AC Sudden Death: Where's the Compressor Relay?
07-22-2006, 11:53 PM
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AC Sudden Death: Where's the Compressor Relay?
I took the '85S for a nice spin in 105 degree weather with fans full on and AC full on.....and it was blowing glorious cold air....then KAPUT! Both the fan and AC quit working. Fan fuse was blown and compressor no longer activates and turns.
On reading Wally's HVAC treatise, he mentions a "too small" relay inside the HVAC Controller head. Where's this relay located and is it easy to get to? Does this mean it's behind the AC switch on the dash rather than on the relay board under driver's feet?
What likely happened? A new fuse fixed the fan but I'm not sure how to find and check/replace the compressor activation relay....or whatever else might have gone wrong. Your informed ideas are much welcomed!
Thank you,
Harvey
On reading Wally's HVAC treatise, he mentions a "too small" relay inside the HVAC Controller head. Where's this relay located and is it easy to get to? Does this mean it's behind the AC switch on the dash rather than on the relay board under driver's feet?
What likely happened? A new fuse fixed the fan but I'm not sure how to find and check/replace the compressor activation relay....or whatever else might have gone wrong. Your informed ideas are much welcomed!
Thank you,
Harvey
07-23-2006, 12:06 AM
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There is a relay on the circuit board in the climate control unit which sits in the console where the a/c button is. The unit is quite easily removed and the relay can be replaced by a similar but more robust relay. Search a/c relay for discussion. also ice cube relay.
07-23-2006, 12:30 AM
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Harvey-
A failed board relay won't blow the fuse. Try replacing the fuse and see what happens. If the new fuse blows, unplug the compressor coil at the connector on the harness across the front of the motor. Single spade terminal, above the front of the compressor. Then another fuse. The clutch coil is a large current draw, around 6 amps and inductive, so it would beat up switches/relays/fuses with pretty equal abuse.
If you do need to replace that relay, the symptoms will not include blowing the fuse. With the relay weak, the contacts can't carry enough current to pop the fuse. Fig Newton's First Law of Electrical Reciprocity says that the expensive component, protected by a fuse, will usually fail, protecting the fuse. In this case the fuse failed, so the expensive component must be OK.
HTH!
There are dedicated fuses for the fans on my S4, and my guess is that there is one for your earlier car too. I can confirm from WSM if you need that, just need to get into my library to do that research.
A failed board relay won't blow the fuse. Try replacing the fuse and see what happens. If the new fuse blows, unplug the compressor coil at the connector on the harness across the front of the motor. Single spade terminal, above the front of the compressor. Then another fuse. The clutch coil is a large current draw, around 6 amps and inductive, so it would beat up switches/relays/fuses with pretty equal abuse.
If you do need to replace that relay, the symptoms will not include blowing the fuse. With the relay weak, the contacts can't carry enough current to pop the fuse. Fig Newton's First Law of Electrical Reciprocity says that the expensive component, protected by a fuse, will usually fail, protecting the fuse. In this case the fuse failed, so the expensive component must be OK.
HTH!
There are dedicated fuses for the fans on my S4, and my guess is that there is one for your earlier car too. I can confirm from WSM if you need that, just need to get into my library to do that research.
07-23-2006, 12:45 AM
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Fig Newton's First Law of Electrical Reciprocity says that the expensive component, protected by a fuse, will usually fail, protecting the fuse.
I love it!!)....I'll have to plagiarize that one!!
I love it!!
)....I'll have to plagiarize that one!!
07-23-2006, 02:48 AM
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So true.
Originally Posted by dr bob
Fig Newton's First Law of Electrical Reciprocity says that the expensive component, protected by a fuse, will usually fail, protecting the fuse. .
07-23-2006, 10:58 AM
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Originally Posted by Chuck Jones
Fig Newton's First Law of Electrical Reciprocity says that the expensive component, protected by a fuse, will usually fail, protecting the fuse.
I love it!!)....I'll have to plagiarize that one!!
I love it!!
)....I'll have to plagiarize that one!!H2
07-23-2006, 12:40 PM
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"The clutch coil is a large current draw, around 6 amps and inductive, so it would beat up switches/relays/fuses with pretty equal abuse." - dr bob -
The compressor clutch current is about 3-4 amps max. Any more than that, indicates
a bad (shorted) clutch or a problem in the wiring.
The compressor clutch current is about 3-4 amps max. Any more than that, indicates
a bad (shorted) clutch or a problem in the wiring.
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07-23-2006, 12:50 PM
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Mine reads 3-4 ohms. Get out yer slide rule.
Edit:
Screwed that up! Loren is correct in the 3-4 amps. I was still thinking inrush amd multiply by the square root of three.
I suspect I do a lot more three=phase AC load estimates than i do DC stuff.
Sorry, Loren!
----
Back to studying the Fig Newtons...
Edit:
Screwed that up! Loren is correct in the 3-4 amps. I was still thinking inrush amd multiply by the square root of three.
I suspect I do a lot more three=phase AC load estimates than i do DC stuff.
Sorry, Loren!
----
Back to studying the Fig Newtons...
Last edited by dr bob; 07-23-2006 at 02:53 PM.
07-24-2006, 10:51 AM
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I printed out about a ream of paper's worth of Search Info on this topic and spent the weekend reading. Lot of repetitive but I think I'm about ready to tackle it now (famous last words). I'm hoping it's a simple fix....(Famous Last Words). After doing the job several times I'll know what to do in the future. Can't afford to get rid of this car....it owns me. And don't want to learn how to fix another. Can you imagine our conversations if we all owned Lambos or Farraris? The 928 fraternity is a great and very informed one...particularly in mechanical fixes. The Audi TT group spends most of its time talking about wheels and wax jobs. Pretty boring after you've done your umpteenth wax job. Us 928ers don't get bored.
Gotta do one of the Nevada runs...but not without AC. Thanks, all.
Harvey
Gotta do one of the Nevada runs...but not without AC. Thanks, all.
Harvey
07-24-2006, 10:55 AM
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Harvey,
If you will send me an email (not a PM) to techAT928gt.com (change AT to @), I will send you a paper that I wrote on the 928 HVAC system that might help...
If you will send me an email (not a PM) to techAT928gt.com (change AT to @), I will send you a paper that I wrote on the 928 HVAC system that might help...
07-24-2006, 11:20 AM
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Harvey--
'can't afford to get rid of this car 'cuz I've spent so much time doing repetitive repairs and I don't want to learn how to fix another'. 'The Audi TT group spends all its time talking about tires and wheels and wax jobs'.
Read what you typed, man! Imagine if you will (a little Twilight Zone melody plays in the background...) a world in which 928 owners around the world could sit back and only need to think about wheels and tires and wax. Instead, we constantly review the weaknesses we have discovered, the cures we have developed, and the things we need to do to keep our toys safe and reliable. And all because of a stupid movie we saw as teens, with a kid playing air guitar in his underwear and a cool-looking car in the background with a Ferrari engine in the sound track.
I love the car, I love the people in the group who make it possible for me to own and drive it. But I don't enjoy being forced to work on it. I enjoy working on it, don't get me wrong, I just don't like the "forced" part.
Many years ago, I gave up my favorite Saab Turbo for a Ford Explorer. Saab had 300k on it, and the (GM-supplied) AC was unreliable and the (GM-supplied) steering rack leaked a bit, plus the longer-term effects of excessive torque were showing up in the gearbox. There was a business reason too. Biggest suprise wth the new Explorer, beyond the relatively dismal performance, was the tremendous amount of extra time I had suddenly. Where the Saab had consumed a "few minutes" every Saturday morning, suddenly a few extra hours appeared. The 928 is at risk of consuming that kind of time for many of us. Yesterday, I went for a quick mountain drive to 'test how well my new video camera mount works'. Good excuse to go drive it. On the way back, I noticed that another fog light bulb needs replacement. With the cleaning and prep, maybe half an hour. The Explorer consumed two headlight bulbs in 240k and 14 years; The 928 has consumed 11 fog and headlight bulbs, plus four headlight bulb sockets, in 50k and 9 years. Then there's the side markers (all of them...), interior light bulbs (all that I leave on, plus the glove box light). Keep in mind, this is just the LIGHT BULB list.
I'll place odds that the Audi TT guys drive their cars a lot more than we do. All they get to talk about is tires, wheel, and wax. That's gotta be hell!
'can't afford to get rid of this car 'cuz I've spent so much time doing repetitive repairs and I don't want to learn how to fix another'. 'The Audi TT group spends all its time talking about tires and wheels and wax jobs'.
Read what you typed, man! Imagine if you will (a little Twilight Zone melody plays in the background...) a world in which 928 owners around the world could sit back and only need to think about wheels and tires and wax. Instead, we constantly review the weaknesses we have discovered, the cures we have developed, and the things we need to do to keep our toys safe and reliable. And all because of a stupid movie we saw as teens, with a kid playing air guitar in his underwear and a cool-looking car in the background with a Ferrari engine in the sound track.
I love the car, I love the people in the group who make it possible for me to own and drive it. But I don't enjoy being forced to work on it. I enjoy working on it, don't get me wrong, I just don't like the "forced" part.
Many years ago, I gave up my favorite Saab Turbo for a Ford Explorer. Saab had 300k on it, and the (GM-supplied) AC was unreliable and the (GM-supplied) steering rack leaked a bit, plus the longer-term effects of excessive torque were showing up in the gearbox. There was a business reason too. Biggest suprise wth the new Explorer, beyond the relatively dismal performance, was the tremendous amount of extra time I had suddenly. Where the Saab had consumed a "few minutes" every Saturday morning, suddenly a few extra hours appeared. The 928 is at risk of consuming that kind of time for many of us. Yesterday, I went for a quick mountain drive to 'test how well my new video camera mount works'. Good excuse to go drive it. On the way back, I noticed that another fog light bulb needs replacement. With the cleaning and prep, maybe half an hour. The Explorer consumed two headlight bulbs in 240k and 14 years; The 928 has consumed 11 fog and headlight bulbs, plus four headlight bulb sockets, in 50k and 9 years. Then there's the side markers (all of them...), interior light bulbs (all that I leave on, plus the glove box light). Keep in mind, this is just the LIGHT BULB list.
I'll place odds that the Audi TT guys drive their cars a lot more than we do. All they get to talk about is tires, wheel, and wax. That's gotta be hell!
07-24-2006, 12:19 PM
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Yea....Bob...after I sent that note...I said to myself, "Self...you're a dumbklutz. Do you know just what you said? What would you give to be able to JUST DRIVE your 928 and talk about waxing it instead of how to de-solder the AC controller relay and jerry rig a Radio Shack part?" And check the Tbelt every 1000 miles? And not spend couple thousand a year on basic upkeep?
Well, that said, the 928 is one heck of a nicer ride than the TT and, I think, a much better looker. The 928 is a true sports car that can pretty easily be made to perform like some of today's better (and much more expensive) sports cars while the turboe'd TT is really no more than an over-revved VW that happens to be relatively reliable (although the books don't rank it very high in that category). I've never had a technical prob. with the TT but I don't have a lot of miles on it either. But I do have more than 100,000 miles and 5 years left on the extended warranty so I don't worry too much. It's my wife's daily driver (although I put more miles on it than she does)..and quite uninteresting, except to look at.
The 968 is nice...but it's no 928 either. But I really like the 968 T belt design and reliability. Plus it's a '94 and a bit newer than my '85...and it resembles my favorite car. 968 racks nice while the TT feels like it's going to tip over...and I'm not going to re-shock it. I'd rather buy parts for the 928.
928...you gotta love'em. If not, spend a wad of money on fixes then sell them to me!!!
Harvey
Well, that said, the 928 is one heck of a nicer ride than the TT and, I think, a much better looker. The 928 is a true sports car that can pretty easily be made to perform like some of today's better (and much more expensive) sports cars while the turboe'd TT is really no more than an over-revved VW that happens to be relatively reliable (although the books don't rank it very high in that category). I've never had a technical prob. with the TT but I don't have a lot of miles on it either. But I do have more than 100,000 miles and 5 years left on the extended warranty so I don't worry too much. It's my wife's daily driver (although I put more miles on it than she does)..and quite uninteresting, except to look at.
The 968 is nice...but it's no 928 either. But I really like the 968 T belt design and reliability. Plus it's a '94 and a bit newer than my '85...and it resembles my favorite car. 968 racks nice while the TT feels like it's going to tip over...and I'm not going to re-shock it. I'd rather buy parts for the 928.
928...you gotta love'em. If not, spend a wad of money on fixes then sell them to me!!!
Harvey
07-24-2006, 03:36 PM
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try one of the spade connectors just behind the firewall on the engine side. (near the wiper motor and some AC lines) i used to have the same issue. however, as long as i got out of the car, popped the hood and cycled this connection, the compressor circuit woud go back in gear! you can hear it drag on the engine when its working! (along with the cold air out the vents)
may have been a bad relay in there , im not sure, but i never found out how to fix it, but i did make the AC work for several years after finding out how to get it to work when it failed occassionally.
MK
may have been a bad relay in there , im not sure, but i never found out how to fix it, but i did make the AC work for several years after finding out how to get it to work when it failed occassionally.
MK
07-24-2006, 03:54 PM
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Mark--
That's the compressor circuit, after the relay, running through the "anti-freeze switch". Disturbing the switch may have been the cause of the fix, as much as jiggling the connections.
That's the compressor circuit, after the relay, running through the "anti-freeze switch". Disturbing the switch may have been the cause of the fix, as much as jiggling the connections.
07-24-2006, 04:11 PM
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Harvey, lots of good info in this thread, https://rennlist.com/forums/showthre...5&page=1&pp=15
The head unit is simple to remove from the dash, but kinda a bitch to open up without breaking the little plastic tabs. Are you good with a solder gun? If you do go the route of replacing the relay on the head unit, let us know. There were quite a few of us there that day that could give you some pointers.
Rich
The head unit is simple to remove from the dash, but kinda a bitch to open up without breaking the little plastic tabs. Are you good with a solder gun? If you do go the route of replacing the relay on the head unit, let us know. There were quite a few of us there that day that could give you some pointers.
Rich