Wich fuse for a/c clutch?
04-10-2018, 12:05 AM
#1
Burning Brakes
Thread Starter
Wich fuse for a/c clutch?
Hi, My a/c clutch is not engaging, and I have no power at either terminal on the pressure switch. My fuse chart doesn't show a specific fuse number for the clutch, but all the fuses that reference anything concerning fans, HVAC etc. is fine.
Please suggest where to look next?
Thanks
Please suggest where to look next?
Thanks
04-10-2018, 01:57 AM
#3
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Check for voltage and current at the A/C anti-freeze switch.
The "fuse" for the A/C clutch is the 2A relay (for the 3A clutch circuit) soldered to the PCB of the HVAC head.
This is a very common failure and one of a few "WTF were they thinking" designs of the 928.
Assuming that the relay died open then the solution is to rebuild the HVAC head with a 5A relay and put a 4A fuse in a weather-proof housing at the anti-freeze switch.
There are numerous threads on how to DIY the rebuild. Or you can send it out.
If the relay died 'closed' and allowed a serious over-current situation, the HVAC head may not be rebuildable.
The "fuse" for the A/C clutch is the 2A relay (for the 3A clutch circuit) soldered to the PCB of the HVAC head.
This is a very common failure and one of a few "WTF were they thinking" designs of the 928.
Assuming that the relay died open then the solution is to rebuild the HVAC head with a 5A relay and put a 4A fuse in a weather-proof housing at the anti-freeze switch.
There are numerous threads on how to DIY the rebuild. Or you can send it out.
If the relay died 'closed' and allowed a serious over-current situation, the HVAC head may not be rebuildable.
Last edited by worf928; 04-10-2018 at 11:04 AM.
04-10-2018, 05:35 AM
#4
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The HVAC control head is another one of those "if it has not failed already it is about to" items. Dave's post tells you most of what you need to know and if you are not already aware, once the control unit has decided you need power to the a/c clutch the wiring takes the circuit through the de-icer switch [that breaks contact if it is too cold] and the low pressure switch in front of the radiator [that will not make unless there is sufficient pressure in the system] before arriving at the a/c clutch. If there is no sign of voltage to earth on either of the pressure switch contacts then almost certainly something in the HVAC head has gone south.
If you go to Dwayne's garage there is an excellent thread on how to replace the [shot?] relay with an auxiliary item. You can also fit an in-line fuse holder for additional security
If you go to Dwayne's garage there is an excellent thread on how to replace the [shot?] relay with an auxiliary item. You can also fit an in-line fuse holder for additional security
04-10-2018, 11:51 PM
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Of the 'good mode' relay failures about 1/3 of them (my data) fail such that the relay will pass 12v to the clutch circuit but will not allow sufficient current through. The bulb in the A/C switch will light but the clutch will not engage.
If the button light lights, but the clutch doesn't engage then, after checking for voltage at the low-pressure switch, the next step is to put a test fixture - basically a socket with a tail light bulb (something that requires 2-3A) - in the circuit after the pressure switch and see if it will glow brightly or not at all or very dimly.
04-11-2018, 03:19 AM
#7
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This is true but not completely sufficient.
Of the 'good mode' relay failures about 1/3 of them (my data) fail such that the relay will pass 12v to the clutch circuit but will not allow sufficient current through. The bulb in the A/C switch will light but the clutch will not engage.
If the button light lights, but the clutch doesn't engage then, after checking for voltage at the low-pressure switch, the next step is to put a test fixture - basically a socket with a tail light bulb (something that requires 2-3A) - in the circuit after the pressure switch and see if it will glow brightly or not at all or very dimly.
Of the 'good mode' relay failures about 1/3 of them (my data) fail such that the relay will pass 12v to the clutch circuit but will not allow sufficient current through. The bulb in the A/C switch will light but the clutch will not engage.
If the button light lights, but the clutch doesn't engage then, after checking for voltage at the low-pressure switch, the next step is to put a test fixture - basically a socket with a tail light bulb (something that requires 2-3A) - in the circuit after the pressure switch and see if it will glow brightly or not at all or very dimly.
I am currently planning a relay change out "A La Dwayne" in the currently defunct GTS module.
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04-11-2018, 03:27 AM
#8
Burning Brakes
Thread Starter
Reading the above - my a/c button light up.
Also I don't know whether this is relevant, but it seem my radiator fans are also not kicking in. More info is that everything worked before I replaced my starter motor brushes. I did disconnect the battery before removing the starter motor.
Also I don't know whether this is relevant, but it seem my radiator fans are also not kicking in. More info is that everything worked before I replaced my starter motor brushes. I did disconnect the battery before removing the starter motor.
04-21-2018, 02:57 AM
#10
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Add an inline relay fed off of the jump post instead of the Dwayne upgrade which can put excessive load on the traces.
An external relay fed from the head unit's relay as a trigger takes almost all of the load off of the internal relay and is easier to change when it eventually wears out.
I did this in my 81 and had no trouble with it until my head gaskets went.
An external relay fed from the head unit's relay as a trigger takes almost all of the load off of the internal relay and is easier to change when it eventually wears out.
I did this in my 81 and had no trouble with it until my head gaskets went.
04-21-2018, 11:57 PM
#11
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This is true but not completely sufficient.
Of the 'good mode' relay failures about 1/3 of them (my data) fail such that the relay will pass 12v to the clutch circuit but will not allow sufficient current through. The bulb in the A/C switch will light but the clutch will not engage.
If the button light lights, but the clutch doesn't engage then, after checking for voltage at the low-pressure switch, the next step is to put a test fixture - basically a socket with a tail light bulb (something that requires 2-3A) - in the circuit after the pressure switch and see if it will glow brightly or not at all or very dimly.
Of the 'good mode' relay failures about 1/3 of them (my data) fail such that the relay will pass 12v to the clutch circuit but will not allow sufficient current through. The bulb in the A/C switch will light but the clutch will not engage.
If the button light lights, but the clutch doesn't engage then, after checking for voltage at the low-pressure switch, the next step is to put a test fixture - basically a socket with a tail light bulb (something that requires 2-3A) - in the circuit after the pressure switch and see if it will glow brightly or not at all or very dimly.
There's a lesson in there, that will improve everyone's life, when messing with vehicle electrical problems:
Always, always check a 12 volt circuit with the load attached. (Make sure everything is connected when testing.)
04-22-2018, 09:30 PM
#12
Burning Brakes
Thread Starter
Add an inline relay fed off of the jump post instead of the Dwayne upgrade which can put excessive load on the traces.
An external relay fed from the head unit's relay as a trigger takes almost all of the load off of the internal relay and is easier to change when it eventually wears out.
I did this in my 81 and had no trouble with it until my head gaskets went.
An external relay fed from the head unit's relay as a trigger takes almost all of the load off of the internal relay and is easier to change when it eventually wears out.
I did this in my 81 and had no trouble with it until my head gaskets went.
04-22-2018, 10:38 PM
#13
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Just make sure it is fused from the jump post.