUT worth the try if your looking for a challenge like I was. If the seals start going, they leak into the coil, then it shorts from their and goes bad. People sell them on Ebay, trying a used one will hopefully work, but up to you if you want to take that chance. I have an extra one out of a '97, some of them are the same, especially the COIL part on top, you can switch them out of they are same size. Mine is the 93, 300d, 2.5. Heat was blowing out side vent with AC on, now it's fixed, see my last couple posts under fm900. My seals were OK, it was my coil that was bad, your BOTH, seals and coil may be bad, have to see. Good luck.