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AC system electrical operation -- is this right?

 
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austinporsche  



Joined: 26 Apr 2018
Posts: 33
Location: Austin, TX

PostPosted: Tue May 21, 2019 2:17 pm    Post subject: AC system electrical operation -- is this right? Reply with quote

Hi all.

FYI, I have a separate post on the general discussion board about where the AC relay is if you want to help out on that as well.

This thread is about better understanding the electrical workings of the 924S' AC electricals.

Based on the schematic on the side of the alleged AC relay as sold by Pelican (see link), it looks like it is a relay with dual contacts, i.e., one input signal throws two output contacts.

https://www.pelicanparts.com/catalog/SuperCat/0992/POR_0992_ELSWCH_pg2.htm#item7

This would seem to be consistent with how the AC in my 924S works as the big black-and-white AC dial in the middle of the dash both operates the blower motor AND presumably the compressor as well.

Do i have this about right?

If so, would it be possible that the contact for the compressor is stuck while the contact for the blower motor is not?

That's my situation: the blower motor runs when the AC knob is turned, but not the compressor.

And if I have this right, does anyone know which output pin is the compressor clutch and which is the blower motor? (87 and A are presumably the output pins to those two loads, but no idea without a full wiring diagram, which one is which).

Pins 85 and 86 are presumably in the AC-Blower switch circuit
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16again2009  



Joined: 04 Dec 2014
Posts: 47
Location: tulsa, ok

PostPosted: Wed May 22, 2019 1:42 am    Post subject: Reply with quote

I went through this exact thing about a year ago. Did everything to the ac and the compressor would not kick in. I replaced the ac relay and that didn't help. Behind the glove box, just to the right of the ac relay is a little black retanglular box. I can't remember what it is called. I took it apart and it had a burned circuit. I put a drop of solder over the burned spot, re-installed it and have had cold ac ever since. Good luck.
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1988 porsche 924s
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austinporsche  



Joined: 26 Apr 2018
Posts: 33
Location: Austin, TX

PostPosted: Wed May 22, 2019 9:26 pm    Post subject: Reply with quote

Thanks. So before the fix, the fan blew but the compressor clutch wouldn't engage, right?

Couple of questions:

1. The 5 blade tall AC relay (the link in my original post) is behind the glovebox? No documentation I have looked at shows it as being there: how did you figure that out?

2. Any idea what the little black box is? maybe an early version of what is now commonly called a BCU -- body control unit? Guessing you don't have pics.

3. Do you have to remove the glovebox to get to the relay and black box or can you get to them from the underside?
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16again2009  



Joined: 04 Dec 2014
Posts: 47
Location: tulsa, ok

PostPosted: Thu May 23, 2019 10:54 am    Post subject: Reply with quote

Yes, that is the same symptoms I had. You have to remove the panel below the glove box and the glove box. About a 20 minute job. It is to the right of the a/c relay. It is a small black square. The electric to your compressor goes through it. The part number is 477959531. I have seen it referred to as a blower motor fan resistor, an a/c adjusting relay, and an a/c amplifier. Take your pick. It is all the same part. I took mine apart, soldered the burned circuit and the compressor started working. I have pictures but this forum does not have a mechanism to post them direct. You have to post them through a photo program which I do not have. But if you will google the part number there are pictures of it online. The whole job took me about 1 1/2 hours. Good luck.
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16again2009  



Joined: 04 Dec 2014
Posts: 47
Location: tulsa, ok

PostPosted: Thu May 23, 2019 10:55 am    Post subject: Reply with quote

Also posting the year of your car would help.
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austinporsche  



Joined: 26 Apr 2018
Posts: 33
Location: Austin, TX

PostPosted: Fri May 24, 2019 6:43 am    Post subject: Reply with quote

1988 . . .
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austinporsche  



Joined: 26 Apr 2018
Posts: 33
Location: Austin, TX

PostPosted: Fri May 24, 2019 7:07 am    Post subject: Reply with quote

Is this the part?

https://www.ebay.com/sch/i.html?_from=R40&_trksid=p2060353.m570.l1313.TR12.TRC2.A0.H0.Xprosche+924+.TRS0&_nkw=prosche+924+477959531&_sacat=0

It says it is 924-944. If so, I have four (yes, 4) 944s from which I am someday going to make three (one has a nonrepairable title from the Texas flood, I wanted it for the manual trans for the best of the other three, which is an automatic; paid $750 with Fuchs).

So may I can cannibalize one of these vehicles first for this part.

It also says that it is for the blower. Again, the blower runs, but the compressor clutch doesn't.
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16again2009  



Joined: 04 Dec 2014
Posts: 47
Location: tulsa, ok

PostPosted: Fri May 24, 2019 12:19 pm    Post subject: Reply with quote

Yes, that is the part. The early 944 uses the same part. It is a blower switch but the wiring for the magnetic clutch on the compressor goes through it. So replace it with one of your spares or fix the one that's in there. If that is your problem it will work. If it doesn't it's something else. Only one way to find out and it is a free fix. Good luck. Let us know how it turns out.
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austinporsche  



Joined: 26 Apr 2018
Posts: 33
Location: Austin, TX

PostPosted: Sat May 25, 2019 4:56 am    Post subject: Reply with quote

Will do. Going to try to get to it this weekend . . . .
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austinporsche  



Joined: 26 Apr 2018
Posts: 33
Location: Austin, TX

PostPosted: Mon Jun 03, 2019 12:14 am    Post subject: Reply with quote

ok, so I got up to where i store my 924 yesterday.


one new piece of data.

when l turn on the AC via the blower switch, the fans kick on, but not the compressor clutch.

does this mean the blower relay in the fuse block is good, but the other relay under the glovebox is bad?
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Fifty50Plus  



Joined: 28 Feb 2008
Posts: 1038
Location: Washington DC area

PostPosted: Mon Jun 03, 2019 1:29 am    Post subject: Reply with quote

There must be refrigerant under some pressure in the system for the clutch to engage. If it has all leaked out, you will never get the clutch to engage regardless of the rest of the wiring and controls being perfect. If your system is 134A, you can go to an auto parts shop and buy a can that comes with a gauge attached to the injection port. The gauge is reusable on further cans of 134A.
You don't have to have the engine running to test. Just attach the can with gauge to the low side of the compressor (fittings are different size so you can't get it wrong) and see what the gauge says. If its over 15 - 30 pounds the clutch should work. If it's zero, squirt the entire can into the system, start the car and see if the compressor clutch kicks in. If so, put another can on the gauge, reattached to the compressor and follow the directions for a rough equivalent of 'full".
If you had internal pressure all along or after adding a can or so of refrigerant, and it still won't activate the clutch, you have a wiring problem.
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austinporsche  



Joined: 26 Apr 2018
Posts: 33
Location: Austin, TX

PostPosted: Mon Jun 03, 2019 3:36 am    Post subject: Reply with quote

Pressure read 50psi, so all good there.

I am taking out this second resistor/relay from behind the glovebox today to see what is going on with it.

So here's what I know:

AC system properly pressurized (though pressure switch could be bad?)
When AC switch (AC fan speed switch) turned on, cabin fan run
When AC switch (AC fan speed switch) turned on, condenser fans run (both?)
When AC switch (AC fan speed switch) turned on, compressor clutch doesn't energize (engage)
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austinporsche  



Joined: 26 Apr 2018
Posts: 33
Location: Austin, TX

PostPosted: Mon Jun 03, 2019 9:48 pm    Post subject: Reply with quote

Can't figure out how to insert an image. A little help.

If there was an image attached, 16again2009, is this the board and the "brown spot" to which you are referring.

Definitely solder is missing, didn't migrate anywhere else and wasn't in the cap of the electronic component, so did it just vaporize?

A dollop of solder on that -- not not touching the adjacent solder point(s) on the board -- should do it then?

[/img]
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