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Old 06-19-2001, 12:19 PM
Registered User
Join Date: Apr 2000
Location: Indianapolis
Posts: 109
I'm struggling with the results of the R134a conversion on our 1989 300TE (done late last summer, seemed to work OK). The mono valve diaphragm and plunger look OK. Tonight I am going to clamp the hoses to/from it to see if that helps get the car cooler, both to certify the mono valve is OK and maybe to get some insight on whether the control panel might be causing some problems.

ANYWAY, I'm really wanting to go back to the dealer in Knoxville who convinced me to do the conversion when we had a new AC hose/manifold installed to put it back to R12. I've seen some comments, from the renowned stevebfl especially, that it is not recommended to convert a pre-1990 model year W124 to R134a.

My question - are there differences between the AC systems before and after 1990 that make post-1990 more amenable to the conversion? I noticed on a neighbor's 1995, for example, that it had two auxiliary fans, our 300TE has only one IIRC. Compressor output? Evaporator size?

Wondering if there are some components to change which would still be fairly economical in the face of going back to expensive R12.


Sweating in Indiana,

89 300TE 165,xxx poor A/C
84 733i 5-speed 150,xxx very poor A/C
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Old 06-19-2001, 04:12 PM
Join Date: Mar 2000
Location: Gainesville FL
Posts: 6,844
The early 124 cars to 1990 had a smaller compressor that just isn't up to the task. The later cars have a larger compressor and the mounting is different which keeps one from using the later compressor.

Before worrying about the conversion be sure there are no other problems. I would worry about the recirc door being open.
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician
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Old 06-19-2001, 04:13 PM
Registered User
Join Date: Jun 2000
Location: Plano, TX
Posts: 2,476
Well, I'll jump in whilst we await Steve's response.

One difference is the compressor. While all the 124 cars use Nippondenso, the earlier cars use lower capacity compressors than the later cars. The early cars use the 10p15 or 10pa15 compressors, while the later cars switched to the 10p17/10pa17 compressor. About 15% more displacement, definately better performance at idle and low engine speeds.

Other changes - the expansion valve is calibrated a bit differently for R-134a vs. R-12, though this is a minor issue.

Whether the condenser and/or evaporator were changed I don't know. As you've noticed, the cars which came with R-134a from the factory had dual electric fans, presumably to increase airflow across the condenser.

It's tough to believe your car is inadequate that far up North. I'm driving an '87TD wagon in Dallas, and while the a/c is not overly powerful, I get by just fine. A bit more cooling around town would be nice, but it's fine on the highway. I recently measured 47F discharge air on a day when the ambient temp was just a hair shy of 100F.

Here's some ideas for improved performance. I'm making these up as I go, so be gentle:

1) Make sure the fresh/recirculate doors are operating correctly.
2) Tint the windows - I've had good results with metallized tint, really cuts down on the radiant heat.
3) Make certain the correct auxiliary fan swith (on the dryer) is installed. Red color switches on the fan sooner than green.
4) Remove the resistor which runs the aux fan at low speed, run it at high speed when switched on by the a/c.
5) Have the R-134a charge checked - a bit low and cooling is suddenly no good.
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Old 06-19-2001, 05:30 PM
Registered User
Join Date: Apr 2000
Location: Indianapolis
Posts: 109
Thanks SO MUCH for the replies. Excellent technical info, and I appreciate the experience of the Dallas poster.

I will dig into the recirc doors tonight -

May I just remove the low speed resistor and jumper it with appropriately-sized wire?

THANKS AGAIN - I'll have to buy something from PartsShop soon - one of my two cars always needs something!

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Old 06-19-2001, 05:42 PM
Registered User
Join Date: Mar 2000
Location: Springfield, MO
Posts: 71
Pressure Switch Changing

I have a green switch on my aux. fan, does the system have to be evacuated to change this to the lower opening pressure red switch i have been hearing about?

I believe my recirc/fresh air door is not working, when I hit the recirc button nothing changes, where is this door located so I can see if it is working?

1987 Mercedes 260e (w124)
1968 Chevy Truck "Goldie"
1985 Brunette "Margaret"
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