I am looking for some info on replacing the Ac expansion valve on the firewall of my E420! I am gettin a negative or vacuum on the low side. The compressor is running so I must getting some pressure in the system. I have a new drier and pressure switch on order. I bought a set of gauges and from what I have read if you get a very low or negative reading it indicates a bad or stuck expansion valve and it is not running cool! I can see the valve on the firewall looks fairly easy to get to but I need some tips on replacement! Thanks


Smokie

 

It can also mean no refrigerant in the system. Either way, the low pressure switch should lock out the compressor because it will self-destruct if it runs in a negative for very long. In no case should the low side go below 27.8 PSI because that's freezing.

You only have half of the equation. What is the pressure on the high side? With an outside air temp of 80°F, the high side should be approx. 124-158 PSI, or about 20-35° above outside air temp (I pasted a link to a TP chart).

If the high side is also low, the system is short of refrigerant. If the high side is equal to or slightly higher than the outside temp, then I would say the expansion valve is stuck shut or has a bad element.

http://assets.fluke.com/appnotes/ele...r/1265816_.pdf