Speedo works, but is wrong

[CatsEyes]

Hi folks,

I've searched, but not found an answer to this.

My speedo works reliably (1986 944 NA). But is always out by 5-10km/h in our measure down here, depending on the speed. My impression is the amount it's out is roughly proportional to speed, but I can't be sure.

Before I rummage around, it'd be good to have an idea of the likely culprit. The speedo cable? The speed sensor? The actual speedo mechanism itself? (Isn't there a magnet involved?)

Thanks in advance.

[kfray]

What size are your tires, if you have installed a different size tire the speedometer will not read correctly. Do you have access to a good GPS ? My GPS has a good MPH readout. On the 1986 944 there is a speed transmitter on the transmission connected to the speedometer.
More information:
http://www.clarks-garage.com/pdf-manual/trans-01.pdf

[smokin_944]

I agree with kfray. If the tire size(height) is different than it was in stock trim, the reading of your speedo will be out. I run 18s on my 944 and the speedo is out considerably.

[looneybin]

i have stock size tires on my '86 and the speedo seems to read about 10% faster that i'm really going.

[wild man]

If the the tire size IS correct, the only other possibility is that the speedometer drive gears in the tranny are not of the correct ratios for that particular transaxle.

[CatsEyes]

Hi folks,

Sorry for the delay. Work troubles.

They're the stock rims (teledials), and standard tyres.

The tranny explanation is a worrying one. Though my impresssion is the car has received very little major work of that kind. There's a full service history going back to 1986 (receipts and all), and no sign of work there.

But here's the thing. Watching the speed closely in very slow traffic yesterday (building works over a long highway up the mountains, with solar-panel based speed readout signs that are used in this country to let you know you're speeding), I noticed I was wrong in my initial statement here.

I think the speedo is always out by more or less 10kmh. Even at very slow speeds.

So I'm thinking it may still be worth my while to check the cluster. Just in case the speedo (or even the dial) has been wrongly re-installed after some surgery...

[AaronM]

Originally Posted by looneybin

i have stock size tires on my '86 and the speedo seems to read about 10% faster that i'm really going.

Ditto to this. The electric speedo in my '86 951 is also 10% optimistic.

Originally Posted by wild man

If the the tire size IS correct, the only other possibility is that the speedometer drive gears in the tranny are not of the correct ratios for that particular transaxle.

No. Firstly, speedos are generally calibrated at the factory to aim for up to about +5%. In some countries, having a speedo that reads slow can subject the manufacturer to fines and other regulatory penalties, so as a practical matter most manufacturers set their speedos fast to avoid being liable. Secondly, even electronic speedos still use a hairspring and a magnetic cup to move the needle. As the gauge ages, the hairspring weakens slightly over time, which causes the gauge to read higher as it ages.

Finally, the electronic sensor in the transaxle reads from the output shaft, so only the final drive ratio matters, and there were only two final drives available in 1986: the 3.889:1 ratio in the NA cars (both the "S" and the 8v cars had the same final drive ratio) and the 3.375 ratio in the Turbo cars. If he had the "wrong" sensor (that is, a sensor from a turbo in his NA car) the speedo would read 15% fast, not 10%.

As a practical matter, the theory that somehow his car has the wrong speedometer gears simply isn't possible.

To the OP: If your speedo is off by a percentage, then it's likely just engineered-in error or the hairspring in the gauge aging and there's not much to do about it. If it's always off by a fixed amount, then the gauge was probably removed at some point to repair the odometer gear and the needle simply wasn't quite lined up right when it was put back onto the pin. If you want to attempt to fix the issue, you need to first remove the gauges from the dash (instructions: Gauges - Removing Late 944 (1985.5 and newer) Gauge Cluster) and then re-set the needle to the zero point (instructions on how to get the cover off the gauges once they're out of the dash: Odometer Repair).

[CatsEyes]

Originally Posted by AaronM

Ditto to this. The electric speedo in my '86 951 is also 10% optimistic.



No. Firstly, speedos are generally calibrated at the factory to aim for up to about +5%. In some countries, having a speedo that reads slow can subject the manufacturer to fines and other regulatory penalties, so as a practical matter most manufacturers set their speedos fast to avoid being liable. Secondly, even electronic speedos still use a hairspring and a magnetic cup to move the needle. As the gauge ages, the hairspring weakens slightly over time, which causes the gauge to read higher as it ages.

Finally, the electronic sensor in the transaxle reads from the output shaft, so only the final drive ratio matters, and there were only two final drives available in 1986: the 3.889:1 ratio in the NA cars (both the "S" and the 8v cars had the same final drive ratio) and the 3.375 ratio in the Turbo cars. If he had the "wrong" sensor (that is, a sensor from a turbo in his NA car) the speedo would read 15% fast, not 10%.

As a practical matter, the theory that somehow his car has the wrong speedometer gears simply isn't possible.

To the OP: If your speedo is off by a percentage, then it's likely just engineered-in error or the hairspring in the gauge aging and there's not much to do about it. If it's always off by a fixed amount, then the gauge was probably removed at some point to repair the odometer gear and the needle simply wasn't quite lined up right when it was put back onto the pin. If you want to attempt to fix the issue, you need to first remove the gauges from the dash (instructions: Gauges - Removing Late 944 (1985.5 and newer) Gauge Cluster) and then re-set the needle to the zero point (instructions on how to get the cover off the gauges once they're out of the dash: Odometer Repair).

Thanks, Aaron. This is exactly the information I was after,as you correctly intuited from my previous posts.

And yes, I had exactly the same thoughts as you - except that in my case they were pure guesses rather than knowledge:

1. The weakening mechanism over time, and maybe fading power of the magnet; or

2. The possibility of the needle being mis-aligned.

And yes, I will investigate 2. by opening the gauge cluster (I also need to fix the odo, which is one reason I asked the question in the firstp lace: two birds with one stone.)

My only other confusion is this. My car lives in Australia, but was originally domiciled in the UK. Its speedo gauge doesn't exactly match any of the pics I can find, either of US-spec or Euro-spec cars, for some reason. (Replaced? But it matches the rest of the gauge-set.)

At stationary, the needle sits at the first notch of the guage. And yet, oddly, following the logic of the rest of the dial, that should be 10km/h rather than 0km/h.

So I have a hunch the dial may have been replaced, and the needle misaligned to 10kph at rest in error. But that's just a guess.

Onto the removal in the next week or so, to see...

Thanks everybody: great knowledge-base here.

[djnolan]

I have seen the needle mis aligned on the temp gauge and a gentle nudge corrected the reading. The gauge has a hard stop at the max reading. So I had to slightly bend the needle until it would reach the max gauge reading and now the gauge reads as it should.

[hangar21]

Hi Guys,
I needed to replace my Odometer gear -1990 944 S2, (144,204.0) The cluster removal went well as did the gear replacement. Re-installation was no problem and system worked well-for about 3,000 miles. I started to notice variations in the speedo needle movement, removed cluster and checked my work- I thought I had screwed something up! After discussing my problem with PCA members, I decided to send cluster to speedo shop to verify my gear replacement was correct. Here comes a big mistake, I drove the car with the cluster removed. This really screwed up the warning circuit. After cluster was returned and reinstalled, Master Warning (!) light, seat belt & air bag lights stayed illuminated. Local shop had to "reset" warning circuit and all works like new. Just my thoughts on my cluster.

Terry

[CatsEyes]

Originally Posted by djnolan

I have seen the needle mis aligned on the temp gauge and a gentle nudge corrected the reading. The gauge has a hard stop at the max reading. So I had to slightly bend the needle until it would reach the max gauge reading and now the gauge reads as it should.

Thanks, djnolan. I'll keep that simple workaround in mind.

My fuel gauge is also out: when absolutely full it reads about 4/5 full. Though the other gauges seem spot-on. So my hunch is when I open the cluster there will be an issue with the two left-hand gagues.

[AaronM]

Originally Posted by CatsEyes

Thanks, djnolan. I'll keep that simple workaround in mind.

My fuel gauge is also out: when absolutely full it reads about 4/5 full. Though the other gauges seem spot-on. So my hunch is when I open the cluster there will be an issue with the two left-hand gagues.

Fuel gauge issue is 99.9% the sender unit in the tank or a ground, not the gauge itself. The most likely of the two is the sender unit. The float runs on a pair of tracks with varying resistance and these get dirty over time, causing the gauge to read low. Pull the sender, clean it all.

Here's a guide for cleaning and testing the fuel gauge and sender: Fuel Level Indication Problems / Repairs

[CatsEyes]

Excellent, Aaron. Thanks for the link.

Though I noted Clarks Garage said the problem might be either the gauge or the sender. No matter: I'll look at both.

[AaronM]

Originally Posted by CatsEyes

Excellent, Aaron. Thanks for the link.

Though I noted Clarks Garage said the problem might be either the gauge or the sender. No matter: I'll look at both.

Clark's is right, it can be the gauge, but that's ridiculously rare in my experience. While I can't speak for every car out there, of the ones I've seen with fuel gauge issues it has always been either a dirty ground or a gunk-ed up sender unit. As electronics, especially in cars, age (miles and use more than simple years usually) connections get mild corrosion and this introduces extra resistance in the gauge circuit, which makes the gauge read low.

Anyway, cheers and best of everything in getting the gauges figured out.

[matty89]

my car is also off by 10kmh faster then im going leave it like that so if im going 120 im going 110 ! its better like that ! the cops wont be bothering you

[RichardE]

I'm with you matty. I'm on standard rims and tyres and always 8kph out but never had a speeding ticket. And my 930 is even worse. More like 12-15k's out. I started to think it was a discrepancy built in by porsche to save us all speeding fines.

[CatsEyes]

Well, it might save us all speeding fines, but only if we couldn't do simple arithmetic!

[wild man]

Originally Posted by AaronM

If he had the "wrong" sensor (that is, a sensor from a turbo in his NA car) the speedo would read 15% fast, not 10%.

As a practical matter, the theory that somehow his car has the wrong speedometer gears simply isn't possible.

I stand corrected then. I didn't realize that the sensor read directly off of the output shaft. Apparently it was an incorrect assumption that there is a pair of changeable gears that the sensor reads off of, for purposes of correcting for different tire/wheel sizes & axle ratios.

[AaronM]

Originally Posted by wild man

I stand corrected then. I didn't realize that the sensor read directly off of the output shaft. Apparently it was an incorrect assumption that there is a pair of changeable gears that the sensor reads off of, for purposes of correcting for different tire/wheel sizes & axle ratios.

As stated, there were only two axle ratios. And the factory-optional wheel/tire combinations were all "plus zero" setups; the original 185/70VR15 tires (the earliest cars) and 195/65VR15 tires have essentially the same overall circumference as the optional 215/60VR15 and 205/55VR16 tires on the NA cars, which means no corrections were needed at all for the factory-available tire sizes. Similarly, both the Turbo and Turbo S cars had tires with the same overall circumference despite the Turbo S cars having slightly wider wheels.

The precise "how" of the sensor mechanism isn't terribly important. What is important here is that there were only two setups; a setup for the Turbo cars and a setup for the NA cars (in '86 anyway, the S2 cars had a different final drive ratio, so they would also have had a slightly different setup).

There may well be some sort of "geared" jiggery-pokery between the output shaft and the vehicle speed sensor, but the fact remains that there was never any need or reason to have any correction for the factory tire options.

[wild man]

I guess you are saying that the only way to correct (compensate for) the error, assuming it is linear, is to asctually PUT on a set of tires that makes it right. My 951S comes up short by 7% with 245/45/16's on 9" rims. When I put new tires on, I'm going with 245/50/16's, which will cut the error down to around 3%, which is within my "spec" of being acceptable. The 245/45 size is now a dead size anyways, so their availability is doubtful. Maybe we should be thankful to GM for making so many camaros and firebirds, as that might be the only reason that the 245/50 size is still alive!

EDIT: It doesn't sound like the OP's error is linear anyways, judging by what he said, after he revised his original post.