Heat stuck on full blast 85 SD - PeachParts Mercedes-Benz Forum

yuke · #1 05-24-2015, 06:33 PM

On way to work today my my heat seemed to stick on full heat even thou I had the selector on min. Setting it was roasting me out. I pulled over and unhooked the 2 wires going to the top of the monovalve it seemed to be at least bearable then. Any ideas what failed or what to look for? My a.c. is not working I can get by but not with my heat on full blast.

mach4 · #2 05-24-2015, 07:04 PM

Taking the connector off the monovalve would cause it to remain open - full heat.

What you describe happened to me once going through Phoenix in the summer - it was the connector. The quick on-the-road fix was to twist the wire so it put sideways pressure on the connector allowing it to make contact. The back-at-home fix was to take the connector apart, tighten the contacts and re-assemble.

You could have a torn diaphragm, though those seldom fail all at once.

Check the connector to see if it's getting power. If it is the problem is on the monovalve side, if not its with the control circuit somewhere.

yuke · #3 05-24-2015, 07:53 PM

Thanks mach I'll check tomorrow.

whunter · #4 05-24-2015, 07:58 PM

Mono valve diaphragm FAILURE is the most common answer to this issue.

The circuit is GROUND controlled = loss of ground or the control unit has failed.

One simple EMERGENCY - TEMPORARY fix = carefully pinch off the auxiliary water pump supply hose near the alternator.

I suggest using automotive hose pinch off pliers to avoid damage.
https://www.google.com/search?q=automotive+hose+pinch+off+pliers&ie=utf-8&oe=utf-8

Diagnosis and repair can wait until you have time.

.

yuke · #5 05-26-2015, 08:40 AM

Ok yesterday got a little time to look more closely at my heat issue I pulled the 2 wires off the top of the monovalve and checked them for voltage with key on yes they had 13.6 volts. I took them out of their holder and crimped them a little tighter to make sure they are makeing a good connection. The results were the heat still stays on maximum heat for about 12 miles then it goes off by itself when I stopped to refuel this morning the heat was off when I stopped but assoon as I restarted to leave heat was back on and in less then a mile I was home. I am leaning to your thought Hunter that its the monovalve. Even though a mechanic replaced mine about 4 yrs. ago I guess another one could have went bad. About 2 weeks ago I did flush some water thru the heater core both ways with a garden hose was wondereing if that is when I might have damaged the monovalve. Are they hard hard to replace? In the meantime I think I might just pinch the hose right at the firewall until I can locate and install the monovalve.

mach4 · #6 05-26-2015, 10:40 AM

Probably the diaphragm. It's a pretty easy fix. Basically take the four screws out on the top and the module slips out. Replace the insert, and screw back together.

This should help - W126 Monovalve repair

The only thing tricky is keeping the parts in order as you take them out.

whunter · #7 05-27-2015, 04:20 AM

Quote:

Originally Posted by mach4

Probably the diaphragm. It's a pretty easy fix. Basically take the four screws out on the top and the module slips out. Replace the insert, and screw back together.

This should help - W126 Monovalve repair

The only thing tricky is keeping the parts in order as you take them out.

I agree, it is an easy DIY, typically 10 minutes or less for me on a cold car.

However, there is a small chance his flush stirred up loads of gunk blocking the diaphragm and/or screen.

If he is VERY lucky, it might be a case of remove, clean, install, test.

Diaphragm inspection is needed in either case.

.

yuke · #8 05-27-2015, 08:31 AM

Thanks for directing me to that replacement article on the monovalve Mach I have one on order. You made a good point Hunter in that it is strange that it started acting up after I did a flush on the heater core going both directions with a garden hose yes it might have stirred some gunk up and partially blocked it. I do find it very strange that the last 3 trips I made with the car heat will be on for about 13 miles and then for the remainder of my trip about 37 miles the heat is off that happens with out touching the select temp. dial at all. Yes it seems like a 10-15 min. job I will pull it out as soon as I get a chance and will let everyone know the outcome. I did read where someone put a shut off valve on the hose going into the heater and that would guarantee no heat getting into the cabin in the summer. Seems like a good thing to install just in case of monovalve failure on a hot summer day. Thanks for the good ideas.

Maxbumpo · #9 05-27-2015, 08:37 AM

I suspect I've got a monovalve problem in my '87 300TD. If I move the temperature control wheel too far from cold, with the AC on, I get hot air from the side vents. Putting the wheel back to maximum cold and the heat will continue to flow from the side vents for five or ten minutes, and then gradually subside.

My point is that your monovalve may only be leaking or passing hot water for a short period of time, but once that heater core is full of hot coolant, it take a long time for the hot air to stop, even though the coolant flow may have stopped.

yuke · #10 06-01-2015, 10:24 PM

Okay the new monovalve insert came in and I took the old one out and inspected it. The screen on it was all gunked up with rust it seemed from my just recent engine flush. The rubber diapham seemed fi e with no rips but since I had new one I installed the new one and went for a ride. Well now I had jjst the oposite problem of no heat at any time and I noticed I had a coolant leak from the monovalve. Took the monovlave apart figureing I did'nt have the rubber diapham seated correctly put it back together and took a test ride I seem to have stopped the leak but still cannot get any heat. I am fairly sure I put the washers back the same way they came out there were 2 flat washers and 1 wavy one. What seemed strange is that the 1st two times I took the valve out it seemed like it was under preasure when I took 4 screws out and left a burst of air and coolant out where as the last time I took it out the engine was just a little under operating temp. and I was trying to be real careful as to not getting burned if some hot coolant should burst out under pressure and to my surprise it was under no pressure at all and I lost no coolant when I pulled the monovalve up. Wonder what could be wrong now was wondering if maybe I have some air trapped in that area preventing the coolant from circulating. Got dark on me will get back on it tomorrow. I even tried to shut car off and restart it thinking it should revert back to heat on when doing that but I still got very little or no heat even took the 2 wires off the top and unplugged it thinking that would put it to the full heat mode and it did not. Puzzled as to what is happening now.

whunter · #11 06-02-2015, 02:38 AM

Quote:

Originally Posted by yuke

Okay the new monovalve insert came in and I took the old one out and inspected it. The screen on it was all gunked up with rust it seemed from my just recent engine flush. The rubber diapham seemed fi e with no rips but since I had new one I installed the new one and went for a ride. Well now I had jjst the oposite problem of no heat at any time and I noticed I had a coolant leak from the monovalve. Took the monovlave apart figureing I did'nt have the rubber diapham seated correctly put it back together and took a test ride I seem to have stopped the leak but still cannot get any heat. I am fairly sure I put the washers back the same way they came out there were 2 flat washers and 1 wavy one. What seemed strange is that the 1st two times I took the valve out it seemed like it was under preasure when I took 4 screws out and left a burst of air and coolant out where as the last time I took it out the engine was just a little under operating temp. and I was trying to be real careful as to not getting burned if some hot coolant should burst out under pressure and to my surprise it was under no pressure at all and I lost no coolant when I pulled the monovalve up. Wonder what could be wrong now was wondering if maybe I have some air trapped in that area preventing the coolant from circulating. Got dark on me will get back on it tomorrow. I even tried to shut car off and restart it thinking it should revert back to heat on when doing that but I still got very little or no heat even took the 2 wires off the top and unplugged it thinking that would put it to the full heat mode and it did not. Puzzled as to what is happening now.

It sounds like you have air trapped in the heater core.

.

yuke · #12 06-02-2015, 08:44 AM

Yes it is very possible there is air in the heater core what is the best way to get it out?

yuke · #13 06-02-2015, 01:01 PM

Okay went out this morning again and got back to the monovalve. The cars climate system worked fairly well before I flushed the cooling system includeing the heater core so I thought that flushing had to cause my problem now. When I pulled the old monovalve out it the screen on it was totally blocked so I cleaned it and thought I'd keep it for a spare as this was the second one I put in it in about 5 years. I ordered a new one from autohauz for a cost of about $20 I put the new one in. I did not have no heat at all now just oposite of what the problem was originally. So I put the old monovalve back in because in my searching I have seen a lot of people have had a problem with the MTC valve which is the one I bought.I have read that the valve fits snuger in the electro magnet body then the my old one and yes it did the tension on the spring pintle seemed to be the same on both but all I can conclude now is that the problem was a clogged up screen and that the MTC replacement valve I got was defective. I guess I will try to return it and get a refund. The climate system works as it should now except that when I put on the defrost it goes to full heat and nothing less. I have read other people have had the same problem and I really dont want to get into that as I can live with that. I have an 85 300 SD hope this can help someone else with same problem. Thanks for the replies.

BillGrissom · #14 06-02-2015, 03:46 PM

I don't know how a clogged screen could cause a "max heat" condition, but perhaps I missed something.

funola · #15 06-02-2015, 04:25 PM

The screen is deadheaded and leads to nowhere. If it did lead to somewhere, it would be sending coolant into the electromagnet, not desirable. AFAIK, no one has figured out the purpose of the screen. If you did, please share.