Oilslick964

Just bought another 1990 C2 coupe (after a year or so on the 951 board). I've had several 964s before, but never tackled a real A/C problem. The compressor wasn't kicking on so I figures it needed a charg, but the shop says it is full and has no leaks! Next I figure a fuse. Nodice, the 7.5 amp fuse in the rear is fine. So now, it is 96f degrees and I don't know what to check next <img border="0" alt="[crying]" title="" src="graemlins/crying.gif" /> . Any suggestions???

DaveK

Does the light on the switch come on? My switch flew out once, and springs / contacts went everywhere. At first, I fitted back upside down and it didn't make the contacts. It's not that likely, but check the light comes on. If it doesn't, it's worth taking the switch out to make sure it's OK.

But - it's a b*!&h to get back in.

Oilslick964

Dave,
The light does come on. Thanks for the tip, though! Can you think of anything else? The shop says that the compressor is not locked up, so hopefully that isn't the problem!

DaveK

I can't I'm afraid. I thought lack of charge was the main cause but you say it's not that.

On a different subject - I'd like to see a pic of the chrome D-90s on here at some point!

Felix

Dear Paul,

When you turn on the AC (when you are inside the car) you should hear some whistling sound as some of the valves open and close. Then you should feel that the air is cooler. If you do not hear anything then there is something wrong with your air con unit situated under the dash. Now, when you turn on the AC and if you are at the engine bay, you should see the compressor being driven with the magnetic clutch engage. If you do not then there might be something wrong with your clutch.

BR,

Felix. :-)

Oilslick964

Felix,
Thanks! I'll check out what I hear when I hit the button in the car. As far as the compressor, The shop says it appears that it is not getting power. He could turn it by hane, but there is no power getting to it. He thought it was probably electrical. I may just have to bite the bullet and pay a shop to fix it, it is just too hot to do without.
Dave,
I'll try and get the pictures of the chrome D-90s up next week. They are chrome on the face and black inside the spokes. I think it looks pretty cool!

Thanks and any more ideas will be greatly appreciated!!!

Paul

Carl Muckenhirn

Have you checked the relay? If I'm not mistaken there are relays for both the condenser fan up front and for the compressor in the engine compartment.

carl.

Oilslick964

Carl,

There is a fuse in the engine compartment and it checked out fine. I did not see a relay in either box for the compressor, though. Am I missing that one? An idea occurred to me that I might try bypassing the low pressure switch, to see if it has gone bad. That would be a cheap fix. Has anyone ever tried that? Is it something any bonehead can do or should I take it to a pro?

Randall G.

Yes, there is a compressor relay in the engine compartment fuse box. As Carl suggests, this could be your problem. As could the 3-level pressure switch. You may also have too much charge, which will keep the compressor from running.

Sounds like the shop hasn't tried applying 12V to the compressor, to see if the clutch engages. It can be done, if you're motivated, by jumpering the proper positions in the relay's plug.

Hope this helps.

Oilslick964

Randall,

So does the relay act as the low pressure switch or is there an external switch for that (I can't find one). If so, would I just pull the relay and jumper the thing hot? Sounds like you have had some experience with this!

By the way, Pretty lofty project with the 993dt in the 964, but I'll bet you are LOVING that 6-speed!!!

Paul

Randall G.

</font><blockquote><font size="1" face="Verdana,Tahoma,Helvetica">quote:</font><hr /><font size="2" face="Verdana,Tahoma,Helvetica">Originally posted by I8A4RE:
<strong>Randall,

So does the relay act as the low pressure switch or is there an external switch for that (I can't find one). If so, would I just pull the relay and jumper the thing hot?

</strong></font><hr /></blockquote><font size="2" face="Verdana,Tahoma,Helvetica">The relay "implements" the 3-level pressure switch. The 3-level pressure switch is an "input" to the relay. If the 3-level pressure senses either a high or low pressure, it causes the relay to remove its "goes-out" power to the compressor.

Once you find the compressor relay (I can look the part # up in PET later tonight, if necessary), remove it, and you should find tiny numbers printed on its plug. If your relay is like mine, it will be a real m*th*r to get out. The compressor relay should be the green relay shown in this pic from a 993 (thanks EJ!):

<a href="http://www.pcarracing.homestead.com/files/wiring_harness/Harness_connection_box.jpg" target="_blank">http://www.pcarracing.homestead.com/files/wiring_harness/Harness_connection_box.jpg</a>

Perhaps someone with PET handy can look up the part #, as a verification.

Check the voltage at position #3 = always hot 12V. If you don't have 12V, either your fuse is blown (but you've checked that) or you have a wiring problem. Jumping #3 to #5 puts 12V at the compressor, and should engage the clutch. #5 is normally powered by relay action.

If the clutch engages, the 3-level pressure switch input can be simulated for testing--I would be wary of leaving it that way long-term.

</font><blockquote><font size="1" face="Verdana,Tahoma,Helvetica">quote:</font><hr /><font size="2" face="Verdana,Tahoma,Helvetica">Originally posted by I8A4RE:
<strong>Sounds like you have had some experience with this!

</strong></font><hr /></blockquote><font size="2" face="Verdana,Tahoma,Helvetica">Not really, mainly just reading the shop manual and wiring diagrams. <img border="0" title="" alt="[Wink]" src="wink.gif" /> I've had little trouble with my A/C (knock on wood), other than converting to R-134a.

</font><blockquote><font size="1" face="Verdana,Tahoma,Helvetica">quote:</font><hr /><font size="2" face="Verdana,Tahoma,Helvetica">Originally posted by I8A4RE:
<strong>
By the way, Pretty lofty project with the 993dt in the 964, but I'll bet you are LOVING that 6-speed!!!

Paul</strong></font><hr /></blockquote><font size="2" face="Verdana,Tahoma,Helvetica">Yes, lofty is a good word. Well, as they say, all's well that ends well.

BS911

How timely. I left off last summer with (what sounds like) this exact same problem except mine only occurs when the ambient temperature is HOT - over about 95 degrees - the compressor will not kick on. After spending the night in the garage (the car, not me) or in the cooler seasons, the a/c works like a champ without a glitch. Back to my hot afternoon commute home in 100 degree weather, after about 10-15 minutes of driving and the condenser fan running (leave the a/c on low all the time to keep that fan running) all of the sudden it works. Does your compressor ever kick in Paul? If not and all else is right, you might be way over pressured.

So today I did a test... This morning, with full function, the unladen/uncompressing low side pressure read 100 psi and cranked down to about 25 once the compressor kicked in. This afternoon, it wouldn't kick in and the low side pressure was about 125 with an ambient at around 94. (Sorry, I don't have a gauge now to read the high side.) About 10 minutes into the drive it kicked on. I immediately turned it off, pulled over, and got a low side pressure reading of 100.

So I'm thinking I am near to a conclusion of a too high problem. My guess is that the shop that "topped off the a/c" before I bought the car put me over the limit. So Randall, could you tell me what pins would indicate what my pressure senders are saying on that relay? Then, if I've got too much in the system w/r134, could I just "vent" the system. Maybe hitting the high or low side valve to blow off some steam until I get an acceptable reading at high ambients? It kind of sucks that I only lose my a/c when it is REALLY hot because if it isn't REALLY hot I just go with windows down and sunroof open.

Thank for any input and glad to find you active in a/c diagnosis already this summer Randall. You are truly the a/c guru here. I can't wait to definitively say that my CCU is juuuust fine. <img border="0" alt="[icon501]" title="" src="graemlins/icon501.gif" />

Randall G.

Hey Brian,

A couple thoughts ...

I wonder if your condenser fan is kicking into high speed? It's supposed to kick into fast-speed when system pressure reaches ~17.5 bar (255 psi). If it's not running in fast-speed when your compressor trips off, that indicates one of two things: 1) it didn't receive its start signal, as it should; 2) system pressure isn't too high. The compressor trips at 27 bar (392 psi, high-pressure side), which is--of course--higher than the condenser fan fast-speed setting.

With regard to your low-pressure readings. Per the shop manual, they're too high. You should be between just above 0.5 bar (7.25 psi) and just over 1 bar (14.5 psi). These values are obtained after running the compressor for 10 minutes, blower at 4, minimum temp. setting, 2000 RPM engine speed. The acceptable pressure band gradually creeps up with ambient temp. Even at 34C (93F), the max. pressure should only be ~17.5 psi.

The high-low pressure switch (internal to 3-level pressure switch) connects to the compressor relay at position #2 of the relay plug. The other side of the switch connects to the CCU. As I understand it, the switch opens to trip the compressor. Under normal circumstances, the switch is closed, and the compressor is powered. I haven't tried it myself, but you should be able to simulate a closed high/low pressure switch by connecting terminal #2 to ground. However, you need to measure the resistance between pins #85 (input from switch, connects to plug position #2) and #86 (control power, connects to plug position #7) of the relay first. If the resistance is high, you can use a plain wire with (say) a 1 A fuse for good measure. If it's very low, you'll need to put some resistance in your jumper wire (or else you'll create a short to ground). Your ground wire would be inserted into the relay plug, with the relay installed over it.

I wrote a lot about sending a trick signal to the relay above. Considering that your pressure appears to be too high, perhaps a better approach for the time being is to present your findings to an A/C shop--let them adjust the pressure, if necessary. It shouldn't cost too much (I'm guessing less than $70). Also, R-134a is really supposed to be charged by weight, which requires a shop's equipment.

Finally, you can find the shop manual tables below. From these, you can see your low-side pressure is too high.

<a href="http://members.rennlist.com/rgranaas/993%20AC%20Table%201.jpg" target="_blank">993 AC Table 1</a>

<a href="http://members.rennlist.com/rgranaas/993%20AC%20Table%202.jpg" target="_blank">993 AC Table 2</a>

Good luck, and let us know what you find.

BS911

Thanks Randall.

I'll take a look at what the relay signals are telling me tonight. (Hopefully) Yes, the condensor fan is kicking in as soon as I hit the a/c button - even though the compressor doesn't. So that also lends itself to a too high pressure syndrome.

I'm pretty sure you're right that I'm going to need a shop to get things worked out, but that whole curiosity thing really has me wanting to at least know and understand what is happening - as long as the real summer heat doesn't kick in first.

Thanks again. More to come...

Randall G.

</font><blockquote><font size="1" face="Verdana,Tahoma,Helvetica">quote:</font><hr /><font size="2" face="Verdana,Tahoma,Helvetica">Originally posted by BS911:
[QB]***snip***

I'm pretty sure you're right that I'm going to need a shop to get things worked out, but that whole curiosity thing really has me wanting to at least know and understand what is happening - as long as the real summer heat doesn't kick in first.

***snip***[QB]</font><hr /></blockquote><font size="2" face="Verdana,Tahoma,Helvetica">Absolutely! If you're fully researched before hitting the shop, you should be able to point directly at what needs fixing/adjusting, and they'll only need to apply their specialized equipment. You're saving yourself lots of money on diagnostic time. And, you might even get lucky and fix the problem yourself in the process of researching/understanding. Furthermore, you may wind up knowing more than the shop about the system's unique characteristics (but lack general expertise or specialized equipment). If the problem is related to charge, the shop really has the proper equipment to set it right. Though some people have had luck adjusting it themselves.

If the compressor won't run because of high pressure, the condenser fan should be running in fast (not slow) speed. That is, the compressor trip point is higher than the condenser fan fast-speed set-point.

Was it an actual shop that did your pre-purchase charge? I'm wondering if the P.O. didn't just cram in a bunch of R-134a until the system blew cold.

It will be interesting to hear what a professional HVAC tech (which I am not <img border="0" title="" alt="[Wink]" src="wink.gif" /> ) has to say about your low-pressure readings, when compared to the shop-manual figure.

<img border="0" alt="[cheers]" title="" src="graemlins/beerchug.gif" />