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Mercedes will not cool at over 60mph?

raventbk on Fri September 05, 2014 4:15 PM User is offline

Year: 1985
Make: Mercedes
Model: 300D
Engine Size: 3.0L
Refrigerant Type: R134a
Ambient Temp: 95F
Pressure Low: 34
Pressure High: 225
Country of Origin: United States

I guess I am hoping for a little more focused information than my usual Mercedes support forum has been able to provide. The specs above are the most current.

I've got an 85 MBZ 300D, that has been converted/updated to a Sanden 7cyl compressor, a 16" x 20" PF condenser, and charged with 1.45# of R134. Only items left original were the evaporator, electric aux condenser fan, and expansion valve. All conversion work and initial charge was done at a local indy Mercedes repair shop.

In an attempt to save on reiterating the same story again, I have included a link to my saga on the MBZ forum I frequent...hopefully this will not violate any forum rules.

The short version goes something like this:

Car cools awesome at idle, and low speeds, but craps out and blows ambient air at high speeds.

There is no blend door, but instead a combo evap/heater core in the airbox. There heater core is isolated, so there is no heat fighting the AC.

I've had suggestions that its overcharged, and occasionally the compressor bound up causing the belt to squeak. I used a robinair recycler and found 1.71# was the initial charge, while the hood sticker specified 2.9# of R12. Using the 80% rule, my calculator came up with 2.3# of 134 should be required. Saving more details that are in the link above, I settled on a recharge of 1.45#, which was the least amount of 134 that gave the best cooling at idle.

I still have the problem of no cooling at high speeds (>60mph). I have verified with my GoPro, the compressor remains engaged throughout while vent temps rise during high speed operation. I've really got no way to verify pressures while driving, but I would assume they're still okay as the compressor never clutches out.

Anybody check out my link for the detailed story, and give me ideas on how to proceed? Thanks!

Edited: Fri September 05, 2014 at 4:15 PM by raventbk

mk378 on Fri September 05, 2014 8:48 PM User is offline

It could be freezing up, either the evaporator air side or the TXV due to water contaminated refrigerant. You can measure pressures during a test drive by taping the manifold to the outside of the windshield, or bring it out from under the hood and in through one of the side windows if the hoses are long enough.

Edited: Fri September 05, 2014 at 8:49 PM by mk378

Dougflas on Fri September 05, 2014 9:47 PM User is offline

check out the new Imanifold by imperial. It allows you to see the readings with your cell phone.

wptski on Fri September 05, 2014 10:12 PM User is offline

Originally posted by: Dougflas
check out the new Imanifold by imperial. It allows you to see the readings with your cell phone.
Or the upcoming iConnect which is the iManifold minus the manifold.

vcsmike on Wed September 17, 2014 11:05 PM User is offlineView users profile

I had this problem on an '82 Alfa Romeo GTV6 recently, but on my subject vehicle, the vent temps would go from 45 at idle to 65 while driving. They didn't go all the way up to ambient, but then again, I just drove it around on surface streets; not at freeway speeds.

In my case, the problem turned out to be outside air getting into the recirculating air intake. The seal where the outside air inlet on the HVAC box meets up to the underside of the cowl had deteriorated. So, even though my flapper door on the HVAC box was doing its job just fine, outside air was pouring in past the seal at the cowl. The Alfa's HVAC box isn't a very good design, as is the case with many European cars of this era, & the recirculating air intake is located very close to the cowl. Therefore, the outside air getting past the cowl seal was pouring into the recirculating air intake.

Once I suspected this to be the problem, I tested my theory by completely blocking off the cowl's fresh air inlet. Once I did this, the A/C performed properly at all speeds, & after I made a new cowl seal, the problem was permanently resolved. Unfortunately, I had to pull the dash to make it happen.

Michael Keith
Vintage Car Solutions
Houston, TX

Edited: Wed September 17, 2014 at 11:07 PM by vcsmike

bohica2xo on Fri September 19, 2014 2:00 AM User is offline

Ok, I went & looked at that thread. All I can say politely is that many of those people should stay far away from A/C systems.

First, you condenser is mounted upside down, and likely plumbed backwards too:
Post #15 on MB board So I question the balance of the work done by such a "shop" too.

How much oil is in this system?

Finally, you can't test at idle if you are having trouble at cruise. A full load test should be done with the cabin fan on highest speed, both front doors open & engine rpm @ 1500 or above. Those pressures are the ones to be concerned with.

Get the condenser orientation & flow corrected first.

"Among the many misdeeds of the British rule in India, history will look upon the act of depriving a whole nation of arms, as the blackest."
~ Mahatma Gandhi, Gandhi, An Autobiography, M. K. Gandhi, page 446.

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