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Newbie here, got my 928 S4 about a year ago, having problems w/ my AC units for the past 3 months. After I replaced the relay in the head unit w/ a 10amp DPDT, the clutch engaged again w/ cold air blowing, then recently I saw smoke out of the head unit, so I turned AC off right away, was still blowing cold air. I found that smoke came from the edge connector cable I believe, the red/white one, if I push the AC button, it will got up to about 150+F, then starts smoking again, I also notice the new 10amp relay gets very hot too, 150F as well. The AC clutch still works and have cold air, but I am afraid to leave the AC on now for obvious reason. Any suggestions on what to look for? bad head unit?
The wire powering the compressor is small and goes across top of the engine. I'm surprised you haven't burned out the freeze switch yet. As the wire gets hotter, the more current it needs from the head unit, so wires are getting too hot there...that's what burned out the on-board relay.
Stick a relay under the hood to change the load circuit to a trigger. I leached power from the jump point. It also powers a new pusher fan I put in front of the condenser with no problems.
if the load is increasing from the compressor due to mixing oils or cross breeding refrigerants,
then it might be prudent to overhaul the AC system and start off with a new compressor and new O rings and exp valve and drier
Definitely do a remote relay in the engine bay. I did it for my 81 and it was well worth it.
It takes nearly all load off of the factory wires to the head unit.
But do as Mrmerlin says and tear the system apart and properly service it. I did it to my 89 and it has been wonderful over the last 4 years.
A major pain for sure, but only made worse due to the compressor failing under the care of a PO. Save yourself the hassle and do it now before something fails.
Thanks for all the feedback, come to think of it, the PO's 5amp relay went out about 3 months ago after I recharged and added oil w/o knowing what was in the system. So looks like I will need to overhaul the system as suggested. I also need to add a quick connect for the high side too, the PO only did the low side.
If you do buy a high side valve, get the Porsche one: 92857396503
It's very nicely made and worth the price. The low side Porsche part however is a cheap POS part that you should avoid and instead buy one that has it's own internal schrader valve.
Newbie here, got my 928 S4 about a year ago, having problems w/ my AC units for the past 3 months. After I replaced the relay in the head unit w/ a 10amp DPDT, the clutch engaged again w/ cold air blowing, then recently I saw smoke out of the head unit, so I turned AC off right away, was still blowing cold air. I found that smoke came from the edge connector cable I believe, the red/white one, if I push the AC button, it will got up to about 150+F, then starts smoking again, I also notice the new 10amp relay gets very hot too, 150F as well. The AC clutch still works and have cold air, but I am afraid to leave the AC on now for obvious reason. Any suggestions on what to look for? bad head unit?
Great forum and thanks,
Danny
You need to check that the compressor clutch winding resistance is not less than 3-4 ohms. Measure the resistance from the clutch output pin of the head unit to ground.
Definitely do a remote relay in the engine bay. I did it for my 81 and it was well worth it.
It takes nearly all load off of the factory wires to the head unit.
But do as Mrmerlin says and tear the system apart and properly service it. I did it to my 89 and it has been wonderful over the last 4 years.
A major pain for sure, but only made worse due to the compressor failing under the care of a PO. Save yourself the hassle and do it now before something fails.
Forgot to mention, I got only ~7V on either antifreeze connector w/ key on and engine off, then ~9v when engine on. The start post is 12.5/13.5, with engine off/on. Is this normal? I will measure the resistance later.
Because the internal relay inside of the factory head unit IS the problem.
It's a 1 amp relay on a 2 amp circuit. It acts as a fuse. Same for the wires, they're marginal at best.
To actually get it to last an external relay is preferred to take the load off of that circuit, which is why the wires are burning up to begin with.
Could easily be a voltage drop due to corrosion in the factory front end harness that could burn the wires up without having any real failure or the compressor is working too hard as others speculate.
We'll know after the system is serviced and charged properly.
Because the internal relay inside of the factory head unit IS the problem.
It's a 1 amp relay on a 2 amp circuit. It acts as a fuse. Same for the wires, they're marginal at best.
To actually get it to last an external relay is preferred to take the load off of that circuit, which is why the wires are burning up to begin with.
Could easily be a voltage drop due to corrosion in the factory front end harness that could burn the wires up without having any real failure or the compressor is working too hard as others speculate.
We'll know after the system is serviced and charged properly.
928 International provides a rebuilt HVAC head unit with an upgraded relay (8 amps) that doesn't require a Mickey Mouse modification by adding an external relay.
You need to properly troubleshoot your HVAC system to prevent any electrical damage, e.g. a compressor clutch resistance less than 3-4 ohms.
Remember, it's better to not have a HVAC system with a highly over-rated relay that could result in an electrical fire, if a system failure occurred - shorted compressor clutch.
In addition to sorting out the head unit ,
the AC compressor and the lines need to flushed out and new O rings and EX valve and drier replaced,
a new compressor comes with a new coil also the two flex lines should also be replaced,
check Griffiths for AC parts, and lines
NOTE they have new Denso compressors for about 450.00 flush everything then take it apart then replace all the O rings with the green versions.
Make sure to note what oil and refrigerant you install into the fresh system
The PO has the Griffiths Kuehl AC kit and its mounting HW, is it hard to convert back to the original style compressor? Since I have to replace the 2 high/low hoses, now is the time to either stay w/ the Kuehl kit and keep the current HW or go w/ Denso and find original brackets, any suggestions?
HVAC AC burnt wire
