PorKen

I drained the battery down on my '85 by leaving an interior light on. Trying to start the car this morning, it was spinning slower than normal, and wouldn't fire. During this slow cranking, I noticed that the tach jumped to over 5000 rpm! After connecting a battery charger for a few minutes, the engine spun quickly, and fired right up, with the tachometer showing the correct speed.

I think I've heard about this before, but had never seen it myself. I have seen before that if the volt meter shows below 10 volts when cranking that the EZF and LH won't work properly, but never noticed this tach weirdness.

In short, if your S3 won't fire, and the tach is freaking out, the battery could be putting out too low of a voltage. Don't automatically assume something is broken. IE. take off your airbox to check the speed sensor connection, like I did...charge the battery, first.

Mrmerlin

Ken I have a few of the LED lights instead of the originals on the interior lights, they will not drain the battery like the factory lights will. FYI

M. Requin

I've seen the low voltage = no start but everything else works on my 86.5 - I've mentioned it in a few posts - but I haven't seen this tach weirdness. Interesting!

Landseer

Good observation, thanks Ken.

When battery depletes down towards 10V on a running 84,5,6, the dash warning lights will all flash.
Seen it on all three.

On the 86, a light tap with a mallet on the alternator caused it to resume charging and immediately extinguished the christmas tree of lights. Repeated the same thing a few weeks later. At that point we replaced the alternator.

Rob Edwards

Maybe related, maybe not-

At some point in the recent past (a week ago? 2 weeks ago? Sigh.), I'd stuck the key in the ignition of the GTS to check something, and promptly got distracted and went indoors. Decided to go for a drive last night and found the key in position II, dead dead dead.

Put her on the charger and walked away. I was just out in the garage and heard a faint clicking noise. WTH is that? Turns out the GTS is making it. Checked the voltage at the jump post, 6.1V.

Maybe I oughta disconnect the battery before trying to charge it from completely flat...

RKD in OKC

Low battery can cause all kinds of weirdness.

The Forgotten On

Low battery causes the factory alarm in my 81 to go off when cranking sometimes. It also does that weird tach bounce like stated above. It just does odd things to these cars.

depami

Just to be clear, what should the tach do while cranking?

Do all years act the same?

joejoe

The 944 tach bounces when cranking, not sure if 928 is supposed to or not. Think so.

Ketchmi

I have experienced this on a few S4's also, seem to be turning over well enough to start but just wouldn't fire until charged.

SeanR

Yep, see this quite often on the LH cars.

danglerb

Low battery, weak battery, can cause all sorts of odd stuff. My least favorite on my US 83 is that the alarm sometimes arms when the voltage dips, then the next bit of flakiness from a door or hatch pin switch and the alarm goes off.

Maybe marginal battery combined with marginal ground connections make for some extra weird stuff.