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On one of the 940s the AC blows cold but often won't work or only come on after several minutes drive. Seems to be getting worse. Any ideas?
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89 240 wagon, 94 940, 200K, 94 940, 135K
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So it seems to be all set
First of all, I don't do anything with the AC; lame but...I took it to my guy and he said he couldn't diagnose an intermittent problem. He said, "maybe this and maybe that". I had to ask him to recharge the system, maybe that would fix it. He put 1.02 of Freon. I guess that's pounds. That's pretty low.
Since then it works consistently well
thanks for the input
--
89 240 wagon, 94 940, 200K, 94 940, 135K
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I'd also look at the refrigerant pressure switch at the accumulator--often it fails in just this fashion, retail about $40. Jumper it and see how fast the compressor turns on. If it comes on immediately while bypassing the switch (providing the refrigerant is at the right pressure), the switch is faulty.
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Dear j.pelchat,
Hope you're well. Yes. The problem could be in the climate control unit. Over time, solder joints - on the main relay and on the power connection (at the back of the circuit board) - develop micro-cracks. These cracks are the result of heat/cool cycles: when power flows, current causes the joints to expand. When the system is off, the joints cool and so shrink.
While far too small to be seen by an unaided eye, micro-cracks can cause intermittent compressor operation or completely shut-down the system.
As removing the climate control unit is a non-trivial task, I'd pursue the possibility of too-little R134a refrigerant, as amarin and hf have suggested. If you find the system has the right amount of R134a, then the next reasonable place to look is the climate control unit.
Hope this helps.
Yours faithfully,
Spook
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Hello Spook,
Hope you're well. Yes I fully agree with you, the relay in AC controller box could be at fault. This must be checked too besides checking for low freon.
To diagnose the AC controller box, pull out the pressostat connector at the accumulator and using a voltmeter (or a bulb could do) test for voltage at the terminals inside the connector. One probe from volmeter (or one wire from bulb) to any one of the terminals and the other probe/wire to ground. Prior to this engine must be running and AC button switched ON.
If no voltage evident at any of the terminals then the relay inside the controller box may be at fault. Usually its solder connections are loose with age. These may be re-soldered. The relay itself is generally OK and long lasting (a sealed type relay filled with nitrogen gas). The FAQ (Heating and Air Conditioning) helps regarding re-soldering.
Regards,
Amarin.
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I agree with Amarin that the solder connections or relay may be at fault. I'd check the voltages as Amarin suggested first before tearing out the MCC. No voltage would most likely indicate bad solder connection(s) or a bad relay. I've replaced several relays that were bad and re-soldered the connections per the FAQs since the MCC was apart. The relays are less than $2 from digi-key and are more rugged than the originals. There is a write-up in the FAQs.
--
Mine: 3-940s running, 2-740 and 1-940 parts cars, and 3 1959 John Deere 630s (1 for parts), dtr1:3-940s, dtr2:1-740, 1
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Hello,
A simple diagnosis of low freon is to disconnect the low pressure pressostat electrical plug at the accumulator and short its terminals together using a paper clip. If the compressor switches on all the while you're doing this then your freon may be low.
Alternatively you could connect AC manifold gauge to the low pressure service port (beside the accumulator) to test for low pressure. R134a pressure correlates well to the ambient temp. If your ambient temp is 70F then the gauge should read around 70PSI (AC OFF). If ambient 80F then the gauges should read around that too (AC OFF). Too low a pressure usually indicate low freon.
Hope this helps,
Amarin.
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I find this post extremely topical and interesting. I have never seen this particular info, relating ambient temps to static pressure. This is exactly what I was looking for, in trying to figure out if my system that I put one small can of R134 in now has enough. It's not cold but it does blow cool air, so better than nothing, and I don't want to overfill. In any case;
So to restate the above, I can drive my car with the AC off, to a shop that has the right connector; they read the pressure and we see how it correlates with the ambient temp. So if it's 75 deg and the pressure is 64, I need some more R134.
How much would you guess, in that example? Any idea? And, there are sets at the parts store that have a hose and a can, and some of those have a gauge. Is that sufficient for this purpose?
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I'd like to clarify a point about using the "ambient temperature chart" to check the rest pressure on anyone's car.
When it says "ambient" temperature this means that the temperature of the Freon gas is at the same temperature as the outside air.
The only way to get a accurate reading is after letting your car (and the gas) sit in the shade...outside..and become completely adjusted (and equal to) to the ambient temperature.
If you drive your car and get the engine warm the Freon gas is going to be in a higher temperature environment because of the engine heat, therefore the pressure will be greater.
Actually, if you're trying to measure the resting Freon pressure on a car that has been driven and is warm you need to know what the gas temperature is.
I know of no way to reliably measure this temperature since the gas temperature varies greatly in various places as the AC system operates.
The ambient gas temperature test is best done on a "cold" engine sitting in a place with a constant "ambient" temperature.
Just my opinion
hope this helps
steve
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Hi Jerry,
Here's a temp-to-pressure chart (if I did it right). For example the R-134a static pressure at an ambient temp of 70°F is shown as 71.1 psi.
--
Bruce Young, '93 940-NA (current), 240s (one V8), 140s, 122s, since '63.
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Hello Jerryc,
Yes you could add a little. No harm done. The question could arise is that if adding a little helps then how much more to add for complete charge. So this gonna be a guessing game later on.
The relation between static R134a pressure and temp (in Fahrenheit) is there but it is not exact. You could refer to R134a temp-pressure chart in refrigeration website. It is just a guidance compared to having nothing else to compare to.
To recharge you must recover the refrigerant, vacuum and charge back with specified weight. There's no way to tell how much (much less how many ounces) R134a to add with this and this pressure reading at the AC manifold gauge.
In case of there's uncertainty of the current system charge, it is better to get proper charge so that if the AC still don't cool well the fault could be in other AC components. This helps in troubleshooting.
Regards,
Amarin.
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Thanks Bruce and Amarin, very interesting.
I am generally familiar with this, I converted one car from R12 to R134, and I have rebuilt/resoldered/replaced relays in the control units, which are actually not hard to r&r, there's a shortcut.
I was looking for a way to get the amount of coolant pretty close without going to the cost and time of remove/vac/replace; ah well, no exact fix here, but I'll add a bit more and see what happens.
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posted by
someone claiming to be hf
on
Tue May 27 14:41 CST 2014 [ RELATED]
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It possible that you are running low on Freon. May be, add a little and see if makes any difference!!!
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