speedometer calibration: can't find it...
 

When my '85 Carrera was brought into the US from Canada a few years ago, they had to change out the speedometer which read km to one reading mph. This is a new (non-cable) speedometer from VDO.
Anyway, it's always read about 10% faster than it should. I spent a coupe hours over the weekend taking it out of its case with the intent of drilling a hole in the case above the adjustment point so I could calibrate it.
I've looked at the various articles regarding the procedure & have noted where the hole should be, but upon looking at the insides, I absolutely cannot find an adjustment point. NOTHING. Are they making the new ones differently? How would they do the adjustment if I sent it away & paid $95.00????
Can anyone accurately describe the adjustment point??? I've been looking for a component soldered onto the board or anything that remotely looks like it could be turned with a mini-slotted screwdriver, but nothing.... help.
regards,
jlex.

Read here: Adjusting Speedometer

I was never able to locate the "trimpot" when I had mine disassembled for the odometer gear replacement.

RoninLB's writeup is from a 77, mine is an '84, which runs about 10% optimistic at 60 mph. I'm just living with it.

Thanks, Kurt, but I've done a lot of searching & haven't turned up anything useful. There's a thread by '84Toy that's nearly identical... he took his completely apart & there's no place to adjust it.
I'm concluding that the later year speedos are adjusted in some other manner: there is NO small screw on a "pot" to adjust. These circuit boards have changed.
jlex.

My SC also reads high. I used a friend's portable nav sys that told me my speed and it was constantly about 10% off. At an indicated 80 mph, I was really doing 73 mph. I just live with it.

I think trying to drill a hole may be a bit much if you don't know what you're looking for. I calibrated my SC's speedo some years ago and ended up removing the bezel, glass and case from the workings to reveal the trim pot. I calculated what the indicated speed should be based on measured rolling radius, gear and final drive ratio, then put the car on stands, fired it up, and made the adjustment based on tach reading.

Don't know if anything has changed with the Carreras over the SC's, but it's fairly obvious once you get the cover off if not. It's a little scary, but not that difficult to pry the bezel back and expose the interior. Good luck,

ianc

QUOTE]Originally posted by Early_S_Man
Well ... the real problem out there for this board is that they just built too many kinds of 911s for too many years to make any generalizations about tachs ...

There are, excluding the cable-driven 356/550 tachs:

1. The real early tachs, before CDI ignition ... 356 and 911/914 ...
2. Early CDI tachs for use with ballast units ... '69 - '70/'71
3. The ~'71 or '72 - '77 tachs for use with point-triggered CDI ...
4. The '78 - '83 SC tachs for use with 6-pin CDI units with 'TD' output terminals ...
5. Turbo tachs for use with 8-pin CDI units, and later, 6-pin CDI units ...
6. 3.2 Carrera DME tachs ...
7. 964 DME tachs ...[/QUOTE]

Thanks, Ianc... but been there, done that. I didn't want to drill a hole in the case either, so I did pry the bezel back, take all the rings, glass, etc. off the front, then took out all the insides after removing the three back screws.... NO LUCK. There is nothing discernable on the circuit board or elsewhere that can be adjusted on this '85 electronic speedometer. (NO TRIMPOT).
I've searched other threads & looked at their pix & it appears the insides of these things have changed from the earlier models... are less components. They seem to have done away with the trimpot.
After all this disassembly work, I don't want to give up & send it away for $95.00. Obviously, there is a way to adjust these if you DO send them away. The question here is how do you do the adjustment on a later model speedo that doesn't have a trim pot????
regards,
jlex.

If the over speed reading is same at 30 & 60 mph then removing the needle may be an option? I've never removed mine so no advice on that delicate maneuver.

Moving the needle is scary... there's got to be a better way. After all, if the people you send it away to can do the adjustment, then someone out there must know how to adjust these later year speedos.....
Now that I've done all the work of taking it apart, I'll be darned if I'm going to pay $95.00 for someone to do the 3 minute adjustment.
Unless someone comes forward with something new, I'm just going to put it back together & drive with it as-is....
Thanks anyway, guys.
jlex.

I have an 87 carrera and it too reads high.
So if I ignore it.. doesn't it affect odometer milleage?

not on mine.

The basic calibration of the 3.2 speedos is done with a resistor from pin 4 to ground on the UAF2115 chip ... that could be replaced with a 100 Ohm trimpot, or you could use the procedure in one of these articles:

http://www.volvoclub.org.uk/faq/ElectricalInstruments.html#SpeedometerCalibration

http://heneghan.members.beeb.net/audi/speedo.html

Mine reads 10% fast too, no effect at all on the odometer though

Yep, my '84 is about 10% too fast as well. I always assumed that the odometer worked off of the speedometer.

Jlex, who does the fix (for the $95 you are referring to)?

has anyone considered smaller tires??

Thanks for the replies, guys...
Warren, I believe you've got the right answer on these 3.2 speedos... is more than just a screw adjustment here... would have to actually desolder & replace an electrical component (resistor) to make it right... I unfortunately have no electrical background... would probably remove & replace the wrong thing....it'd have to be pretty obvious to me before I'd attempt it.
If anyone does it, be sure to take & post pictures!!!!
Regarding the quote: was thru email from N. Hollywood speedometer...HOWEVER... I understand Pelican Parts also does calibrations, so check with them if you're going to send it away. I'm sure they're competitive & competent.
Thanks again, Warren; you just about always have the right answer.
regards,
jlex.

Speedo correction by tire size
 

If the speedo is reading high, larger (bigger rolling radius) tires are need to correct the problem.

If you change the tires, it will change the odometer though. Bigger would put less miles on the car than actual, now would that be honest?