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I have 12v at clutch wire, but sometimes clutch is not engaged. WSM electrical diagram seems to show a ground strap, but I don't see one.
is there a ground strap, or it ground provided with compressor mounting?
the compressor clutch ground wire is a single wire that comes out of the back of the clutch and is secured to the compressor body with a screw .
NOTE the power wire also comes from the same area and it is much longer.
The compressor being bolted to the engine block is how the system is grounded check the engine ground wire.
Where are you measuring the power.
Its possible your low on freon or the freeze switch has failed.
This is about what the clutch magnet looks like,
and you can see the two wires attached to the rear side of the clutch,
the short wire is ground and its screwed to somewhere close to the line connections,
if the clutch was not grounded then it would never work
Classic HVAC head relay failure: 12v at clutch wire but compressor will not engage because fried relay can’t pass 3A to pull the clutch.
Put a small bulb (marker light; something ~2A or less) in series with the clutch wire to ground. (It’s easiest to use the connector on the low pressure switch for this.) If you’re got 12v from the clutch lead (obviously the ‘car side’ of the circuit) to ground but it won’t light up a small bulb then the HVAC head relay is toast. If it will light the bulb then the a/c clutch is toast. Some theoretical uncertainty in verdict with a low-draw bulb. But, I’ve never found a zombie HVAC head that would pass 0.5A but not 3A.
The clutch ground wire typically terminates under a screw on the top of the compressor body where the hoses connect, so it isn't really accessible (or damageable...) with the compressor in place.
Meanwhile... The HVAC control head relay is undersized for the duty. Easiest diagnosis is a voltage chack on the circuit with everything connected and energized. Easiest access to the trigger circuit is at the freeze switch, under the plastic cowl cover at the base of the windscreen. Little silver aluminum box maybe 1.5" cube, with two spade connectors for wiring, plus a capillary tube that comes out, loops in a coil, then continues on to pass through a hole in the cowl into the air duct and the AC evaporator. Focus on the electrical connectors. Test for battery voltage at both terminals, referenced to ground. With everything connected and gas in the system, you should see voltage there. If the relay is dufo, voltage will be less. There are other things that cause can voltage drop in other places, but the initial testing is easily done at the freeze switch. If the clutch still doesn't work but you have full voltage at the freeze switch terminals, we can dig deeper.
... but the initial testing is easily done at the freeze switch.
Personally I think it’s a lot easier to test at the connector for the low pressure switch. That comes off a lot easier and with less leaning over the fender. The males spades on the freeze switch are weak too.
no time this evening for proper testing, but I have 13.6v at the low pressure switch, if that helps?? assuming
that would mean I have voltage at the freeze switch?
can the relay in the head unit act intermittently? for the hell of it, I took a drive for a about 1/2 hour (earlier today)
and AC ice cold, and compressor on the whole time.....?
can I cheat and use a test light instead of rigging up a bulb?
Last edited by merchauser; 01-19-2019 at 08:30 PM.
other day turned on AC and clutch disengaged after about 2 minutes, and on a drive last week, AC stopped blowing cold;
pulled over and clutch was not working. certain I have a good charge of Freon.
what ever is occurring, is very random
other day turned on AC and clutch disengaged after about 2 minutes, and on a drive last week, AC stopped blowing cold;
pulled over and clutch was not working. certain I have a good charge of Freon.
what ever is occurring, is very random
Relay failure is the most likely root cause. The thing works when cold and with heat build up it then packs up albeit the 2 minute timing is not much time to heat things up but given it is a random failure distinctly possible I suppose. Mine used to fail after about 10 minutes of running, I would switch off the a/c and it would re-engage when switched on a few minutes later. Changed out the head unit and installed the slightly different one from my late S4 and problem solved.
At this point now that you describe a problem with the AC operation,
I am with Dave and Fred the HVAC head needs a relay refresh,
while your in there replace the freeze switch and install a 3 amp fuse at the freeze switch
NOTE Greg Brown rebuilds these and makes sort of a kit to refit this HVAC system
Relay failure is the most likely root cause. The thing works when cold and with heat build up it then packs up albeit the 2 minute timing is not much time to heat things up but given it is a random failure distinctly possible I suppose. Mine used to fail after about 10 minutes of running, I would switch off the a/c and it would re-engage when switched on a few minutes later. Changed out the head unit and installed the slightly different one from my late S4 and problem solved.
I am leaning towards that head unit relay. of all the components, clutch, pressure switch, freeze switch, and head relay, which one would be most likely to act intermittently? thinking the relay?
regardless, the dr bob write up for the external relay seems like a no brainer.
01-19-2019, 10:46 AM
AC compressor/clutch ground?
