One of the benefits of the new millennium is that it has become reasonably cheap to check your speedometer using a GPS. If you don't own one, I'm sure that your neighbor does!

When I checked my speedo against the GPS (Garmin Streetpilot) it revealed that the speedo read 10% too high. I.e., 110 km/h on the speedo was only 100 km/h in reality.

In order to verify this, I checked the speed against the odometer. At a certain speed, 1 km takes a certain number of seconds to pass. Example: 1 km @ 60 km/h takes 60 seconds. This test also showed a 10% difference between indicated speed and true speed (assuming that the odo is more accurate).

Having this much of an error, I felt very motivated to investigate the possibility of a calibration. Since the speedos of the 700 series are electronic, another approach than replacing an internal driver wheel had to be found.

By doing a lot of reasearch on the Internet I found that in most cars it actually could be done, merely by a change of a resistor. The speedo itself is build around the ubiquitous ITT UAF2115 chip (A datasheet could be found here: http://heneghan.members.beeb.net/audi/uaf2115_1ds.pdf ). And the resistor to be changed is the one that is connected to pin 4.

Enough theory! How to calibrate?

1. Remove the instrument cluster as shown in the FAQ, i.e. remove the plastic covers and unscrew the two screws.

2. When the cluster is removed, open it by removing a number of philips screws on the rear side. Notice that the silver colored screws are longer than the golden ones so make sure that you where they belong upon reassembly.

3. Remove the speedo by unscrewing four screws on the plastic circuit board side. Two of the screws are also holding two connectors. Do not mix them!

4. Carefully remove the meter needle by turning it counter clockwise while pulling. After that remove the "number plate" which is glued onto the meter. Don't worry, you will probably not need more glue to reattach it, it is quite sticky!

5. Unscrew the three small screws which now should be visible. After that, the meter core should come lose.

6. Unscrew the two screws on each side of the the step motor (silvery thing, 3 cm diameter) and the circuit board should come lose.

7. Locate the resistor shown in the picture. Heat up your soldering iron, this is the little buggar that is to be replaced!

8. Replace the resistor. In my meter, the value of the original resistor was 51 ohm. In order to decrease the speedo reading by 10%, I increased the value of this one with 10%, i.e. 56 ohm. Of course, you should increase the value with the same amount as your error.
I double checked this by hooking it up to a signal generator, but that is not necessary unless the error is REALLY big.
It is of course possible to use a potentiometer instead of a fixed resistor, but I prefer resistors since the do not change that much over time.

9. Reassembly is almost the reverse of the assembly. It could be tricky to realign the meter needle, but I did it by rotating it counter clockwise until it rested at 20 km/h (as it did from the beginning).

10. Buy a bottle of beer.

11. Test drive your car and enjoy an extremely accurate speedo reading!

12. Drink the beer!

And my result? The GPS and speedo now read the same within 1 km/h at all speeds up to 130 km/h (didn't test at higher speeds)!

of course one might question the gps..

let me explain..my freind and i both have in car gps based sat nav systems..these are on different hardware but use the same gps reciever and the same sat nav software and are portable between cars..and we both noticed a difference between the speedo on the reading o nthe sat nav...so we did a littel swappign around and test driving (note different cars, 6 years apart in age, different manufacturers, different tyre sizes..DIFFERENT)

in ALL combinations the sat nav read 3mph HIGHER than the speedo's (both speedo's) conclusion...more liekly the sat nav is up the spout than 2 seperate speedos' both ah ave the same non linear errors....

Here’s how you can get a third opinion:

Get on the highway, set your cruise control to EXACTLY 110 mph. When you get pulled over, simply ask the friendly officer whether he had you clocked at 110 mph or 113 mph. Then you know for sure which one is right!

Just trying to help! (and of course, just kidding)

Jeff Pierce
--
'93 945 Turbo ( one kickass family car ! ), '92 Mercedes 190E (my daily driver), '53 Willys-Overland Pickup (my snow-plow truck/conversation piece -- sold to a loving home), '85 Jeep CJ-7 w/ Fisher plow

Of course the GPS could be erratic as well but that isn't very likely.

What GPS did you use? According to Garmin's specs, the StreetPilot III has a speed accuracy of 0.05 m/s = 0.18 km/h at steady speeds. That was enough for me...

Because it seems odd the electric meter would suddenly become 10% more sensitive. But I suppose the timing capacitor could dry out by 10% too.

Circuit looks a lot like the one in the 240 .
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Art Benstein near Baltimore

The speedo was not accurate from the beginning, i.e. no sudden change.

The -88 740 speedo is somewhat different than your -91 240 and the speedo reading is not affected by an external timing capacitor. It is fairly similar to the circuit shown in the UAF2115 datasheet with the motor connected to pin 5 and a current setting resistor between pin 4 and GND.

BTW, maybe you guys on the other side of the puddle (US) had stronger regulations when it comes to allowed error on the speedo?

I believe I threw you off with my pin counting scheme. Not having found the data sheet back when (thank you for the audi club link!) I was counting pins in a fashion I'd been used to with other dip circuits with heat sink tabs, as in this Motorola example.
So the pin numbers don't match up with the ITT sheet. And the functions were deduced from part values and actual signals found. Looks to me like the 240 uses the same circuit when you account for my bollixed up pin numbers.

Now, I checked three boards from random junkyard pickings; they all sport the same 51.1 ohm resistor so I'm thinking VDO probably adjusts the meter movement mechanically for final test calibration rather than test matching the resistor. Anything else I've seen VDO do (voodoo) with "selected" resistor values, they've put the selected part up on standoffs.

Have no clue about regulations, but I'd think the speedometer head itself would need much better than 10% tolerance if the end result adding in tire variations would be useful at all.

A few years ago my daughter would seem to be often ferrying her friends around in her parent-supplied and maintained 244. I could have made use of that "Taxi Meter" output function for a good laugh! Thanks again for the link to the ITT app note.

Cheers,
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Art Benstein near Baltimore

OK, now I am with you!

No, I'm not the original owner, but I bought it from a friend of mine who had had it for 8 years and he claimed that he had clocked the speedo many years ago and found it to read too much.

Thank you for the info on the VDO voodoo. 51.1 ohm resistors must possess magical powers which make all VDO speedos in the world accurate!

Keep up the good work!

I have a friend with an '87 740 Wagon and the speedometer (that was in the car when he bought it) reads somewhat accurately, but the odometer reads 5x faster than the actual distance covered. Driving about 2/10 of a mile yeilds a full mile of travel on his odometer. Earlier this summer I picked up a replacement instrument cluster from a local salvage yard ($12) for him, but it would have been interesting to keep driving with the malfunctioning odometer. -Irv Gordon, here I come!

As for my 240 Wagon, I put a set of 195/65R15 tires and wheels on it last year, and then clocked it with my GPS. The odometer reads just about 2% faster than actual speed, and I'm glad it's not too far off. (original tire size for 240 wagons is the old Metric 185R14 tires)

God bless,
Fitz Fitzgerald.
--
'87 Blue 240 Wagon, 247k miles.

This might not be accurate because of my faulty memory, but it reminds me of someone's problem with a retrofit of, I think, cruise control, during the early ABS years. The sender in the diff needed a frequency divider between it and the speedo to provide fast enough speed info for the braking system, yet match the existing scheme with cruise and speedometer.

But like you said that would not explain the meter working so well unless the exact multiple input locked the phase comparator for the analog speedo output with, say, every 5th pulse.

87 sounds too early for this, even for the cutting edge 700 series. And fixing it with a replacement cluster makes it sound more like a problem in the original speedo's signal conditioning, or odo motor pulse decoupling, delivering a crappy waveform to the chip's input. Even at 5x, that would be noteworthy accomplishment getting a 700 to 2 million.
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Art Benstein near Baltimore

I might not be correct, but there was an extra perpendicular circuit board in my speedo (unfortunately outside the picture I took) which had some surface soldered circuitry. A 40xx IC with some passives around it. I'm quite sure that this board is a divider. Reading on the Audi site, that speedo had the input directly on pin 9 whereas my speedo had the input connected to the perpendicular board. The audi speedo had dividing ratio K=6644 whereas my speedo had K=25295, almost a four fold difference.

Art, you seem to have opened lots of speedos. Could you please see if you could give us more info on that perpendicular board and K-values. I do not want to open my speedo again!

And I have ABS and cruise control.

Sorry, I shouldn't even be posting in the 700 forum, not ever owning one. I don't recall any surface mount stuff in any of my 240s, but the 240s have a separate board that counts up miles to illuminate the service light. Nothing in the way of logic that comes between the differential sensor and the 2115. I have no ABS cars, but the 92 manual (240) shows the diff pickup being shared between ABS and speedo - literally.

My recollections of the divider board discussion are probably from John Sargent in this or other threads: http://www.brickboard.com/RWD/index.htm?id=601650. Another instance of replacing the resistor as you did (by a Volvo tech I believe) was mentioned in http://www.brickboard.com/RWD/index.htm?id=388452 (this thread) . The knowledge is on this board somewhere, but I don't have it.




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Art Benstein near Baltimore

Yes, one of the threads suggests the existence of an ABS signal converter (divider board). Thanks for the links, btw! Seems like I wasn't entirely alone of having a badly calibrated speedometer.

Yes, I thought you might notice the company. What I didn't see was a clear indication the odo was in disagreement with the speedo; the only reason I'd think to mess with the resistor value.

I have no experience with the particular meter in the electronic speedo, but maybe 30 years ago I thought it was common practice to adjust both zero reference and spring tension (full scale current) on the moving coil meters themselves both in assembly and during recalibration.

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Art Benstein near Baltimore

Art & Volvouser, regarding the speedo in my friends '87 740 Wagon that read 5x high, -I didn't have time to swap the instrument cluster. I just picked up the cluster at the salvage yard and dropped it off to his house (a couple months ago). Knowing him, he probably hasn't installed it yet. However, I would agree with your assumption that the odometer and speedometer are having signal communication issues (either a bad/noisy output signal or a poor phase lock on the odo input). Since the speedo reads correctly, it's probably not an issue with the impulse sender. The car does not have ABS brakes (just a regular '87 wagon), and shouldn't have a signal divider. If the replacement cluster doesn't solve the problem, I'll just grab the proverbial "ferrite salt shaker" and sprinkle some inductive chokes throughout the vehicle. (hey, it could be electrical noise from the fuel pump or ignition being picked up on the impulse sender line, and the speedo is smart enough to filter it out and the odo isn't)

Just out of curiousity, how many Engineers and Electronics Techs do we have on this board??? There seem to be an unusually high percentage versus the typical demographics that I'd expect to see on a vehicle forum.

God bless,
Fitz Fitzgerald.
--
'87 Blue 240 Wagon, 247k miles.
'88 Black 780, PRV-6, 145k miles.

>There seem to be an unusually high percentage versus the typical demographics that I'd expect to see on a vehicle forum.

Fitz, the 80's volvo is an ideal car for old electrical tinkerers: it is full of chronic reliability challenges, many solvable without needing to know proprietary microcode designs done in 150-pin SMT ASICS.

Good thought on that fuel pump with the lousy brushes. Just bring your scope, you'll find it without throwing ferrite at it.
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Art Benstein near Baltimore

I totally agree with Art here. The Volvos of the 80's are ideal for us EEs who like to do some "hands on" work as well ...

Good luck with the scope!

I am inclined to agree with the title of your response.

We currently own four 700 series turbo cars, and I can find no inaccuracies with the speedometers in spite of their age (up to twenty years). Yes, I have checked all of them with both the stopwatch and GPS.
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john

Thanks! Great tips for the FAQ. Time to get a GPS.