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#1
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When I first start by car the ac seems to work from the side vents, after several miles of driving the ac seems to shut off and I just get fresh air. Also, the center vents do not come on at all now. I checked the fuse panel, the fuse marked for ac, is OK. Does anyone have any ideas how to fix this?
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#2
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As soon as you notice that only fresh air is coming in, pull over to a safe place, leave the engine running, open the hood (put it in park, set the e-brake) see if the compressor is engaged. If it isn't it could be for a variety of reasons. Haivng just experienced the same problem, I can tell you what it was on my 87 300TDT. First, due to wear, the gap on the compressor clutch was too large.
When I would back off on the throttle, the voltage drop would cause the compressor clutch to disengage. Second, there was a restriction somewhere in the high pressure side that caused the compressor to shut off. That was solved by flushing the system. Third I had leaks, so over time the freon leaked out, no freon,no cooling. Fourth, a bad pressure switch! Basically the system was shot except for the compressor itself so instead of costing $1500 to fix,I got off with about $750 or so for new clutch, pressure switch, receiver-drier, expansion valve, seals, system flush, vacuum, and recharge. Hope this sad story helps, but now the A/C runs cold. I forgot to mention the vacuum pods in the dash were all bad--that was big bucks. |
#3
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Let's look at the most common and easy fix first before looking for other causes:
(1) For the venting problem, one of the causes is a disconnected small vac hose near the power brake booster. Carefully check the area on top of the power brake booster under the hood. (2) AC not working after few miles of driving may be caused by a slipping fan belt. The rpm sensor detects the slipping and shuts off the compressor. Check the condition of the fan belt and the belt tensioner. You may have more than one problems with the AC system, but at least, you have to make sure the belt is working properly before moving on to other areas. David |
#4
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I own an '87 300TDT as well. I've been through the
a/c system for the same problems. Here's a simple test. When the a/c stops cooling, pull over, shut off the engine, and immediately restart the car. Does the a/c now work? If restarting the car always causes the a/c to cool, at least for a while, you have a problem with the RPM comparison circuit. Basically, the a/c control electronics compare the speed of the a/c compressor to the engine. If there is a substantial difference, it shuts off the a/c until the car is restarted. This system is intended to protect the single serpentine belt which also drives the water pump, alternator, etc. It's intended to protect drivers who would otherwise continue driving when the a/c compressor seized, broke the belt, overheated (and destroyed) the engine. Good in theory, a pain in the butt in practice. A couple of common causes: 1) Is your plastic splash panel (below the engine) in place? Splashing water on the serpentine belt will cause enough slippage to trigger the cutout. 2) Excessive clutch gap on the compressor. If you have more than about 50K miles on the a/c compressor it may need the clutch gap adjusted. Commonly overlooked by even experienced a/c shops. Specification is .50mm +-.15mm. Brain dead simple to check. 3) Oil on the clutch material. Just degrease the clutch using Simple Green, or your favorite degreaser. 4) Failed Klima relay. Mine went berserk after about 13 years. Cost about $85 from a mail order vendor. 5) Bad electrical connections from the compressor to the Klima relay or from the engine speed sensor to the Klima relay. Corrosion/dirt can accumulate after all these years and cause the Klima relay to falsely trigger the cutout. |
#5
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Very thorough analyses. Thank you, jcyuhn.
David |
#6
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Thanks for the positive feedback.
I have one more possible cause to toss in: the serpentine belt is slipping, causing an actual difference in speed between the engine and compressor, which the Klima relay is correctly detecting. There are two possible causes of this particular malady: An old, stretched, or greasy/ dirty belt. Also, the belt tensioner is occasionally known to fail. The original tensioner design on these 80s diesels had problems, they all failed. It was replaced with an upgraded version. It's possible this is the cause, but in all liklihood the tensioner has been replaced on these cars a long, long time ago. - Jim |
#7
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jcyuhn,
Can you pinpoint exactly what parts of the tensioner assemble are likely to fail? The whole tensioner assemble has spring, bushings, shock, etc. Do you have the part number(s)? Thank you David |
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