No temperature control from thumb wheel

[JeffSpicoli]

Hi All,
I am the new owner of this 500:

FOR SALE - FS: 1993 500E, Black/Black, 162kkm, $26k, Japan import (Atlanta, GA) | Cars for Sale - 500E / E500 ONLY

Good evening! No affiliation, looks interesting! Japan market 93 500E recently imported. VIN = WDB1240361B909399 https://atlanta.craigslist.org/atl/cto/d/norcross-1993-mercedes-benz-500e/6968898080.html 1993 Mercedes-Benz 500E - $26000 (Atlanta) Up for sale just imported euro spec...

www.500eboard.co

Great car and great experience so far. I will start an owners thread soon to update all the interesting adventures I'm enjoying. But I digress, today I am asking for help diagnosing a pesky HVAC issue.

On my 1993, with the down arrow pushed on the dash unit, I get A/C out of the vents, all four, ice cold. Normally in Fort Lauderdale this is great, however in our "winter" sometimes the temps plummet into the 60's and having ice cold A/C as the only option is not ideal.

I did the searches and reading and did the tissue test on the small vent near the dome light. It failed. I changed the aspirator this weekend. Unfortunately no change but the tissue test now passes.

So, even with the thumb wheel on full hot, no heat comes out of any vent on the down arrow and the up and down arrow. BUT, if I put it on defrost with the thumb wheel on any setting I get hot from the left and right vent and defroster, and cold from the center vents.

From what reading I've done on here (incredible resource btw) it looks like I need a new monovalve, but I'm reaching out to the experts for their advice before I dig in.

Thanks for reading!

 

[gsxr]

Try disconnecting the wires to the monovalve first. If you get heat back from at least some vents, the monovalve may be OK.

 

[JeffSpicoli]

I have heat from the left and right vent if the push button setting is on defrost. On that setting it also puts the fan to high regardless of the fan control button, is this normal?

 

[gsxr]

I have heat from the left and right vent if the push button setting is on defrost.

Hmmm. Sounds like a control issue of some sort, but if you are getting cold air out the center and heat out the sides, the monovalve is probably OK. If the monovalve was bad (stuck on full heat) you'd never get AC. What happens if you click the temp control wheel to the detent at max cold or max heat? Any change?

On that setting it also puts the fan to high regardless of the fan control button, is this normal?

Yes, max fan in defrost mode is normal (explained in the owner's manual).

 

[JeffSpicoli]

No change at all.

 

[gsxr]

You might need to find a specialist who knows these cars well. I'm not sure what would cause the symptoms you describe. If it were me, I'd swap in a different pushbutton control unit, which is the brains behind the climate control system. Used ones are pretty cheap and easily available, and fairly easy to swap out.

For grins, check all the fuses (rotate them each in their sockets), and temporarily unplug the auxiliary pump for the coolant, located directly in front of the CAN (computer) box under the hood. I'm not expecting any change, but if that doesn't do anything, I'd lean towards swapping the PBU next.

 

[JeffSpicoli]

I need to purchase an owners manual. This car was a Japanese import, and the books that were included were in Japanese (although the owners manual wasn't one of them).

 

[JeffSpicoli]

Ok, it's somewhere to start. Thank you!

 

[sheward]

You might post an inquiry for an owners manual/books in the wanted section. Gerry and others have provided scavenged sets from parts cars in the past. I may have one available in the next couple of weeks as well.
Congratulations on the purchase! Is the car still here in Atlanta?

drew

 

[JeffSpicoli]

No, we found it from Fort Lauderdale and made a road trip out of it. Car was flawless for the drive back home.

 

[JeffSpicoli]

Oh, and Thank you!