Help needed with misfire when radiator fans are at high speed
 

I've been having a growing problem in my 944S2 on hotter days when the engine cuts out occasionally, sometimes for several seconds. At idle, it might stall out completely, but on the highway, it might make acceleration or even keeping up speed very difficult.

When the misfires happen, I can see the voltage gauge drop from a normal 13.5V - 14V to down near 12V instead. It coincides with the radiator/condensor fans cycling to full blast, which usually happens right away if I turn on the AC.

I did replace the brushes and install a variable voltage regulator on the alternator, so I could try turning that up again, but I feel like it's keeping the voltage high enough most of the time. The battery is reasonably new (about 1y), and I've cleaned all the engine grounds. I can think of a few other reasons this is happening, but I wanted to see if folks recognize this behavior enough to give guidance.

1. The alternator is just not keeping up with the load. The voltage regulator did help keep it higher in general, but if the voltage is still dropping under periods of high current draw, maybe the alternator just cannot keep up.

2. The fans are drawing too much current. I have no idea how to test that. Do folks see their fans cause a noticeable drop in the running voltage of the car as they cycle from normal speed to high speed?

3. The ignition system needs a makeover. I don't know the last time the spark plugs, plug wires, ignition coil, or the distributor/rotor were replaced. I could just buy a bunch of parts and replace them to see if that game of whack-a-mole fixes my issue.

4. Upgrade the ignition or fan system? Are they both just really inefficient and are there more efficient options for each that are less power-hungry?
-Neil

Cross-posted to Rennlist: https://rennlist.com/forums/924-931-944-951-968-forum/1301711-help-needed-with-misfire-when-radiator-fans-are-at-high-speed-2.html

This sounds like a diagnosis that might be best approached by someone with a 4 channel oscilloscope (like a Picoscope). I would monitor the ignition waveform, the injection waveform, the ground at the engine, and the amperage from the battery. The best thing about a DSO (digital storage oscilloscope) is that you can see how things change in relation to other events at the same time.

You can monitor the amperage to the radiator fans by using the clamp ammeter.

For the first free thing to try, remove and clean all your ground connections. Can't hurt, might help.

I haven't been able to reproduce this except when driving on a hot day. I will try running the air conditioner more often, even just up on the lift now, to see if that forces the situation more often, but it has only had an issue when driving so far.

And I've already cleaned all the grounds.

It doesn't do a ton of good to monitor the current to the fans because I don't know what the appropriate numbers are for those. And I imagine the resistor packs that control fan speed may also be a sink for some of that current. It may not all be going to the fans.

Does anyone know roughly what overall current the alternator should be able to put out?
Does anyone know roughly what current the fans should consume when at full blast?
Does anyone know if there are good upgraded fan options that are more electrically efficient?
Does anyone know if there are good upgraded alternators available?
-Neil

My alternator puts out 13.5-14V based on the gauge on the dash at idle. If yours does not, I would start with the alternator and the wiring to/from alternator to battery/ground. The fact you are getting misfires when fans are on leads me to believe your alternator cant keep up, but could definitely also be caused by issues with your ignition system or fans.

First step would be to confirm alternator is working properly and correct voltage output, checking both at the alternator itself, and then again at the battery to be sure the wiring to from alternator is working as intended.

Your alternator is a Bosch 115 amp alternator (when new and all parts are working properly).

The alternator (with the adjustable voltage regulator) puts out 13.5-14V normally. It drops 1-1.5V when the fans kick on high. I guess the question is whether that's because the fans are causing an unnecessarily large current draw (which I would expect to potentially blow a fuse) or if the alternator just cannot maintain that voltage under higher loads. I will try to reproduce the issue on a lift where I can more easily measure current from the alternator. If I'm not getting near 115A when the voltage drops, then I guess maybe I just need a new alternator.

Do folks find that these alternators fail even with new brushes?
-Neil

Alright, I turned everything on I could (defroster, stereo, headlights, fog lights, AC and fan at full speed, etc), and I measured current out of the alternator, and I only got about 72-74A. At that point, the car was running decent, but it occasionally stuttered a bit and the lights flickered. I kept probing around for some voltages across the starter and the chassis, trying to find if there are differing voltages in places. It was pretty consistently in the 12.2-12.7V range, so definitely not where I'd like to see it.

Once the car was sufficiently warmed up again, it was stuttering worse. At this point, the current out of the alternator was struggling to get 55A, occasionally bursting to almost 60A.l It seems likely my alternator is weak.

@944 Ecology, are you sure it's supposed to generate 115A? That seems like a lot higher than I'd expect. Clearly, mine is not generating enough current regardless, but I find it hard to believe it's lost that much of it's functionality.

Anyway, I'm off to do some alternator research.
-N

115A is printed on the label.

Do you have the alternator belt adjusted as per the workshop manual? If you did not use the factory tool, it is probably not tight enough.

I'm less confident in my diagnosis from last night. I think it makes sense that the alternator should draw more current right after starting than 20 minutes later when it's warm. When it first starts, it should be providing both the power to run the car _and_ the power to recharge the battery after starting. 20 minutes later, the battery should be topped up, and the alternator should be able to relax the current output.

It's definitely still not hitting the desired voltage though, even with the regulator turned all the way up. So it might be the alternator, even if not for the expected reason.

I did not use the factory tool to tighten the alternator belt. I just used a standard tensioner, but that was when the belt was new a couple thousand miles ago. I can try and see if it's still tight enough. I guess it makes sense that it might have loosened up, but I would expect to hear some squealing or something if it was slipping that much. I will try tightening it tonight.
-N

Unplug the fans and see if you still get misfires when hot.

*edit* If so, it might be the ignition module.

That sounds like a good way to overheat. And if it does work with them unplugged, that still doesn't answer the question about whether it's the fans drawing too much current or the alternator producing too little.
-Neil

If your theory is that the fans are causing a misfire then it makes sense to test that theory. If it gets close to overheating, jump out and plug the fan relay back in.

It's probably heat and not the fans but why not test? Things like coils and ignition modules are sensitive to heat.

Have you run through the ignition system tests on Clarks garage?

The alternator belt was a bit loose, but tightening it didn't help at all.

I didn't try unplugging the fans yet. I couldn't reproduce this on my lift today - only while driving, so it wasn't a good time for that test anyway. Actually, I guess I could do that by pulling the fan relay still, so maybe tomorrow I'll try.

No I haven't looked through Clark's garage ignition tests, but I don't remember it being anything too unusual. I'll go glance again. I'm getting close to just buying a new coil, wires, distributor cap/rotor, and spark plugs. Or a new alternator. No idea really...
-N

On an S2 I would get an ignition module long before a coil, and get a good quality one also. Bosch 0227100124 I believe, a used one off a Volvo or Saab would be a cheap test.
The OEM coils are quite robust but heat can always cause a part like that to fail.

Easy to check the quality of the spark when it's misfiring, see if the spark can jump 10mm.

This is the first I've heard of the ICM issues with the S/S2 cars, and those symptoms match my behavior perfectly. For $50 and a simple swap, it seems worth a shot. Of course, I have a Pelican coupon for $20 off $250, so I overspent and got a new ignition coil, distributor, and rotor too, just in case.

Crossing my fingers that something in this box will do me some good!
-Neil

I would change one part at a time so if it fixes it you know which one it was.

That is great advice....

However, I'm not likely to take it. I've been having severe back pain lately, and I want to spend more time driving and less time wrenching. I will replace the easy stuff, since I don't know when any of it was last replaced (I've only owned it for a year). Being fresh and new is just good insurance.

I suspect the coil is tough to reach, so that may stay where it is for now, unless the problem persists. And if I replace all this and the problem _still_ persists, I guess it's time to look more closely at the alternator again.
-Neil

I would apply thermal compound to the new ICM after cleaning the mounting surface. My Bosch ICM didn't come with any for some reason.

I cleaned up the positive and negative terminals on the battery and went for a drive today. Neither looked problematic, but there was actually a bit of oxidization between the "layers" of connections on the positive side. Every other connection was tight and clean already. I'd say that made a marked improvement. It's not super hot today (about 70 degrees), but I drove around with the AC on, fan at 4, headlights, foglights, stereo, and rear defroster on. I even moved my windows up and down a few times and had a USB charger in the 12V socket.

I didn't get a real meter out, but the voltage gauge was higher than expected for all of that. There was some light stumbling when the fans came on that maybe dropped to low-mid 12V range on the gauge when idling. If I was moving it was almost always above 13V even with everything. I never had even a single ignition stumble while driving.

Again, this wasn't a high heat day, but even on a day like today, I wouldn't expect it to be so high. It was over 14V as soon as I started it up with no extra accessories turned on yet, and that's more than I've seen on the dash gauge before.
-Neil