Is it possible to adjust the speedometer?

[jhh925]

The car is an E28 M5 with the original cluster. Wheels are original and tires are the stock size (225/50-16) all the way around. The speedo and the tach work perfectly well in the sense that they come on and give readings in the expected way.

Diff was fully rebuilt. The speed wheel is original to the diff. The speed sensor is new.

Unfortunately, the indicated speed on the speedo is about 9-10% higher than actual. That is, compared to a GPS app, I'm reading 9-10% too high on the speedo ... 100mph indicated on speedo = 90/91mph on the GPS.

Is there a way to adjust what the speedo reads out to try to get a bit closer to reality?

[Blue Shadow]

Is the trigger wheel inside the diff missing a tooth?

[turbodan]

It is fully electronic, no adjustments can be made.

If it's reading 100 at 91 you're on the high side of the margin of error that I would consider normal. I'm not surprised that BMW would build in some optimism on the indicated speed though, especially in the M5. I really don't see it as a problem, run ~10 percent above the speed limit and you'll be fine.

I have found that the tach isn't spot on accurate either compared to the dashboard I have on the laptop through megasquirt. Is what it is though.

[jhh925]

It is fully electronic, no adjustments can be made.

If it's reading 100 at 91 you're on the high side of the margin of error that I would consider normal. I'm not surprised that BMW would build in some optimism on the indicated speed though, especially in the M5. I really don't see it as a problem, run ~10 percent above the speed limit and you'll be fine.

I have found that the tach isn't spot on accurate either compared to the dashboard I have on the laptop through megasquirt. Is what it is though.

Thanks. I was afraid of that. And I get it, but it's hella annoying. It's also my strong suspicion that the odometer is relying on the same signal from the diff, so that if my speedo reads +10%, then my odometer is also indicating +10% - isn't that the case?

[Mike W.]

Not exactly an answer to your question, but it might be of use.

I have found the OBC readings to be very accurate. Granted there is no instantaneous MPH reading, but set it at a steady speed, put it on CC, reset the start of average speed and bam, you have current speed, within a very small margin. Curious that OBC and the speedo both rely on the same input, and both are electronic, but one is more accurate.

Adjusting the speedo. I have not looked into it, but E28s are simple enough there might be enough analog components, such as a resistor, that it might be possible to recalibrate by changing a component. Like a 10K resistor replaced by an 11K. Might, I have never looked into it.

[Kerrvillian]

The E28 speedo is the same setup used in the Mercedes W126/R107 (and probably other European cars) of the era. There is a single chip on the board that controls the galvanometer coil on the speedo needle and issues a stepper motor pulse to the odometer.

If your odometer is accurate, it's just the galvanometer calibration is out of adjustment. There's a single resistor on the board that sets the magnetic strength of the biasing coil to affect accuracy. I haven't got a cluster in front of me to be more specific than that since it's been a while since I've fiddled with one, but I've been through this EXACT issue with more than one Mercedes and was able to dial in the speedo to match perfectly with a GPS signal with a 25 turn potentiometer installed where the original resistor was.

If I get a chance to grab a cluster in the next couple days, I'll post a datasheet to the chip that does the odometer indexing and speed signal output. If you can read a schematic, it's self-explanatory what you're looking for.

[Ju@n]

It's exactly like Kerrvillian says.
Here you have a post, I know it's in spanish, but don't really feel like translating right now haha.
Motometer speedometers in E30s already have that trimpot instead of a fixed resistor, in my car I simply replaced the resistor, because I had one that matched what I wanted, if not one of those 25 turn trimpots is your friend.
Best way to do this is to create a pulse generator, the speedometer unit just needs 12v and a signal. Record what the speedo says at several speeds (20-40-60) using a GPS, then simulate that in a bench.

[Blue Shadow]

Digikey has that variable resistor and shipping is free (to US and Canada) if you send em a check with your order. Helping the hobbyist still to this day. Once we know the resistor value, easy to select the proper VR, Bourns for sure, but get the vr set up correct so it fits and you can access the adjustment.

[jhh925]

If your odometer is accurate, it's just the galvanometer calibration is out of adjustment. There's a single resistor on the board that sets the magnetic strength of the biasing coil to affect accuracy. I haven't got a cluster in front of me to be more specific than that since it's been a while since I've fiddled with one, but I've been through this EXACT issue with more than one Mercedes and was able to dial in the speedo to match perfectly with a GPS signal with a 25 turn potentiometer installed where the original resistor was.

I have not checked the odometer reading vs. actual known distances, but this is a great question and your info above is very helpful & encouraging. Weather is crappy here right now, but I should be able to get some data on the odo later this week and report back here.

If I get a chance to grab a cluster in the next couple days, I'll post a datasheet to the chip that does the odometer indexing and speed signal output. If you can read a schematic, it's self-explanatory what you're looking for.

That would be great. It would also be fantastic (if this is even feasible) to give some guidance on which resistor in the cluster is actually providing that "calibration" function.

[Galahad]

... If I get a chance to grab a cluster in the next couple days, I'll post a datasheet to the chip that does the odometer indexing and speed signal output. If you can read a schematic, it's self-explanatory what you're looking for.

Is there a chip in the speedo unit or are you talking about the chip on the SI board? I haven't been able to find a datasheet for the latter.

... Curious that OBC and the speedo both rely on the same input, and both are electronic, but one is more accurate.

This isn't all that odd - OBC calculates speed based on total pulses / elapsed time, so the only error sources are inaccuracies in the clock (which is off by what, a minute a month tops?) and being somewhere in-between pulses from the wheel in the diff, which is on the order of a foot of tire rotation.

The speedometer calculates speed by balancing an input magnetic field against a return spring, both of which are analog and have much higher maximum error. Given all the forces are small, any issue with either the magnetic field or the spring is going to cause larger errors. Modern speedos use a stepper motor to avoid this issue.

[Kerrvillian]

Is there a chip in the speedo unit or are you talking about the chip on the SI board? I haven't been able to find a datasheet for the latter.

It is on the speedo head itself. It's a DIP chip with bonded legs to act as a heatsink. I haven't had a chance to go grab a cluster, so I can't provide a picture or more information than that. I've slept several hundred times since I last worked on calibrating a speedo.

[Kerrvillian]

Ok, here's a link to the datasheet for the ITT UAF2115 chip that drives the odometer and speedometer. It's the same design for pretty well all electronic VDO speedometers of this era, so it's pretty generic.

https://pdf.datasheetcatalog.com/datash ... 248_DS.pdf

What you're looking for is a resistor between Pin 4 on the IC and ground (Pin 13). This is your calibration resistor for the galvanometer current strength going to the speedo needle. If the odometer and the OBC are reporting correct mileage, this is what you need to adjust to make the speedometer gauge match. It's trial and error, this is analog circuitry we're dealing with.

Hope that's helpful.

[tn535i]

fwiw and iirc... there used to be large fines to the manufacturer if the Speedometer was in error on the low side, meaning people drove faster than they realized. So the Germans errored to the high side accounting for some variations in tire sizes and everything else that might effect speedo reading. I understood it to be by design while the odometers are generally reflective of German precision with new OE size tires.

Be glad yours still works in any case... ha ha

[Mike W.]

fwiw and iirc... there used to be large fines to the manufacturer if the Speedometer was in error on the low side, meaning people drove faster than they realized. So the Germans errored to the high side accounting for some variations in tire sizes and everything else that might effect speedo reading. I understood it to be by design while the odometers are generally reflective of German precision with new OE size tires.

Be glad yours still works in any case... ha ha

I do remember something about that, but I thought it was after the E28 era, not sure though. My '85 seemed very accurate until it got a little cranky around 310K and did an assortment of odd things.

[Fx323i]

I do not know the E28 speedometer's construction but on E30 this was possible. On the E30 cluster there was written a coefficient K=(number) dependent of if in miles or kilometers per hour. Basically it means, how many teeth it sees per kilometer driven.

Knowing this coefficient, it is possible to calculate the frequency to send to the gauge. The gauge on E30 has a potentiometer to adjust how the needle reacts to the signal. Terminals are described (on E30) on the gauge's connection to the board itself.

+ = +12V
31 = ground
31b = speed signal input

My case (Euro car, km/h) was having K=4838 (this means 9 teeth in the differential sensed by the sensor per revolution, multiplied by the circumference of tyres; keep in mind that BMWs of that era were showing +10% of speed by 50 km/h so a little adjustment in calculation needs to be there). My case was, I was helping a friend, whose car was on stock wheels and showing 40% more of the speed.

1 km/h = 4838/3600 = 1,343 Hz

Following values are approximate as my pulse generator can only produce whole Hz values.
50 km/h = 67 Hz
100 km/h = 134 Hz
150 km/h = 202 Hz
200 km/h = 269 Hz
240 km/h = 323 Hz

I attached the cluster and signal generator (a very simple one can be obtained for several Euro/Dollars) with a desired frequency for a mid-speed (let's say 100 km/h), adjusted by the potentiometer as close as possible. Then I switched to the highest speed reading and refined. Then I went down in steps and checked if the shown values were at least a bit close to normal reading.

Video will appear after a while, it is in my language, but you at least see the principle.

https://youtu.be/lr2oqTBcY1M

If you desire to calibrate the cluster because of different tyre size, you need to calculate your K-factor and start from there. Keep in mind that the cluster is in some position in the dash and you see the needle under an angle, also the readings are not fully proportional.

Hope it helped although I do not know if E28 tachometer is the same. Also, if anything is not understood, please allow me some mistakes as English is totally not my mother tongue.

[jhh925]

Ok, here's a link to the datasheet for the ITT UAF2115 chip that drives the odometer and speedometer. It's the same design for pretty well all electronic VDO speedometers of this era, so it's pretty generic.

https://pdf.datasheetcatalog.com/datash ... 248_DS.pdf

What you're looking for is a resistor between Pin 4 on the IC and ground (Pin 13). This is your calibration resistor for the galvanometer current strength going to the speedo needle. If the odometer and the OBC are reporting correct mileage, this is what you need to adjust to make the speedometer gauge match. It's trial and error, this is analog circuitry we're dealing with.

Hope that's helpful.

Very cool! So reading the data sheet, it looks like I'm looking for a 14-pin chip on the back of the speedometer itself that has two wide cooling pins and that looks like this:



The data sheet says, "The mean output current from Pin 5 can be adjusted externally by means of a resistance from Pin 4 to ground so as to equalize tolerances of the meter."

So, when I find that chip, I'll count counter-clockwise from the notch to pin #4, then I'm going to look for a resistor that should be directly connected to that pin (via the PCB). I would then pull that resistor out, wire in a potentiometer, go for a drive, and adjust the pot in such a way as to get the speedo to match my GPS? Here's a pic of the data sheet test circuit, so I think I'm looking for BMW's equivalent to the circled resistor:



Sound about right?

[Galahad]

...
Sound about right?

Yeah that sounds right. It would be easier if you had a frequency generator like Fx323i suggested but driving around should get the results you want

[Blue Shadow]

well if you do that, dial in the exact resistance you need you could just replace the resistor with one of the value you need, saving about 2 bucks vs the price of a nice bourns 3296 multi-turn.

Looking forward to your results.

[Galahad]

well if you do that, dial in the exact resistance you need you could just replace the resistor with one of the value you need, saving about 2 bucks vs the price of a nice bourns 3296 multi-turn.

Looking forward to your results.

You'd still need the multi-turn to get the value dialed in - I'm really doubting he has a resistance decade box sitting around

[1st 5er]

Has anyone pulled the needle and adjusted it's position when pressing back on?

[Fx323i]

This is where the adjustment potentiometer lies on an E30. I do not have a disassembled E28 cluster on hand to see if it is present there as well. But I can imagine that in the factory they were calibrating it this way so I suppose it could be there.

[Blue Shadow]

Has anyone pulled the needle and adjusted it's position when pressing back on?

For the fuel gauge, years ago. Goes to full now and the light comes on before the needle goes below empty.

[Mike W.]

Has anyone pulled the needle and adjusted it's position when pressing back on?

For the fuel gauge, years ago. Goes to full now and the light comes on before the needle goes below empty.

Yeah, I did the same on a temp gauge on an E24 long ago. Couldn't figure out why it was always running so hot. Then I noticed when stone cold it was almost out of the blue already and manually turning it, it went way past red.

[Kerrvillian]

Has anyone pulled the needle and adjusted it's position when pressing back on?

Tried that on a Mercedes with a similar issue to this. The problem is that it's not a linear relationship, so the speed will still be way off at anything other than what you adjust the needle for. The error compounds the faster you're going. So at 70, the error is more than twice that at 35.

The Mercedes I had was doing 95 down I-45 one day and I got passed by a 240D. His speedo said he was doing 69 when he passed me.

[jhh925]

I'm finally getting around to trying to get my speedo to read a bit more accurately. I pulled my cluster and removed the speedo from the cluster. It was pretty easy to locate the chip that matched the info given above ... but unfortunately, the "adjusting" resistor shown in the diagrams above is buried in a very inaccessible spot. The arrow in the pic below points at the resistor.



To get to the resistor, I had to pull that board off of the bottom of the speedo unit. To do that, I had to de-solder five connections, shown here.



Having pulled the board off, you can clearly see that this chip matches the info above (UAF2115). And the traces on the other side of the board exactly match the diagrams above for the UAF2115 chip. The "adjusting" resistor is the one labeled "R14" in this pic.



I pulled the R14 resistor out and it measures exactly 75 ohms. I have some 100 ohm potentiometers on the way, so hopefully I'll be able to post the results here in the next couple of days. Given how inaccessible this resistor is, for now I think my plan will be to wire in the potentiometer with some longer leads, set the adjustment while driving, then stuff the adjusted pot back into the dash. At some point in the future I may buy a matching resistor to replace the pot.