First too much heat, now no heat. Tale of w/o (heat...) - PeachParts Mercedes-Benz Forum

micalk · #1 09-29-2008, 10:34 AM

Thanks to all who responded to my thread re: monovalve. I wired it straight to 12V/gnd and the heat went away.

Decided to get adventurous and resoldered all the connections in my ACC Control Unit. Plugged the monovalve back into the original connector. Now I have no heat. Disconnected the monovalve and still no heat.

Reading the service manual, the circulation pump is on when the "legroom flaps" are energized. Operating the CCU, when the temp is moved from cold to hot, the vents all change so I'm assuming that this part of it is working.

Does any hot water flow to the heater core without the circulation pump or is it absolutely necessary that the circulation pump operates to get any heat at all? I haven't had it on the freeway yet for extended high rpm drive, but will this morning.

Jeremy5848 · #2 09-29-2008, 11:12 AM

The aux coolant pump is a "helper" for low-driving-speed, cold weather conditions. You should get heat even if it is not working, unless it is frozen and/or clogged up. You can test the pump separately by connecting it to 12 volts and listening to hear it run (you can connect the wires either way, it doesn't matter for the test which way the motor spins). You can replace the pump with a length of pipe and do without it completely, especially if you are in a warm climate.

SD Blue · #3 09-29-2008, 11:13 AM

Sounds like you need a ~$20 monovalve rebuild kit. Super-simple repair on the W126....four screws, replace innards.

qwerty · #4 09-29-2008, 12:21 PM

Quote:

Originally Posted by SD Blue

Sounds like you need a ~$20 monovalve rebuild kit.

Have you priced one lately?

Graplr · #5 09-29-2008, 12:32 PM

As I see it, really there are only two options for this (Now no heat)

1- The monovalve is stuck closed- To fix order a new repair kit

2- You messed up your ACC when you resoldered it. This is actually very common. Don't ask me how I know.

- To fix you would need a new ACC.

I would lean towards #2 since it is a bit odd the monovalve stuck closed all of a sudden, but not impossible, and becuase it worked before you resoldered and didn't work after you resoldered.

micalk · #6 09-29-2008, 03:21 PM

Drove it into work, about 35 miles mostly freeway. No heat the whole way. And that's is the thing that tells me "It's not the control unit. Yet." I'll disassemble the monovalve again, and see if it is stuck. And now that I have a connector wired up, I'll see if it's operational outside the housing before installing it again. I have another monovalve that I can try as well. This was the newer of the two so that's why it was installed.

IIRC, monovalve rebuild kit was ~$18 from a local place in Glendale, CA.

micalk · #7 09-29-2008, 04:17 PM

IDRC (I don't recall correctly).

Monovalve repair kit was ~43 at local place.

qwerty · #8 09-29-2008, 07:54 PM

Quote:

Originally Posted by micalk

Drove it into work, about 35 miles mostly freeway. No heat the whole way. And that's is the thing that tells me "It's not the control unit. Yet."

Remove the electrical connector from the monovalve. If that results in lots of heat, the problem is a control issue and not the valve.

micalk · #9 09-30-2008, 01:09 AM

Quote:

Originally Posted by qwerty

Remove the electrical connector from the monovalve. If that results in lots of heat, the problem is a control issue and not the valve.

Tried that earlier today, and still no heat. But after reading some of the posts here I banged on the monovalve with a screwdriver handle. Had heat again while connected to battery.

With the CCU on Max heat, I replaced the connector from the CCU to the monovalve, and the heat remained. Turned the temp wheel just off max, and the temp went cold. Driving home I fiddled with it.

So now I can turn it on max heat, bang on the monovalve and the heat comes on, turn it off max, and the heat goes off. I've been able to get it to cycle a few times before it gets stuck in the energized position. I can also get it to release by hitting "max" a few times and then leaving it there.

I'll play with it for a while before replacing it, and see if I can cycle it into submission.

Does the end of the valve sit against a seat, or does it slide inside sleeve? I'm guessing seat.

Graplr · #10 09-30-2008, 09:48 AM

Looks like it is definitely the monovalve then.

Buy a new replacement kit. You don't want to bang on it every time you need heat.

PM sent.

micalk · #11 09-30-2008, 11:31 AM

When I drove in to work this morning, the temps seemed moderated, but in this mild weather its kinda hard to tell yet. I probably won't change it out til winter when those morning temps can get way down into the thirties and I have to heat the glow plugs to get the engine started. By then I'll know for sure if it is still intermittent.

I just bought this car, and as you can see in the signature, it has low, low miles. So I'm thinking that it just needs to be used a bit to loosen things up. When I first got it, it groaned whenever there was any suspension travel - it was especially noticeable when stopping as the nose dove down.

Thanks, everybody, for sharing your thoughts.