Tried a good temp controller and it did not make any difference. Will be sending push button unit to JD to see if he can do anything with it. I did find a good bit of oil in push button unit where it looks like the oil line going to instrument cluster has been leaking, got into harness to push button unit and put oil to it.

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Jim

FSM 83-600 has comprehensive step by step diagnostics on troubleshooting ACC problems. It may give you a headache and eye problems afterwards though.

Edit: Have you put your hands on the in/out hoses of the monovalve to see what they feel like? That will tell a lot as well as putting a light bulb (careful! elec short warning) across the monovalve and observe it while you drive.

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

I asked this same question a while ago but no one knew the answer. I found the answer in FSM 83-600 after I shorted the monovalve by accident/ blew a PBU trace and had to troubleshoot.

The monovalves's check valve's function is to keep the heater core from filling with hot coolant when the engine is shut off.

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

Found the wiring harness connector at the pushbutton unit full of oil. Shipped pbu to JD today for refurbishment. Will try to resolve oil issue and clean harness up.

Attached Thumbnails

 

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Jim

Have the same issue with my 85 right now. Replaced the thermostat a couple years ago because the car always running too low of temp. That fixed that. But on interstate I have little heat and I noticed my cars running half the temp it should, (has to be below 40F) to see this happen... But as soon as I slow down, or take an exit, it's blasting heat out.

BTW, how did oil get into that harness? The only oil I know of coming into the cab is from the oil pressure sensor and thats not even close?

Not as much as being close as to being above. From my initial observation (will pull instrument cluster this afternoon) the harness is running underneath the steering column below the area of the instrument cluster where oil may be tracking down to the harness.

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Jim

I have dealt with this issue in the past. I have heard a theory about new monovalve inserts having weak springs that do not allow it to open against the fast coolant flow at highway speeds.

I forget... is there a way to run without the monovalve insert in place? Or a way to prop it open? Maybe tearing the rubber off an old one and running it as a test?
Just some ideas....

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I could not own any MB without the help from you...fellow forum members.


83 300td 207,XXX

I had an issue where the traces on the climate control circuit board actually burnt out from high current, of course in the dead of winter. So in a pinch what I ended up doing was removing the monovalve and instead installing an aux pump I found off something in the junkyard; as well as wiring the blower motor to a potentiometer (power coming from the fuse box & fused) installed on the dash. Worked fantastic since I was slightly overvolting the blower causing it to blow harder.

Figured I would update this thread.

I received Jim's oily push button unit. There was no electrical damage from the oil. Just messy. I did find a few cold solder joints here and there as I've come to expect on these units.

Here is a photo album of the work. I am going to let it sit under my fans for the night just to be sure all of the cleaning agents/alcohol is dry.

https://imgur.com/a/kfVcv

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Cruise Control not working? Send me PM or email (jamesdean59@gmail.com). I might be able to help out.
Check here for compatibility, diagnostics, and availability!

(4/11/2020: Hi Everyone! I am still taking orders and replying to emails/PMs/etc, I appreciate your patience in these crazy times. Stay safe and healthy!)


82 300SD 145k
89 420SEL 210k
89 560SEL 118k
90 300SE 262k RIP 5/25/2010
90 560SEL 154k
91 300D 2.5 Turbo. 241k
93 190E 3.0 235k
93 300E 195k

I should have updated my part of this thread too. I found where the oil line to the oil pressure gauge had leaked int he past. It has been fixed but there are still oil stains on the carpet to the right of the drivers foot area.

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Jim

great alternative!!!

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83 300CD- sanden, dual p/f condensers, 160a alternator, ect
91 300TD- 722.6, #22 head, 3.5L IP, w140 manifolds, ect

This worked without having a regulator for the blower motor?

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Mac
2002 e320 4matic estate│1985 300d│1980 300td
Previous: 1979 & 1982 & 1983 300sd │ 1982 240d

“Let's take a drive into the middle of nowhere with a packet of Marlboro lights and talk about our lives.” ― Joseph Heller, Catch-22

Either it was a beefy pot or (assuming early ACC) there is a blower motor control unit that I believe takes in a 0-x V signal and switches through some resistors (using its relays) under the brake booster.

__________________
Cruise Control not working? Send me PM or email (jamesdean59@gmail.com). I might be able to help out.
Check here for compatibility, diagnostics, and availability!

(4/11/2020: Hi Everyone! I am still taking orders and replying to emails/PMs/etc, I appreciate your patience in these crazy times. Stay safe and healthy!)


82 300SD 145k
89 420SEL 210k
89 560SEL 118k
90 300SE 262k RIP 5/25/2010
90 560SEL 154k
91 300D 2.5 Turbo. 241k
93 190E 3.0 235k
93 300E 195k

Regarding the monovalve insert: As one poster stated the MTC brand spring is too weak to overcome the water pressure at road speed. Probably because the MTC unit uses a coarser screen than the factory unit. Only difference I can see. Springs feel the same.

So I put a ~3/4 inch long compression spring under the rubber valve down inside the valve body, which effectively gives the spring inside the insert more power. Now the MTC unit works at road speed. Without the extra spring -- no heat any time. I hope the spring doesn't escape and find its way to the water pump. I probably should have locwired it to something.

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98 E320 4matic Wagon
84 300SD (2 ea.)
51 Chev 1-ton PU
Assorted old motorcycles

After doing some more reading I've about decided to order a complete monovalve assy from the dealer.

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Jim