traxtar928

Sean's talking about me! HAHA...

The charging system on my 928 has been the bane of my existence for the past 3 years. I've installed a new battery ground cable, new 0-gauge marine power cable from battery to alternator, tried a new battery to see if anything changed (nothing changed), new bosch alternator (not reman), replaced the tattered cooling hose to alternator, new 14-pin harness from Sean, new engine ground cable, cleaned all fuse panel grounds and checked the others, cleaned the connections on the back of the gauge pod, checked my relays... I'm probably forgetting some other things I've done.

I'm STILL getting a serious voltage drop (12v outlet will drop from 12.3V to 10.0V) at red lights when the HVAC blower is running and the air conditioning is running. I've also looked at the HVAC controller and refurbished it when I was doing my diaphragms and looking at why the car was having AC warm-air issues, and replaced the HVAC blower motor ~3 years ago when the original fan self-destructed.

When the HVAC blower is off, there is plenty of amperage to keep the voltage in the car at 12.5V and 13.5-14V at the jump post.



At this point, I'm wondering if the AC compressor could be draining my voltage somehow, or maybe the HVAC resistor pack? Clearly I'm running out of things to check... I have no idea what to do next, and am considering just having Sean go at it for diagnostic work to find the problem I'm so fed up with it. Sigh... Really bothers me that I can't figure this one out.

Alan

Sounds like a CE panel to HVAC connection issue - where the HVAC blower is causing a voltage drop. When this happens what is the jump post doing?

I'd clean all the CE grounds, and clean the loom connectors on the bottom of the CE panel. Maybe unplug the HVAC blower for a bit (maybe wait for cooler weather) and see how everything behaves then?

Alan

Carl Fausett

No problem Alan, I am not speaking as an EE as you are, just an old mechanic with 31 years of experience. Its a situation I have seen played out multiple times, where the alternator "wont charge" and hovers around in the low voltage range just above 12. Pull the old wet-cell lead acid battery, replace with fresh battery, start it up, whattya know, now she's charging at 13.6 no problem!

I also note that some alternator manufacturers will void your alternator warranty if a fresh battery is not installed with the new alternator at the same time. I figure that stands for something, too.

Do AGM batteries change this? I don't think so. However, Sean says he has had problems with Optima Reds in 928's, and I have had very good results with them in 928's. For what its worth.

Bigfoot928

I always use the Interstate MTP-91 battery. 700CCA - 875 CA. I think the original fitment was for a MB diesel. it fits without issues in the S4 and has plenty of cranking for stroker high compression engines.

Alan

Good - So we are agreed that the alternator is not actually on speaking terms with the battery then. That's a good start.

To the warranty question - I think we can agree it is pretty well unenforceable in practice...

But it probably does indeed help getting less alternator returns due to failing to charge an old battery. I mentioned this above (and like the case you noted in your reply too)

Would a lawyer just laugh at it - probably - but if it mostly works as a deterrent why not go with it - its not likely they would ever try to follow though on it. Just posturing I think.

I would hope most people would logically start with testing the battery before replacing an alternator anyway?? But battery testing isn't always performed by well trained technicians as I noted - so YMMV!

Alan

traxtar928

The blower isn't the main culprit... it causes a drop of 0.2-0.3V at the 12v outlet. As soon as AC kicks on... down to below 10V when the car isn't moving. The compressor is good, but is it possible that the AC clutch has a short?? Can the compressor clutch be changed out without removing the compressor itself?

When stopped while in drive with the AC on and blower running, I have to shift the car into neutral to prevent the voltage from dropping below 10V, otherwise my radar detector will auto shut off due to low power. The rpm's do not change between drive and neutral when stopped... stays steady at ~700 rpm or so. So, I don't think it's an alternator speed issue, either. This fluctuating voltage issue also means that I eat through car chargers since they burn up from the ever-changing input power. Sigh...

Alan

Pull off the connector for the refrigerant pressure switch - this will disable the compressor clutch - does it still do the same thing?

Alternatively you can measure the actual current to the compressor clutch by inserting a 10A ammeter (DVM in amp mode on 10A terminal) into the circuit by breaking the circuit at one terminal of the freeze switch (either one).

I expect the compressor clutch is ~2A.

Alan

traxtar928

I already have the HPS and LPS disconnected from the system as part of troubleshooting my AC issues this summer. I will measure current going through the freeze switch and report back... most likely this weekend. As always, thanks for the assistance, Alan.

James Bailey

What happens as the insulation of the coil winding in the A/C clutch starts to fail and ultimately try to become a solid lump of copper ? Obviously at some point it no longer engages the compressor...

dr bob

True. There's a very simple test for this with the Ohms function on a common DMM. IIRC I measured the coil resistance at about 4 Ohms when I first started looking at my control head relay failure. Divide the system voltage (12.4 - 14) by your resistance value and you get the current (Amps) that will pass through the circuit.

Remember that originally there is no fuse specific to the AC clutch circuit, so if the coil fails via an internal short-circuit, the little traces on the control-head circuit board become the sacrificial section. You can see whey GB adds a 3A inline fuse to his repaired control heads. For those circuits with less than 2.5 Ohms of loop resistance...

Jim M.

To the OP, I had SeanR install one of his harnesses on my GTS and it solved a multitude of problems. Worth every penny!