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Old 06-13-2010, 10:10 AM
Registered User
Join Date: Aug 2006
Location: Houston, TX
Posts: 450
Reluctant AC '92 300SE

Freon level is to spec.

A few DTC's - all in the intermittent category:
ECT sensor - open short (041)
In Car Temp Senaor - open/short (019)
Coolant Pump - open/short (075)
The open/short for coolant circulation pump has been here and gone since I got it 5 years ago, even after replacing the pump and having the HVAC control rebuilt. I finally decided to bite the bullet and replace the HVAC wiring harness. While I was at the parts counter at my local dealer, the 140 guru tech from the service department came in. He asked why I was ordering that harness and I told him. He said "save your money - I've never seen one actually fail, and that trouble code shows up in 95% of the 140's I work on. As long as the pump is working, ignore the message..."

So - the problem:
On occasion, you can start the car and the A/C blows cold. Other times, no. When it doesn't blow cold, the compressor isn't engaging. When it does engage, everything works as it should, no abnormal noises from the compressor or anything. If the A/C works when the car starts, it will continue working as long as the car is running.

I'm wondering if the coolant temp sensor could be at the root of the problem? I know part of it's purpose is to prevent A/C at very low temps.

Here's what I'm thinking for diagnostics:

1) Next time it occurs, pull the connector from the coil on the compressor and see if 12v is present there. If that's the case, then the coil has an intermittent short.

2) If #1 reveals nothing, pull the connector from the pressure sensor on the dryer. Bridge those connectors and see if the compressor comes on. If it does, then the pressure sensor is at fault.

Any other suggestions? It's gettin' hot here in Houston - we need A/C...

W. Brian Fogarty

'02 S55 AMG (W220)
'92 300SE (W140) - sold
'76 450SEL 6.9 Euro #1164
'76 450SEL 6.9 Euro #521

"Bond reflected that good Americans were fine people, and most of them seemed to come from Texas..." Casino Royale, Chapter VII
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