pkt1213

About 8 years ago I sold my 89 944. I texted the guy who bought it from me a few months ago and he was interested in selling it. My wife flew out to Denver a few days ago and are taking our time driving around in it and having an adventure (anniversary adventure if she asks). There is a pretty long list of little things to fix but the first one I noticed to day, driving in the mountains, is the tach.

I know the later cars have a shorter final drive, but at 70, I am showing 4600 rpm. I ran through a gear calculator and 3rd show over 5k and 4th should show just under 4k. Also the RPMs do drop everytime I shift. I am starting to wonder if something is wrong with the tach. There have been a few times today that I have burried it up past 7k because it is apparently so short with no fuel cut at 6500. I am wondering if the ECU knows what is going on but the cluster doesn't.

Jfrahm

Wow those wheels look amazing!
I might have a tach, I have an extra cluster or two, I had a tach read totally wrong on one of my cars and swapped tachs in the cluster.

pkt1213

I'm a little upset that I sold my extra cluster 8 years ago.

It had Club Sport wheels from a Turbo S (when I bought the car the first time it had Fuchs which totally rubbed and swap them because I love the CS style) but needed some new rubber as well as a 245/50 is a little tall of a tire. Really the CS wheels are too much rubber for an NA car. So I picked these up from a guy local to the car and put new 225/50 all the way around.

Is there any other way to confirm RPM than with a timing light? Find an inductive tach tomorrow?

Jfrahm

If your DMM will do Hz you can look at the signal to the coil and divide it down. If you are still in the area I can look at it.

pkt1213

We're down in the Englewood area now. I wasn't thinking about using a DMM on it. I need to buy a better one anyways.

Jfrahm

Also it looks like 40mph is at 3000 in 3rd and 62 is 3500 in 4th, this should be close enough with the slight tire change:


This is in a 1988 manual but should be correct for 1-4 if it has a 944 or 924s trans (?)

pkt1213

I forgot the manuals had that. Back when manufacturers wanted you to know more about the car than how to start it, put gas in it, and how to bring it back in for repair. I'll have to check and see if it is still in the glove box.

I did download the spreadsheet from 924.org (I think) that had all of the 924 and 944 gearbox information. I calculated RPM at 70 for both 3rd, 4th, and 5th as well as calculating back from RPM and diff ratio to get a mathematical 5th. None of that matched up and rpms drop at each shift so it its staying in 4th or something.

I bought a digital multimeter yesterday that does RPM. My tach is reading high (25-30%), maybe not as high as I expected (50% would have put it right on the money) but we are also sitting in a parking lot at neutral. I've seen so many little thing on cars be exacerbated or dissappear at idle as opposed to at speed it is hard to say.

I was doing to reading last night and it could be corrosion between pin 21 and the DME or a bad tach. Is there any way to diagnose this other than swapping in a new cluster? I am also wondering if it could have bad ground because the temp gage seems to read low but the the operating temp and cooling seems fine. I was reading some stuff in the turbo section and it looks like the tach get +12v, grnd, and then the rpms are controlled by an oscillating ground in or by the DME?

pkt1213

After my check this morning where the tach was only reading 20‐30٪ high, the same thing held true while crusing down the interstate. After a few hours I caught the tach needle moving around and watched it go from 3800 up to 4400. 🤦‍♂️

jeyjey

It's worth un-plugging and re-plugging the 3 connectors on the back of the instrument cluster. They definitely do oxidize.....