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Old 04-23-2001, 10:44 PM
Posts: n/a
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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Old 04-23-2001, 11:07 PM
steve hutson
Posts: n/a
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.
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Old 04-23-2001, 11:28 PM
Registered User
Join Date: Jun 1999
Location: Los Angeles, Calif, USA
Posts: 521
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.

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Old 04-24-2001, 10:15 AM
Registered User
Join Date: Jun 2000
Location: Plano, TX
Posts: 2,476
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.
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Old 04-25-2001, 11:59 PM
Registered User
Join Date: Jun 1999
Location: Los Angeles, Calif, USA
Posts: 521
Very thorough analyses. Thank you, jcyuhn.

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Old 04-26-2001, 09:56 AM
Registered User
Join Date: Jun 2000
Location: Plano, TX
Posts: 2,476
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
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Old 04-26-2001, 12:56 PM
Registered User
Join Date: Jun 1999
Location: Los Angeles, Calif, USA
Posts: 521

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

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