A/C Compressor wiring? EDIT: Shop set car on fire
05-04-2007, 01:56 PM
#17
Banned
Thread Starter
Tails thank you for that excellent write up, however I am having difficulty locating plug T29 to test, are they any specific markings that would identify it? I attached some images, can you locate the plug in the image?
05-04-2007, 02:30 PM
#18
Race Car
Have you put a gauge on the system yet?
You must have pressure for it to engage the clutch.
The compressor is switched ground not power.
See if you have any pressure at all 10-20-30 lbs with motor off low or high side.
Your pressure switch can be bypassed to allow a charge if compressor won't turn on due to low pressure.
I will share the trick with you, but start from the beginning before you fry your compressor.
You must have pressure for it to engage the clutch.
The compressor is switched ground not power.
See if you have any pressure at all 10-20-30 lbs with motor off low or high side.
Your pressure switch can be bypassed to allow a charge if compressor won't turn on due to low pressure.
I will share the trick with you, but start from the beginning before you fry your compressor.
05-04-2007, 02:38 PM
#19
Banned
Thread Starter
I don't have a set of gauges, but when the rear A/C hose was replaced the system was charged. I do not have 12v at the freeze switch, relay IV tested good, fuse 6 was a 7.5 amp fuse and fine but I replaced with a 5 amp fuse anyway. I need to locate plug T29.
05-04-2007, 02:45 PM
#20
Race Director
Originally Posted by whitefox
The compressor cycles on and off in ~30 second intervals, it blows cool air out the front, if i turn the rear A/C on, the front will just blow warm air while the back blows cold. "American Foreign Automotive" is the name of the place.
This sounds like low refrigerant to me.....when the rear A/C kicks on it pulls some R12 from the front system....this is why the total volume of R12 in rear A/C cars is higher......
Another thought on the warm front air is a vac leak causing the heater valve to open....or a bad heater valve....also quite common, but easy to fix
05-04-2007, 02:50 PM
#21
Banned
Thread Starter
Since the low pressure switch and freeze switch was bypassed, I suspect that might be the reason it was cycling on and off. I replaced the heater valve with a metal one from NAPA.
05-04-2007, 07:13 PM
#22
Banned
Thread Starter
Just found a picture of an 88 wiring diagram, T29 has two yellow wires, violet and yellow wire and two red and white wires running to it. Looks like my thumb was pointing right at T29, out to test. Wish me luck.
05-04-2007, 09:03 PM
#24
Ross
Terminal 2 plug T29 should have 12 volts when ignition and A/C switch is on (pressed in).
Check that when you press the A/C switch on (with ignition on) does the light in the switch turn on as this is an indication that power is passing through the switch.
If you don't get 12 volt at terminal 2 of plug 29 then the internal relay of the control switch may have failed.
When A/C switch is pressed on it supplies power to relay coil inside the A/C control switch, closes the relay and supplies power via terminal 2 of plug T29 to the thermo switch evaporator freezing protection.
If this relay coil has failed then this is probably the reason for no power (12 volts) at the double with spade connection of the thermo switch.
Tails 1990 928 S4 Auto
Terminal 2 plug T29 should have 12 volts when ignition and A/C switch is on (pressed in).
Check that when you press the A/C switch on (with ignition on) does the light in the switch turn on as this is an indication that power is passing through the switch.
If you don't get 12 volt at terminal 2 of plug 29 then the internal relay of the control switch may have failed.
When A/C switch is pressed on it supplies power to relay coil inside the A/C control switch, closes the relay and supplies power via terminal 2 of plug T29 to the thermo switch evaporator freezing protection.
If this relay coil has failed then this is probably the reason for no power (12 volts) at the double with spade connection of the thermo switch.
Tails 1990 928 S4 Auto
05-04-2007, 09:17 PM
#25
Under the Lift
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Is this a different relay than the one that supplies power to the compressor clutch? It looks like the same to me with the current path going from the relay through T29-2 to the freeze switch to the low pressure switch to the clutch. I'm just a little puzzled since he says the compressor runs, although it cycles. I wouldn't expect it to run with no power to the freeze switch.
05-04-2007, 10:20 PM
#26
Three Wheelin'
Originally Posted by Bill Ball
I wouldn't expect it to run with no power to the freeze switch.
05-04-2007, 11:16 PM
#27
Bill & Ross,
Brian has noted that the compressor has been hard wired in (as advised by Ross).
As a matter of fact, the PO of my car had hard wired the circuit to the compressor clutch from the thermo switch outlet to the the high/low pressure switch. However this is another story.
Bill and Ross if you check the Control Switch located through the field of C,D & E - 31 & 32 of sheet 5 electrical drawing you will note that the 12 volt power enters the switch from "connection point 8" (field location G33) via connector 1 of T29 and enters control switch at terminal 5. The wiring from connector/terminal 1-T29 also branches off to terminal 9 (inlet) of the push button A/C on/off switch. When the switch is pushed "on" power goes to the control switch terminal 7 to the internal relay and activates the internal relay solenoid and closes the circuit from terminal 5 and 6 connecting terminal 1 of the thermo switch via terminal 2 of T29, hence there should be 12 volts also at Temminal 2 of plug T29 when ignition and A/C switch is turned on.
So if the fault is in fact in the solenoid coil of this internal relay and the internal contact points do not close then there will be no power to the Thermo Switch.
As a check on whether the relay coil is not operating, bridge terminal 5 and 6 (by aligator clips and wire) and check whether there is 12 volts at terminal 1 of the thermo switch, i.e. the spade connector that has the two wire connected to it. If 12 volts is now present at this connector then I would suggest that the internal relay is at fault.
Once power is obtained at the thermo switch, the system should operater via the circuit to the pressure switch to the compressor clutch.
Ross let know how it goes.
Tails 1990 928 S4 auto
Brian has noted that the compressor has been hard wired in (as advised by Ross).
As a matter of fact, the PO of my car had hard wired the circuit to the compressor clutch from the thermo switch outlet to the the high/low pressure switch. However this is another story.
Bill and Ross if you check the Control Switch located through the field of C,D & E - 31 & 32 of sheet 5 electrical drawing you will note that the 12 volt power enters the switch from "connection point 8" (field location G33) via connector 1 of T29 and enters control switch at terminal 5. The wiring from connector/terminal 1-T29 also branches off to terminal 9 (inlet) of the push button A/C on/off switch. When the switch is pushed "on" power goes to the control switch terminal 7 to the internal relay and activates the internal relay solenoid and closes the circuit from terminal 5 and 6 connecting terminal 1 of the thermo switch via terminal 2 of T29, hence there should be 12 volts also at Temminal 2 of plug T29 when ignition and A/C switch is turned on.
So if the fault is in fact in the solenoid coil of this internal relay and the internal contact points do not close then there will be no power to the Thermo Switch.
As a check on whether the relay coil is not operating, bridge terminal 5 and 6 (by aligator clips and wire) and check whether there is 12 volts at terminal 1 of the thermo switch, i.e. the spade connector that has the two wire connected to it. If 12 volts is now present at this connector then I would suggest that the internal relay is at fault.
Once power is obtained at the thermo switch, the system should operater via the circuit to the pressure switch to the compressor clutch.
Ross let know how it goes.
Tails 1990 928 S4 auto
05-05-2007, 01:55 AM
#28
Under the Lift
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I read it was hardwired but that didn't make sense either - if it's hardwired to 12V, why is it cycling on and off?
BTW, I really appreciate the guidance on the circuits, Tails. I've done a bunch of AC service for other locals, but other than the controller relay itself and the usual leak here, leak there, I haven't had to get into T29 and the other circuits. This will come in handy when I encounter a more complicated situation. Thanks.
BTW, I really appreciate the guidance on the circuits, Tails. I've done a bunch of AC service for other locals, but other than the controller relay itself and the usual leak here, leak there, I haven't had to get into T29 and the other circuits. This will come in handy when I encounter a more complicated situation. Thanks.
05-05-2007, 03:50 PM
#29
Banned
Thread Starter
It works! The compressor no longer kicks on and off, steady on with terminals 5 and 6 jumped. Relay it is. Thank you! How should I go about replacing the relay?
05-05-2007, 09:24 PM
#30
Ross,
It appears to be the relay, but we need to confirm this.
You need to do a continuity test on the relay coil. Remove the 15 pin plug to the Control Switch , set you multimeter to max ohm resistance reading and measure the resistance of the coil using pin 7 and 4 on the Control Switch. If the coil is opened circuited you will get an infinity reading, if coil is OK you will get a ohm resistance reading.
If you get the infinity reading then this signifies that the relay coil has failed.
I have never removed the Control Switch, so I don't know whether the relay is an integral part of this switch or a plug in unit.
From your electrical drawing you can see that the Control Switch is a relative complex unit with resisters, diodes, a potentiometer etc that connects the temperature sensors, the vacuum solonid valves the temperature control unit, etc.
There are two things that you can now do:
1. Remove the Control Switch and see whether it can be dis-assembled. If so, then locate the relay and see whether it is replacable with a unit from an electronics or electrical supplier.
2. If it cannot be dis-assembled then it will need replacing. If it is easily removed then I would first try and get a loan unit to install and see whether it works the A/C compressor and HACV system as designed. There is a test procedure in the WSM.
If it does work everything OK then your options are a new unit or a second hand unit or see if someone is willing to repair it for you. Always remember that someone had to assemble it in the first place, so it can be dis-assembled.
Tails 1990 928 S4 Auto
It appears to be the relay, but we need to confirm this.
You need to do a continuity test on the relay coil. Remove the 15 pin plug to the Control Switch , set you multimeter to max ohm resistance reading and measure the resistance of the coil using pin 7 and 4 on the Control Switch. If the coil is opened circuited you will get an infinity reading, if coil is OK you will get a ohm resistance reading.
If you get the infinity reading then this signifies that the relay coil has failed.
I have never removed the Control Switch, so I don't know whether the relay is an integral part of this switch or a plug in unit.
From your electrical drawing you can see that the Control Switch is a relative complex unit with resisters, diodes, a potentiometer etc that connects the temperature sensors, the vacuum solonid valves the temperature control unit, etc.
There are two things that you can now do:
1. Remove the Control Switch and see whether it can be dis-assembled. If so, then locate the relay and see whether it is replacable with a unit from an electronics or electrical supplier.
2. If it cannot be dis-assembled then it will need replacing. If it is easily removed then I would first try and get a loan unit to install and see whether it works the A/C compressor and HACV system as designed. There is a test procedure in the WSM.
If it does work everything OK then your options are a new unit or a second hand unit or see if someone is willing to repair it for you. Always remember that someone had to assemble it in the first place, so it can be dis-assembled.
Tails 1990 928 S4 Auto