W124.128 300D A/C Questions

[JamesDean]

Hey everyone,

So when I bought my 300D back in July of last year the ac was working nicely.

The previous owner had it converted to R134A sometime in 2010. It has the quick connect fittings, label etc. It was doing a nice job of cooling when I test drove it as well. I was pleasantly surprised.

I went to kick on the a/c yesterday and I get no engagement of the compressor clutch.

Now I know that could be either no voltage or low/no refrigerant. I'd like to check the voltage at the coil first but I couldn't see an easy to access test point/connector. I figured I'd ask here to see if anyone knew of the best point of access.

Assuming the clutch is getting proper voltage, I might be low on R134a. I know most of you dislike using 134A in these R12 cars but for the time being that what I've got and I'll stick to it until my 190E is ready for daily driver duty. My question is..If I buy a few cans of 134A what pressures should I expect to see on my manifold?

Do I just follow the 134A pressure/temp chart? I know you are supposed to use less 134A when in an R12 system or something along those lines..

I was also going through the fuse box and I found that fuse "D" was missing. The sheet on the box lid indicated that D was for the aux fans.. Interesting. Maybe I removed it..I'm not sure. I re-did the whole fuse box with nice copper fuses anyway.

[JimFreeh]

As a general rule, R134a is filled at around 80% of R12's weight.

An effective way to "guess" at the proper fill level is to observe the large return line to the compressor (near master cyl) for temp. Keep adding freon until you feel the line go cold, then stop. The onset of cooling happens quickly. If you overfill R134a, your heat transfer will fall off, so no benefit to bumping in too much.

Your pressure is dependent on temp and humidity, but in lieu of any numbers, I've found 50/250psig to be a default starting point. If you go negative on suction, you are low.....

Jim

[JamesDean]

Thanks Jim,

I gave it a quick go last night. I looked up the pressure/temp charts for R134a.

From memory, it said low side 30-40. High side 160-170 for 70F ambients. So I put that in for now.

My vents are blowing cold, not sure how cold but i've got another can still so maybe I'll have to add more.