How much 134a in my 90? A clutch cycling question too.

[drdave]

This is actually a two-part question.

My 90 has a retrofit sticker in it that was done in 98. It said they used 2.1lb of refrigerant and 8oz of lubricant. I'm guessing that is around 32 - 33.6oz of refrigerant.

My high-pressure valve was leaking so I had it serviced. They said they only pulled 10oz of refrigerant and charged me for an additional pound. That only amounts to 26oz of refrigerant in my book.

What is the correct amount of 134a for my 90?

Second question;

Before the ac service, my ac clutch would cycle every 30-45 seconds when first started, but would stay engaged all of the time when at operating temperature. Now it doesn't cycle at all, just stays engaged all the time. I am told not to worry about this, that 134a acts differently than the r12 did. But I know this is not right.

Can the cycling switch be replaced without removing the refrigerant? How should the clutch act with too much or too little refrigerant?

Too many questions, way too little time (and money)!

<small>[ July 07, 2002, 01:55 AM: Message edited by: drdave ]</small>

 

[olympic]

If the refrigerant is low, the pump will cycle on and off constantly every few seconds. R134 is less eficient htan R12 at transferring heat so it may be normal for the pump to run all the time.

 

[drdave]

Bump

 

[SHOTOGO]

I just did the retrofit this past weekend to my 92. Everything I have read recommends that when an R-12 system is retrofited to R-134a that it be filled to 85% capacity. My 92 specified that the capicity was 36oz, I'm not sure if a 90 is the same or not, I would think so. The capicity should be on the sticker on the black cover over the fan. So for mine the correct amount was 30.6oz, I put about 28-29oz in and it is working great. My clutch also stays engaged all the time also, I always thought it was supposed too, not sure though. If there was to little refrigerant it would cause the clutch to cycle on and off repeatedly, not sure what would happen with too much. I'm not sure if you can replace the cycling switch without removing the refrigerant. The cycling swith pressure is adjustable though, there is a small screw on the top of the switch when you pull the plug off that allows you to raise or lower the pressure at which it activates. I've read that it is recommended that the pressure be lowered for the 134a. I left mine alone and it has been fine so far.

 

[shodazzl]

I've read where the amount of refrigerant to be used for the r12 systems ,change to a lower oz.(don't know how much lower) once you convert to 134a.They also recommend to change the accumulator reciever.I think an overcharge might make the a compressor cycle .I had mine retroed and it cycles sometimes as you said,I think they might have overcharged it, but NO problems with AC and it seemes to run fine.

 

[clintonk]

The clutch cycling is controlled by the low-side pressure, which corresponds directly to the temperature of the evaporator. On a hot day, the evaporator may not get cold enough to lower the pressure enough to kick out the compressor, so it runs all the time. Try setting Max A/C and turning the fan speed all the way down. The lower airflow will remove less heat from the evaporator, likely allowing it to cool enough for the compressor to cycle.

Clinton Knight
'90 White 185K
'95 Green 53K

 

[drdave]

I tried adjusting the cycling switch but I don't think it made much of a difference. The seat of my pants tells me that it cycles while driving, but this could be a placebo affect since I'm trying to see a difference now.

 

[sdpatt]

The cycling switch will disengage the compressor when the low side pressure gets below the cut-off point. The lower the refrigerant charge, the more often this happens. I have a R134a retrofitted system in my 1991 and inqured with several shops and web sites regarding the required pressure in the system. The general consenses was that the low side pressure should be around 40-50psig and the high side pressure around 250psig at idle.

These pressures will increase as the engine compartment and ambient air temperatures heat up, but the 50/250 numbers at idle are a pretty good target. Too little pressure will cause frequent or rapid cycling of the compressor (not good). Too much pressure may cause popping of the relief valve (not good) or poor performance of the A/C system (also not good). This is because the low side pressure is too high to allow the R134a to expand and absorb as much heat as it is capable of.

My compressor stays engaged full-time with the R134a charge while the A/C is on. If it is cycling, you should check to see if the low-side pressure is falling too low. That would say that you need to add a little more refrigerant to the system. Your description sounds like the system is a bit low on pressure as it cycles when first started, but stays engaged after the engine heats up and expands the gas a bit. Try another couple of ounces of R134a to get the low side pressure above the cut-off point at cold idle.