Instigator56

I have an ’89 Porsche 928 S4 with a manual transmission. Since I purchased the car, I’ve had done a lot of corrective maintenance and upgrades to make the car more reliable and enjoyable—see Just Joined Today! Meet my Old/New Car.

In mid-June 2023, my mechanic gave the car a clean bill of health before I took a drive to Peabody, MA (from Upper Marlboro, MD) at the end of June. I had no problems during the drive up (on a Sunday) and back (two days later)—but lots of rain both ways.

Currently, over the past couple of weeks, I’m having the following problem: the voltage drops from 12V to 11V (or maybe lower?) about a second or two after I step on the clutch.

Scenario:

• I’m driving any distance and I approach a stop sign or traffic light. If I either coast or apply the brake—no change in displayed voltage. However, a second or two after pressing in the clutch petal, the voltage drops down to approx. 11V or lower (as shown on the voltmeter needle) and then immediately comes back to 12V.
• Sometimes, when this condition occurs, the car stalls out! However, I’m able to do a rolling restart—i.e., put the car in neutral while rolling and turn the keyed ignition switch—with no problems—e.g., not restarting.
• Moreover, I determined that I can induce the voltage drop phenomenon if, after the car is above 2000 RPMs, I hold the clutch pedal down for approx. 2 seconds.

Since the posting nearly two years ago, along with some troubleshooting, my mechanic has the following:

- Bypassed leaky rear evaporator (to fix the A/C system).
- New battery (Interstate) and a battery maintainer ).
- New idle air control valve & O2 sensor.
- New intake gaskets and seals, crack case valley cover seal, valve cover gaskets, and spark plug tube seals.
- Rebuilt MAF.
- New coolant level sensor.
- New upper control arms.

After doing some “R&R” (Rennlist Research)—928 S4 engine shuts off while driving (Posts #1, #2, #5, #7, #9) & GTS Engine Cut Out - Intermittent (Posts #1, #8, #13, #19, #31, #32, #43)—my “guess” is that I have bad 53B relays [since I don’t have any evidence from the maintenance history that any of the previous owners (going back 8 years) replaced any of the relays]. So, I plan to have all three replaced. Also, I plan to ask my mechanic use his IR gun to see if any of the other relays are warmer than usually.

Questions:

• Is there something obvious that I’m missing?
• Has anyone else experienced this problem? If so, what was your fix?

Thanks, in advance, for any suggestions that you can provide.

Alan

Sounds like you just have an idle control problem you haven’t yet fixed. When the idle drops you will get a lower voltage - in hot conditions (ambient and car) this will be worse. Add in some ISV operational issues and you will stall out or really drop the voltage due to low idle speed. AC on will likely make this worse. I had an issue on my GTS where the ISV misbehaved eg (low idle stall) only when it was really hot - cold it performed perfectly OK.

Alan

Instigator56

@Alan, I thought the same thing; but dismissed the thought since I have a new ISV (installed: Mar 2022). Is there something other than the ISV that controls idle speed?

Alan

I'd still suspect that as the #1 possibility - are you certain it was changed rather than just checked out (do you have the old one?) - mine was checked twice and seemed perfect (cold) before I demanded it be replaced with new - as it was to me the most likely cause (and most other things had by then been eliminated). Car has run perfectly since then.

Alan

Rob Edwards

Could there be something wrong with the cruise control switch on the clutch pedal that's causing the voltage drop? Which brand of ISV got installed?

Petza914

So if you're stationary with the engine running and the trans in neutral, if you use the accelerator to say hold the revs at 2200-2500 rpm for 10 seconds the let them fall back to idle and never touch the clutch the voltage will drop to 12v and stay there for as long as the car is running?

I can't think of a link between the hydraulic clutch system and the electrical system that would explain what you're seeing. Because you mention it can also stall, that voltage drop you're seeing is because the idle has fallen almost to zero and then recovers.

See if it behaves the same with the AC on and off.

Instigator56

@Rob Edwards , the shop says that they installed a new Porsche OEM ISV as a replacement.

Also, thanks for the cruise control switch suggestion. I haven't had a manual cars that also had cruise control. How does that switch affect idle speed control?

Instigator56

@Alan , my shop tells me that they installed a Porsche OEM ISV back in Mar 2022. As I recall, the original one was causing the car to stall at idle.

Glad to hear that your car is running smoothly. 👌 😊

Mrmerlin

I would inspect the adjustment of the throttle cable , ( the one that goes around the wheel ) It might be too tight.
Verify that the TPS is clicking both cold and hot.

Did the crank position sensor and the knock sensors and the hall sensor at the RF cam get replaced?
Was a the Flappy valve stop replaced so the flappy wont jamb?
Did the 2 plastic junctions that plug into the MAF boot get replaced?

Instigator56

I believe that I had the knock sensors replaced in 2019. I'll have to check my records about the others

Instigator56

So, my mechanic has located the problem…

Good News: The problem was not engine-idle related. The CPS and TPS are fine as well as the ISV and the MAF. Also, I’ve learned A LOT about alternators in the last couple of days. 😊

Bad News: The regulator in my 3-YO alternator [175 A high output alternator from 928 Motorsports (P/N: ELE-HOA-175A-88/95)] is no longer functioning.

Back in July 2020, I ordered a high output alternator to replace my 30 YO OEM alternator; it was installed by my mechanic in Sep 2020. Since I was replacing the OEM radiator fans with 928 Motorsports’ Dual High Performance Fan Kit (P/N: COL-DRF) and adding condenser fans (to help the A/C work better in city traffic); I believed that the OEM alternator wouldn’t be able to keep up with the load. So, the new alternator was my insurance policy to ensure all the electrical things would stay happy. My assumption was that this new alternator was supplying 13.5 V to 14 V (cold, measured at the alternator terminal and alternator frame) when new.

As my mechanic was troubleshooting the car’s clutch-related (or so I believed) voltage drop, he load-tested the alternator. Unfortunately, it is putting out only 13V with “no-load” and 12.2V under load—which means that I’ve been running off of battery power under load for a while. ☹ The shop believes that the voltage regulator inside of the alternator failed due to heat. When I had the new alternator installed, I deleted the cooling hose and shroud b/c I believe that modern alternators didn’t need this—and I didn’t know how to adapt it to the new alternator. In hindsight, that deletion might have allowed that alternator to receive its cooling air from the hot radiator.

My mechanic recommends that I go back to the OEM alternator—I assume that it’s a Bosch unit. I believe that he’s referring to the one with the external voltage regulator; however, that one only puts out 115 amps IIRC. So, I’m uncertain if its up to handling twin condenser fans, twin radiator fans, a A/C blower set to level 2 (during the day in traffic) and headlights (at night).

Questions:

1) What should I choose as a replacement since 928 Motorsport (under Carl’s ownership) is no more? For example, should I go OEM or should I look for a “better” high output alternative?

2) Should I find a better way to provide cooling air to the alternator?

As always, I welcome your advice and recommendations.

Petza914

The 928MS alternators were Mechman units which are very good alternators. I would contact Mechman directly and buy a new voltage regulator for the alternator you have - it's probably $50.

Instigator56

Thanks for the recommendation! Unfortunately, the Mechman representative just told me that they no longer make this alternator or carry any parts for it (e.g.,voltage regulators). It seems that only 928 Motorsports (i.e., Carl) ordered this version that was for 928s and the last order made was... CY2019.

Petza914

Well that sucks as it's probably the same one that's in my 928 Spyder.

Mrmerlin

Greg Brown from Precision Motorwerks was selling Custom made uprated output alternators for the 928,
they are plug and play, from what I have heard they work well.

That said, also there was a thread on RL by Bluvot about fitting another brand of alternator that required little in the way of modding to make work,
and was quite user friendly in the cost department.

Lastly you could buy a stock alternator and take it to the repair center and have them build you a Higher output version,
that way you can use the airduct ,