Climate Control Unit Relay Failure - Common?
#17
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Dan
'91 928GT S/C 475hp/460lb.ft
#18
Chronic Tool Dropper
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The clutch coil doesn't know, at least electrically, how well connected the friction faces are, or what torque is applied to the compressor. The clutch coil is mounted to the compressor housing though, where additional load often means additional heat that could migrate to the coil. It's a stretch though, since the clutch also sits in the airflow from the radiator fans.
The original relay contacts are rated for AC operation. Alternating Current, not Air Conditioning... The clutch coil is an inductive DC load, where the collapsing magnetic field on relay contact opening generates a really decent electrical spike. Result is arcing at the relay contacts. This is very similar to the effect on distributor points, in vintage points-and-condenser setups that feed an ignition coil. Porsche added the "suppressor" in the CE panel to help soak up that spike, but it doesn't quench it completely. It really needs that condenser, instead of the diode-in-a-can, but that would add to the inrush load on initial relay close. Lots of things conspire to make relay failure inevitable, at least for those of us who use the AC.
#19
Drifting
I did a rewire at the CE Panel per Alan's instructions for my 91 GT, it works perfectly and takes the stress of off the head unit relay.
If anybody is interested I can post it or send a PM to Alan and he would give you instructions for your model year.
If anybody is interested I can post it or send a PM to Alan and he would give you instructions for your model year.
#20
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Bilal: Pls do. I am currently installing a relay in the engine bay. Since it is nearly done, I will test it when I get back from vaca. On the chance it doesn't work as advertised (b/c of my inexperience at this) I may have to try a second time. I started the CE route but it was just too confusing when I got to re-wiring the plugs; w/o a step-by-step I was lost.
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FYI: A new out-of-the-box clutch needs 3 amps to engage the compressor.
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#26
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Hey Ran,
Took me a while but I found Wally's wisdom. He also posted for adding a relay for the fans in case we ever have a problem with powering them. Same principle. I added something I have regarding a standard relay's pin layout.
Craig
https://rennlist.com/forums/6708030-post3.html
https://rennlist.com/forums/8415630-post7.html
Supply - 30 - power in to relay
Trigger - 86 - 'switch' - the on/off of the relay
Load - 87 - power out from the relay
Simple, and I like this approach. Leave out the diode fan stuff until you need it.
Took me a while but I found Wally's wisdom. He also posted for adding a relay for the fans in case we ever have a problem with powering them. Same principle. I added something I have regarding a standard relay's pin layout.
Craig
https://rennlist.com/forums/6708030-post3.html
https://rennlist.com/forums/8415630-post7.html
Supply - 30 - power in to relay
Trigger - 86 - 'switch' - the on/off of the relay
Load - 87 - power out from the relay
Simple, and I like this approach. Leave out the diode fan stuff until you need it.
Last edited by MainePorsche; 06-26-2013 at 04:40 AM.
#27
Drifting
Here is Alan's procedure for installing a relay in the CE panel and utilizing the non used relay and fuse sockets for a stock look. I have done it and it works perfectly. Thanks Alan.
This is for a 91 GT.
For the 91:
The clutch connection from the HVAC head is on CE pin G21 - the wiring on the CE panel rear connected to this should be moved to connect to your new relay's terminal 87 (output). Terminal 30 on your new relay should connect via a fuse (pick one of the unused ones) to either the battery bus (30), the X bus (X) or the ignition bus (15) doesn't really matter which. Some of the unused fuses may have connection to one of these busses already (depends on car options).
Now add a new wire from CE pin G21 to drive your new relay's coil on relay pin 86 & ground the other coil connection on relay pin 85.
Spare relay socket blocks (unwired) on a '91 should include III (wired but unused - DRL), VI & XX.
Fuse 31 & 32 on a '91 should be unused & available for your use they are totally unconnected inputs (top) and outputs (bottom). Fuse 43 is also wired for an optional telephone (it also supplies the Cat Monitor) but you should be able to use this assuming you don't have a telephone installed.
Its an ignition supplied fuse with its output on CE pin N24. The stock 7.5A fuse should be OK.
Alan
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This is for a 91 GT.
For the 91:
The clutch connection from the HVAC head is on CE pin G21 - the wiring on the CE panel rear connected to this should be moved to connect to your new relay's terminal 87 (output). Terminal 30 on your new relay should connect via a fuse (pick one of the unused ones) to either the battery bus (30), the X bus (X) or the ignition bus (15) doesn't really matter which. Some of the unused fuses may have connection to one of these busses already (depends on car options).
Now add a new wire from CE pin G21 to drive your new relay's coil on relay pin 86 & ground the other coil connection on relay pin 85.
Spare relay socket blocks (unwired) on a '91 should include III (wired but unused - DRL), VI & XX.
Fuse 31 & 32 on a '91 should be unused & available for your use they are totally unconnected inputs (top) and outputs (bottom). Fuse 43 is also wired for an optional telephone (it also supplies the Cat Monitor) but you should be able to use this assuming you don't have a telephone installed.
Its an ignition supplied fuse with its output on CE pin N24. The stock 7.5A fuse should be OK.
Alan
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Thanks guys.
Decided to go with the hot point trigger relay approach due to the heavy current leading seen at the control head.
That was followed by a complete AC compressor replacement.
Decided to go with the hot point trigger relay approach due to the heavy current leading seen at the control head.
That was followed by a complete AC compressor replacement.
#29
Race Car
Dan
'91 928GT S/C 475hp/460lb.ft