Another NHS customer. Maybe two weeks max and it seems to be within 1 mph. Cleaned it, new bezel, changed odo gear and replaced the IC that drives the cruise control signal output while in there.

Another option is to install 12OclockLabs SpeedDRD-U1 which alters the pulse rate from the transmission reed switch. You can speed it up or slow it down. Great for people who have different size tires street to track.

Will this method work on a mechanical versus your electronic speedo?

So, in reading the entirety of this thread, and several others, I have reached the following conclusions, (based on a 1986 Carrera (170mph) speedometer):

1) There is NO calibration/adjustment mechanism (i.e. potentiometer).
2) Rotating the needle on the shaft is the only DIY method of calibrating without soldering a different resistor onto the board.
3) I will go blind if I continue to look for a potentiometer adjustment mechanism.

So, QUESTION: Is there a calibration mechanism of some sort that I am overlooking???

A very specific descripton by someone intimately familiar with this specific speedo, or pics of that mechanism, would be helpful. (Please, no more NHS and PAS referrals- I already know how to contact them. I need DIY money-saving assistance).
Thank you to all who have posted technical info.

Pics of my speedo posted here...

http://forums.pelicanparts.com/uploa...1487027223.jpg
http://forums.pelicanparts.com/uploa...1487027266.jpg
http://forums.pelicanparts.com/uploa...1487027294.jpg

Odd, I had mine done at NHS to repair the gear, and they did that and calibrated it at their shop with my car there, and it is still off, probably reading about 5-10 MPH high. It has always read high, but I've never checked against GPS. It seemed unlikely that it was off because they had it on a stationary roller, but I still notice that it feels like it is reading high.

This has always concerned me because of mileage on the odometer and devaluing the car.

AonCurly- I am curious to know how your calibrated speedo compares to a GPS. Today I will be "calibrating" my speedo using the "twist" method and my GPS.

recommend that you assess your odometer's accuracy separately from the speedometer. Apparently, german/european authorities penalize manufacturers if speedometers are pessimistic (and can be traced to the cause of a speeding car). So, speedometers are designed to be slightly optimistic, while the odometer is supposed to be calibrated to be accurate.

I'll try to check in the next couple of weeks and post my results.

Success! For my calibration exercise I installed my speedo without the clear plastic face so I could pull over to easily and VERY carefully adjust the needle (using the "twist" method) then immediately get back on the road to re-check the calibration. Only took one adjustment, then back home to replace the clear face. Accurate to +/- 1mph.

The odometer circuit just counts the pulses from the transmission to get the distance (x) so it shouldn't change unless the tire size changed. To get the derivative of distance with respect to time (speed) it generates a fixed current fixed time pulse with every switch closure. More pulses per unit time means the current in the meter deflects the needle more.

The last post on this page (#73) describes how one person calibrated his speedometer with a pot to change the pulse width.
http://forums.pelicanparts.com/porsche-911-technical-forum/332604-smokey-speedo-4.html

Thanks, Rick. Porsche life is never as simple as one would hope. I just discovered yesterday I have THREE windscreen-headlight washer systems in ONE Porsche! I'll check out in detail that adjustable pot for the speedo. Thanks again! -Matt!

Still running this to ground to find the best way to correct speedometer and odometer errors. Does anyone know whether or not the sensors are +DC pulse, -DC pulse, or AC sine?

The sensor is just a reed switch that closes as a magnet goes by in the transmission.

The UAF2115 IC has a counter from the sensor signal that serves as a frequency divider and pulses a stepper motor every so often incrementing the odometer. This would be very difficult to adjust.

The calibration of the speedometer involves both a gain and offset error. The gain is the width of the constant current pulses as the vehicle moves (R3 on the schematic). The offset is where the plotted line indicating speed verses frequency of pulses crosses zero (R2).

OK, I gotta join Steve above: IS there a fix for odometer error? Seems NHS and others can only fix speedo. And please, not all of us are EEs...

I am working with a group to get this issue squared away. They are looking at what it will take to get both the speedometer and odometer errors corrected. I will post the results when it is worked out. They need a couple of days to do some research and will get back to me. It is why I asked the question above about +- Dc and sine. I gave them a link to this thread so they can see responses. FWIW, NH did tell me they can correct both, BUT if tire size changes, then the correction is no longer accurate. The other group allows tire size correction down the road.

For the odometer, every 32 pulses from the reed switch it increments the gear on the odometer with a stepper motor. With the current IC you have the choice of 32, 64 or 128 counts to increment the gear.

To change the calibration you could change the gear or insert your own single chip micro controller to drive the stepper and make the divisor anything you want.

Hopefully no one will get too upset I am reviving this...

This actually works and very well. Sadly/Luckily I have two speedos, one from my original 911 (which was written off) and the "newer" 911. Both are 1986's...

The speedometer I want "adjusted" has 200,000 miles on it, is off +7-10mph depending - and was willing to experiment on with this method.

The other speedometer from my original 911 only has 60,000 miles on it and is dead on around town and on the highway. I really wanted to keep this one as a spare instead of transferring to daily driver w/ 200,000mi.

So...with that out of the way (as a mild disclaimer) I took the glass off the one to be adjusted, grabbed my phone (with speedometer app) and mounted my Garmin gps. Installed the speedometer and headed around town and on the highway.

I brought clean mechanical gloves to adjust with (just in case) and hit up different speed zones, visually matched according to gps/phone then stopped and VERY carefully adjusted how I saw fit using the above method at different speed comparisons. My needle rests below zero now a bit but overall it seems to be dead on especially around town and up to about 65ish. It was originally off by 5, 8, 10mph and seemed to get progressively worse the faster I went - 90mph on the speedo was gps tracked as 78.
It still has a bit of this and at 80mph it was showing 75-76 but I can live with that as most of my speeds are around town and back roads so 35-70mph was my major concern.


**** I am going back out tomorrow and test things and wanted to state a word of caution I feel is important. Even today just seeing if this actually works I stopped 4-5 times thinking I could find the sweet spot so if you have any "perfectionist" OCD types of personality in you this could get to be a rabbit hole of sorts :) ...and remember it's a bit delicate so I feel the more you mess with it the more your chances increase of doing damage.


Photos:

With glass off:
http://forums.pelicanparts.com/uploa...1640811303.jpg


Phone App (Garmin GPS was at 46mph also:
http://forums.pelicanparts.com/uploa...1640812149.jpg


Speedometer at the same time:
http://forums.pelicanparts.com/uploa...1640811303.jpg

Bitte,

Thanks for the detailed update on this. Timely for me as I was considering sending my Speedo off for calibration. Since I don’t need a rebuild, I’m not inclined to send my unit out at great expense. Just looking to be a bit more accurate in the 60-90mph range.

Which phone app are you using? iPhone or Android?

I have an older Iphone (XS,which I guess is now old by todays standards :) )and will send a screen shot of the different apps I am trying for speedometer readings:

http://forums.pelicanparts.com/uploa...1640876522.png

They all seem accurate when measured against each other and my Garmin GPS.

Hope this helps. Be patient, take your time and "measure twice" before putting everything back together...I am actually about to head out again this morning just to see if it's good enough. Will probably leave it alone but any excuse to drive the car.

Happy New Year!
Erik

Danke!