AC Charging Questions
 

On the 95 E300D I'm selling, the AC doesn't work. However, I have confirmed that the compressor engages when the low-pressured cut-off switch is jumpered. So that's a good sign.

I tried to add some refrigerant to see if I could get the compressor to engage. But, after almost a full 12 oz. can, it wouldn't. I saw a static low side pressure of about 70....but I gather that's a meaningless number.

Should I try to add another can? Or is that risky?

I know that the proper way to see what's going on is to evacuate and recharge the system. However, I'm not willing to spend money on getting the AC diagnosed....I will leave that to the new owner.

Thanks.

The 70 psi static reading isn't meaningless.... it tells me the ambient temp when you took the reading was somewhere around 70F.

The safety cut-out switch doesn't activate until the system pressure gets very low, like around 20 psi. Since you can jumper it and get it to engage it would appear at least your 1st issue is in the compressor control circuit or a bad safety switch.

Another can of R134 won't help - 70 PSI is plenty high enough to energize the safety cut-out circuit.

Maybe the Klima relay is preventing it from engaging somehow? Can you get a gauge on the high side and see what it is doing?

It could be a bad low pressure switch. Unfortunately you have to evacuate the system to replace it. On a car with over 300K miles, I'd be tempted to jump the low pressure switch, with full disclosure to the buyer of course, and enjoy the summer. I've never heard of a high pressure failure in these cars though you might have tilted the table with the extra 12oz :) Cooling capability will be notably diminished before the low pressure trigger so you'd know to do something about it if it fails in the summer.

Sixto
87 300D

Try another Klima relay.

That's an interesting idea.

That was my thought originally....kinda pricey though. I wish I had a known good one I could try. That's why I love having the 91 and the 92....I can diagnose by swapping parts.

Sixto, in a situation like that, if the compressor fails due to low refrigerant (and a jumped low pressure switch), does the clutch disengage or does it stay engaged and thereby mess up the belt?

If the compressor disengages, no belt issues. If the compressor stays engaged, like if the low pressure switch is jumped, and the compressor locks up, well belt damage can occur.

That is why I like the old V belts. You can cut the compressor belt and not worry about the water pump, alternator, etc.

Not true on the 95. The system looks at the speed of the compressor and compares it to the speed of the engine and disengages the compressor if there is a speed difference.

Try another over voltage protection relay (this is what I meant to say in reference to the Klima relay). They should be interchangeable between the 95 and 93.

Weirdest thing....woke up, armed with gauge and charging gun, read zero static pressure, charged the system, and it blows ICE COLD. Must be losing my mind.

So, we'll see how it goes. Low pressure cut-off appears to be fine though.

Ha it healed when it was afraid of going under the knife. Seems to me that sometimes the systems on cars need 24 hours to settle for the refrigerant to circulate and equalize. Odd