1991 Air conditioner Problem

[john510]

Hve a 91 RD686 Mack Dump and developing a problem with the AIr.. It was changed to the 134a system while back by Mack and worked so so ever since but want to get prepared to maybe make it work little better .. seems to cycle a lot and not as cold. I did a vac of the system and reloaded it and seems better but it almost as if its not at it full potential I've included some pics of the compressor , dryer, switch on the dryer but where is the orifice tube at? is it in this block going into the firewall?

How often should the compressor turn on for then off for?

I do have the gauges , vac and tools to fix it..

[Rob]

Hve a 91 RD686 Mack Dump and developing a problem with the AIr.. It was changed to the 134a system while back by Mack and worked so so ever since but want to get prepared to maybe make it work little better .. seems to cycle a lot and not as cold. I did a vac of the system and reloaded it and seems better but it almost as if its not at it full potential I've included some pics of the compressor , dryer, switch on the dryer but where is the orifice tube at? is it in this block going into the firewall?

How often should the compressor turn on for then off for?

I do have the gauges , vac and tools to fix it..

You do not have an orifice tube in your system. The system uses a mechanical expansion valve. It is the block with the circular top on it made of aluminum. One side of this block should be hot, and the other cold as this is the point of expansion in the system. It is the inlet to the evaporator core in the dash. When the system was converted to R-134A from R-12, was it flushed? Was the condenser removed from the truck and flushed independently? Was the receiver/dryer replaced? I've seen literally dozens of systems converted and they work great for a while, then either cease to funtion, or become marginal in operation.

The PAG oil necessary for operation with R-134A is not compatible with the mineral oil used with R-12. Intermixing of the two will congeal forming restrictions in the system making it less than desirable in operation. This is prevelant in the bottom of the condensor, and evaporator, and at time plug the receiver/dryer. I suspect this is what's happening with yours. It will need to be broken down into pieces and flushed clean with a solvent made for this, (we used to use R-11) and reassembled, then start over with the charging process. The Sanden compressor you have is easy to flush be removing the plug in the side of the body. The receiver/dryer needs to go in the trash. The expansion valve can be cleaned with solvent. Both the condensor, and evaporator are best cleaned with an a/c kit specifically made for this purpose.

When the system is converted right, then charged to an 11 degree superheat setting, it will give you a headache when it is running.

Rob

[john510]

You do not have an orifice tube in your system. The system uses a mechanical expansion valve. It is the block with the circular top on it made of aluminum. One side of this block should be hot, and the other cold as this is the point of expansion in the system. It is the inlet to the evaporator core in the dash. When the system was converted to R-134A from R-12, was it flushed? Was the condenser removed from the truck and flushed independently? Was the receiver/dryer replaced? I've seen literally dozens of systems converted and they work great for a while, then either cease to funtion, or become marginal in operation.

The PAG oil necessary for operation with R-134A is not compatible with the mineral oil used with R-12. Intermixing of the two will congeal forming restrictions in the system making it less than desirable in operation. This is prevelant in the bottom of the condensor, and evaporator, and at time plug the receiver/dryer. I suspect this is what's happening with yours. It will need to be broken down into pieces and flushed clean with a solvent made for this, (we used to use R-11) and reassembled, then start over with the charging process. The Sanden compressor you have is easy to flush be removing the plug in the side of the body. The receiver/dryer needs to go in the trash. The expansion valve can be cleaned with solvent. Both the condensor, and evaporator are best cleaned with an a/c kit specifically made for this purpose.

When the system is converted right, then charged to an 11 degree superheat setting, it will give you a headache when it is running.

Rob

When the system was converted to R-134A from R-12, was it flushed?

I dont recall

Was the condenser removed from the truck and flushed independently?

I dont think it was.

Was the receiver/dryer replaced?

Yes it was and and the newer style pump

-----------------------------I plan on buying a new condensor I think this one has little tomuch wear on it lots of fins bent looks like its ready for another. While at it I will get a new reciever dryer. Any sugestions on the pump to insure it still functions properly?

I take it that expansion valve is the block and can be cleaned in solvent and replaced.

you mention the evaporator, where is that?

what other solvents can i clean the system with?

Edited May 7, 2012 by john510

[Rob]

When the system was converted to R-134A from R-12, was it flushed?

I dont recall

Was the condenser removed from the truck and flushed independently?

I dont think it was.

Was the receiver/dryer replaced?

Yes it was and and the newer style pump

-----------------------------I plan on buying a new condensor I think this one has little tomuch wear on it lots of fins bent looks like its ready for another. While at it I will get a new reciever dryer. Any sugestions on the pump to insure it still functions properly?

I take it that expansion valve is the block and can be cleaned in solvent and replaced.

you mention the evaporator, where is that?

what other solvents can i clean the system with?

The evaporator is the coil in the dash that the air blows through and becomes cold. The receiver/drier is something that should be replaced and never reused. The expansion valve is the aluminum block this is attached to the inlet of the evaporator. Remove these parts from the truck, (evaporator, condensor, expansion block) and clean/flush each independent of one another. Most "good" automotive parts stores will market a flush kit and they come with solvent. I like to use PAG 100 oil in conversions. No good way to check compressor without being installed. All a compressor does is compress the refrigerant to a higher level, and make it hot through the heat of compression. As long as it doesn't leak, or knock, it is probably good.

Rob

[randyp]

Wish i could offer advice, but cant, dont know squat bout it,,dont really even bleeve in air ditioning. Do without and you will get used to it after a while,,,35 years or so,,maybe, ifn dont die of a heat stroke first,,,,randyp

[Rob]

Wish i could offer advice, but cant, dont know squat bout it,,dont really even bleeve in air ditioning. Do without and you will get used to it after a while,,,35 years or so,,maybe, ifn dont die of a heat stroke first,,,,randyp

I'm right there with you.

Rob

[john510]

The evaporator is the coil in the dash that the air blows through and becomes cold. The receiver/drier is something that should be replaced and never reused. The expansion valve is the aluminum block this is attached to the inlet of the evaporator. Remove these parts from the truck, (evaporator, condensor, expansion block) and clean/flush each independent of one another. Most "good" automotive parts stores will market a flush kit and they come with solvent. I like to use PAG 100 oil in conversions. No good way to check compressor without being installed. All a compressor does is compress the refrigerant to a higher level, and make it hot through the heat of compression. As long as it doesn't leak, or knock, it is probably good.

Rob

thank you for all your help..

[john510]

I just noticed the dryer unit is before the expansion block?

should it not be after it comes from the firewall and before the pump?

[Rob]

I just noticed the dryer unit is before the expansion block?

should it not be after it comes from the firewall and before the pump?

No sir; it is in the high pressure side of the system. If there is a cannister in the low pressure side, it is called an accululator but serves a different purpose. Some systems incorporate both.

The receiver/dryer catches and filters any trash the compressor produces in the way of congealing oil, metallic particles, and by products from wear. It holds impurities in a filter bed to keep them from entering the condensor coil and circulating within the system hastening it's own demise. An accumlator is placed in the output of the evaporator to catch any liquid refrigerant that is not completely flashed to a gaseous state within the evaporator core. As we remember from basic physics a liquid cannot be compressed and to attempt to compress liquid refrigerant with the compressor will instantly render the valves scrap.

Don't believe a word that randyp, or otherdog characters say bout not knowing nothing bout nothing. I learnt all I know from those guys. They be my idols.

Rob