1999 Mack MR688S inefficiency

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1999 Mack MR688S inefficiency

Postby Mamrak76 » Tue May 15, 2018 8:47 am

I have a truck that is not cooling properly. I have an ac machine so recovery is no big deal.
I have made sure it had a new reciever dryer, replaced a couple of old crummy looking hoses, new expansion valve, and a new condenser as the old one was leaking also a new belt.
I made sure it was under vacuum for 20 mins and charged to the listed spec of 2 pounds.
Pressures at 78-80 ambient. were 32 low and 200 high.
There is no cabin air filter on the Evap but it is clean.

The main problem is I cannot get bent temps below 62 degrees.
I posted this on the old forum and never figured it out. Replacing the txv, dryer, belt, hoses, condenser, were all attempts to figure out the problem. On a hot day sometimes 60 is all you can get and I understand that but I stay above 60 no matter what the ambient is.
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Re: 1999 Mack MR688S inefficiency

Postby cool2bcool » Tue May 15, 2018 8:52 pm

Is the return line from the evap back to the compressor cold? If so could be a reheating/blend door type issue. Where either the air isn't fully directed over the evap or air is going through the heater core at some point. Make sure if you have a heater core valve its fully shut and not returning hot coolant back to the engine. Also make sure the evap itself is clean of debris/dirt.
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Re: 1999 Mack MR688S inefficiency

Postby ice-n-tropics » Wed May 16, 2018 2:15 pm

Your problem is typical of older trucks that have had several compressors changed without flushing old oil out of the system. oil insulates the internal surfaces of both heat exchangers and reduces heat transfer. Typically truck systems have 8 + ounces of oil and replacement compressors come with a full oil charge, therefore, some higher milage trucks have 25 to 50 ounces of oil. Each excess oil charge impacts the louver temp 5 degrees or so. Recirculation air mode should achieve 45 degrees at 55 mph at ambients under 95 deg.
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Re: 1999 Mack MR688S inefficiency

Postby bohica2xo » Thu May 17, 2018 6:29 pm


I just pulled the bottom line off of the condenser on a 2000 Malibu. Drained a solid 20 ounces of oil. The local Firestone monkeys had put 3 compressors on it & charged it with the magic machine each time. Every receipt she had showed a charge for "refrigerant oil". It would not cool at all.

Flush that Mack back to bare metal & load the correct oil charge.
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Re: 1999 Mack MR688S inefficiency

Postby Mamrak76 » Fri May 18, 2018 7:59 am

This is weird as I have been dealing with this problem for over a year. I found the problem but it was not what I thought! I worked on a ford car yesterday and the condenser fan was unplugged. The high side went close to 400. On my truck the high side was normal around 200-250. Yesterday my Mack’s fan broke so I replaced the fan and the fan clutch. That was my problem. The fan clutch was not working at all. It never overheated or got high on pressures but it was not working. I replaced the fan clutch it was like night and day difference! Can anyone explain how the pressure stayed normal? I know the signs of a fan not working but apparently I was wrong. The vent temps dropped to around 50 which is perfect for an older truck.

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