Hi Scotty,
Thanks for taking questions, and congratulations on your channel doing so well.
In your video on using AC pressure gauges, you said that if the reading on both high and low sides were about the same, then the compressor wasn't doing anything and needed to be changed.
Here are the details of my situation;
> 97, F150, 4.2l, manual/5 speed, ~ 125k miles
The AC had been short cycling for a long time (on/off multiple times per minute, multiple seasons) but had been super cold, so nothing was done until ...
Initially (after noticing no cold air), I observed the ac clutch was engaging very intermittently (usually only when first started up for the day). Soon it ceased to engage completely.
Here are the steps I took, in order;
1. Replaced low pressure switch = same results
2. While awaiting arrival of pressure gauges, tried jumping the terminals on the low pressure switch connection, this to see if clutch would engage, indicating a probably low charge of refrigerant. Results = clutch still did not engage.
3. On AC clutch plate, removed a single shim and cleaned up some crud that may have been adding space there = clutch now engages as it should, but still no cold ac.
4. Used AC gauges to see pressures at idle with ac on = 45 - 50 psi on both sides
5. Observed (now that the clutch is engaging), there is no short cycling, and there are no grinding, squealing or other strange sounds coming from the compressor. Also, the compressor can be turned by hand
=== Questions
1. Is there any other possible cause for this issue (with pressure at idle the same on both high and low sides) other than the compressor failing? Sensors, electrical, anything? It just seems odd to me that there are no sounds from the compressor even when the clutch is engaged, so I'm looking for confirmation.
2. If the compressor has failed without locking up (I checked this with ac & engine off, should ac be on?), and is not making sounds, would that indicate it hasn't thrown metal bits into the system, and so changing other parts (accumulator, condenser, etc) wouldn't be necessary?
3. I've also heard you can check the compressor oil on removal and if it's clean, changing other parts wouldn't be necessary. Your thoughts?
Thank you again. Much appreciated.
Rick
If the compressor clutch is engaged and there is no change in pressures then the compressor is not working. That compressor does not have an electronic control valve, so if a sensor were to cut off the AC it would do so by disengaging the clutch. Does the pressure change at all when the compressor clutch engages? When revving the engine? Does the clutch stay engaged? Usually compressors will make noise when they fail internally but it's possible to have something like a bad valve that wouldn't necessarily make noise.
There's some more information here:
https://www.2carpros.com/questions/ac-low-high-side-equal-pressures
Whenever a compressor is replaced, at a minimum the condenser, expansion valve (or orifice tube), and drier should all be replaced and the rest of the system flushed out. Even if you don't see any debris you are still risking that fine particles not visible to you will cause damage later. Also, you will likely find that the warranty on the new compressor requires this to be done. (Technically, the drier should be replaced any time the system is opened up.)
Thanks for the info Chuck, especially about a sensor failure resulting in the clutch disengaging. I thought this might be the case, but thinking and knowing aren't the same of course.
The pressure does not change when the clutch engages, and the clutch stays engaged as long as the ac control in the cabin is set to on. It was only me so didn't check when revved. If that could make a difference I could check it tomorrow?
I see on the link you provided, the image in KASEKENNY's reply says if the readings are 50 / 50, then the clutch isn't engaging. Mine definitely snaps in and starts spinning when the ac is turned on. If the front clutch plate is spinning, there's no way it could not be engaged, right? In other words, in and spinning equals engaged, right? No way around it?
What if whatever the clutch plate snaps into inside the compressor is broken? I'd expect noise from that (and am hearing none), but I'm really not familiar with the inside of compressors.
Thanks again for your insights.
Rick
@un_rick The clutch is external to the compressor and it drives the crankshaft via a plate connected to the crank.
https://www.intl2008.com/n1855320/How-does-an-AC-compressor-clutch-work.htm
@Chuck Tobias Yea, I actually changed the clutch on this about 7 years ago, so I do get that it's external. I was trying to ask, in an open ended sort of way, what if the crankshaft were broken, or what if the place where the 2 meet was worn out/stripped. Would it be possible for the clutch to appear engaged, but to not actually be turning the compressor? Or would the details of how these are constructed make that impossible, so that in and spinning = engaged 100% of the time?
@un_rick Very unlikely the crank is broken. If that were the case I would expect to hear bad noises from the compressor.