1998 V70 | MCC | Strange AC Behaviour Topic is solved

yanga001 · Post by **yanga001** » 14 Jul 2021, 15:38

Hi Everyone,

So i have had a bit of a fussy AC over the past few months however i cannot figure out what is going wrong on it.

The behaviour is sometimes after the car has been running it will not turn on if the car has been recently turned off, i.e. still hot. This behaviour also can randomly occur while driving.

I checked and rechecked charge levels and the AC does blow cold when it is able to turn on.

Initially i had believed it to be an issue with the compressor clutch however i noted in Vol FCR that the clutch does turn on and off properly when it is enabled. The system also registers the correct pressures (around 500 KPA of charge when off) which goes up to 1000-1500 KPA when the compressor is on. I know there is a very slight leak (needs a topup every 1-2months in the summer, however the system does maintain pressure when left over the winter. I have added oil when adding the R134A substitute.

I replaced the MCC with a spare after i had issues with the selectors and i believe that is when the behaviour began.

Essentially i will select AC on and it will sometimes come on or sometimes not come on.

The failure modes often occur after the following events:
-the car was previously run, then turned off, then turned back on
-Ac was running, then turned off, then turned on
-very hot and humid days

What stumped me recently is that holding the AC button down seems to trigger the compressor to turn on in instances where it previously failed.

If i hold down the button for 10-15 seconds then it seems the compressor is enabled, puts up an initial pressure charge and is then able to sustain itself.

Could this be a problem with the MCC board?

Work done recently:
-fixed alternator voltage regulator (now have about 13.4-13.6 at all times
-replaced AC relay with a known working one
-charged the system

Is there any known literature that says holding down the AC button will force the compressor to turn on?

I found this odd as i gauged the compressor clutch recently and verified its the correct tolerance.

Any ideas would be supported, ill see if i can get a video in the failure mode to demonstrate what i am talking about.

Post by **jreed** » 14 Jul 2021, 16:39

Another classic test (and this one can be dangerous to you and whatever stick or tool you use to do it with -- wear protection for eyes and hands) is to press in on the AC pulley with a broom handle or something similar like a stick to see if it will engage when the symptom is occurring. I did this once when my clutch gap was too wide due to wear and the pressure from the stick closes the gap enough for the magnetic force to pull the pulley in the rest of the way and engage the compressor.

yanga001 · Post by **yanga001** » 14 Jul 2021, 17:00

Already did the push in test multiple times, non caused it to start which is why I am stumped

Post by **scot850** » 15 Jul 2021, 16:08

If the A/C is off, does the heater fan work ok? Thinking it could be a bad fan motor not blowing but the A/C is ok.

The only other thing I can think of is the compressor is bad.

Neil.

yanga001 · Post by **yanga001** » 15 Jul 2021, 22:48

The heater fan seems to works, are you referring to the blower motor or the one after the radiator. Both of which work which stumps me. I would understand if something acts broken but its very intermittent. The compressor and system was redone 2-4 years ago by the previous owner. I am able to reliably jumpstart my AC when it does not want to run by holding the buttons so ill take it as is it for now.

Post by **abscate** » 16 Jul 2021, 04:04

It sounds like an intermittent in the MCC control unit. Those are expensive new and hard to find used so be careful if you open it up.

yanga001 · Post by **yanga001** » 16 Jul 2021, 09:51

I got two but one of them has a broken setting selector. Seems to randomly choose between defrost, shooting cold at the floor or heat at my face.

The second one has this issue.

I am not one to shy away from taking it apart and resoldering the buttons though so we will see.

yanga001 · Post by **yanga001** » 22 Jul 2021, 19:57

The SAGA continues...

I think i am low on refrigerant now, reading dead 25 psi which seems enough to trigger the AC compressor and keep it on but not enough to blow cold air?

When reading the pressure it seems to drop and i get hot air however the clutch does not disengage, is this normal?

Going to add some charge to get it to 28-29 psi which i know should blow cold air.

yanga001 · Post by **yanga001** » 23 Jul 2021, 11:00

Update:

The AC charge hose blew up on me again, managed to charge the system a little bit to get it to work but not nearly enough.

I am fed up with these cheap charge hoses and will elect to get a quick connect system. My theory is that these hoses and fittings are not meant to hold sustained pressure over long periods of time and will fail at a high pressure on the next bottle.

At this point ive gone through atleast 20-30$ of lost R134A and 2 hoses each a nominal 20$ of the cost so ill set my sights a bit higher if possible.

Anyways moral of the story is it was low freon charge.

Symptoms:
Compressor was able to engage but could not cool

yanga001 · Post by **yanga001** » 23 Jul 2021, 14:47

Took apart the hose that was leaking and found that the lower connector was loose. It seems between manufacturing the casing to hold the hose connector and the link to the valve that the screw in did not match up. Therefore there was a 2-3mm gap where the gasket could not seal. Tightened it down and forced it shut. Will hedge another 10$ of refrigerant tomorrow on the hose. Hopefully i do not regret it.