I have no hot air coming from my HVAC. I replaced the mono valve a short time ago and all was fine. Checked the mono valve and it is working. Is this a vacuum pot problem? Would re-circulation pump faliure cause this? No matter what button I push there is only cold air.

This might be a problem with the HVAC push button unit. I've seen a few come through my shop here that have cold solder joints on the board. One was actually right on the temperature control wheel.

If you leave the monovalve disconnected you should get full heat all the time. Someone correct me if I am wrong.

Does your coolant level look OK?

Vacuum does not come into play with heat in a W126. Aux water pump MIGHT be suspect if you had low heat at low speed but not no heat at all.

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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

JamesDean,

You are correct. A lot of times, people don't realize that the monovalve is switched at the ground going back to the PBU. People will test for 12v and assume the ground is bad and run it straight to the frame of the car. That causes no heat because the monovalve is always powered and closed. I saw that little "fix" on one my 300SDs.

__________________
1991 F250 super-cab 7.3 IDI. (rebuilt by me) Banks Sidewinder turbo, hydroboost brakes, new IP and injectors.
2003 S430 - 107K
1983 300SD - Tanoshii - mostly restored ~400K+.
1983 300SD - Good interior. Engine finally tamed ~250K.
Monark Nozzle Install Video - http://tinyurl.com/ptd2tge

drove the car for awhile there is some warm air. checked the vacuum lines and have a bad vacuum leak. I just ordered replacement diaphragms and will rebuild the pots (oh joy). let you know how things work out. Thanks for the info. If this does not work I have a spare control unit to install.

If you are going to rebuild the vacuum pods be aware of this:

W124 climate control vacuum pod replacement

__________________
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'll keep that in mind. Rebuilt the pots on an 85 and did not see that problem but that was years ago. Thanks for the warning, I'll keep an eye out for this.