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Old 09-24-2007, 12:15 AM   #1
03IRM3
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E30 Speedometer Repairs - Failure Modes

E30 Speedometer Repairs

So it seems the E30 suffers from odometer failures from time to time and many people have inquired about them when it happens. Quite a few of the answers are to replace the gears, however I have seen a number of them that did not suffer from the gear failure in the forums. I just spent the last two days goofing around with several clusters and figured I might as well share what I have found.

Several failure modes of the E30 speedometer when the odometer quits working:

1. Gears
2. Solder joints
3. Capacitor
4. IC (or Integrated Circuit)
5. Stepper motor
6. Coil wires (not ignition)


Symptoms: Odometer does not work, speed indicator works.


The first item is fairly straight-forward and seems to be the most common issue. However, if the gears are fine and the needle still moves, some of the later items listed could be the cause. The needle movement indicates we have a signal to the speedometer head which is a good thing, otherwise a person would have to diagnose why there is no signal.

Solder joints. I had one solder joint that was cracked around a mylar type capacitor where it was connected to ground. Not good. I re-soldered all the joints on the board as an assurance that this would not become a problem for the next 15 odd years (cross fingers). A quick check to make sure the connections are clean is also good while you are in there.

Capacitor failure. It is a common issue that occurs on the VDO speedometer. I don't know if there is a history on the MotoMeter units or not but quite possible as they have some commonality in the circuits, and this part being one of them. On the board of the speedometer head lives a 220uF electrolytic capacitor. Often times this some-bitch will decide to piss itself. If you notice a reddish looking or rust colored substance on the surface of the board where most of the electronics resides, this could very well be the problem. For some reason these particular caps have a habit of leaking which in turn means the circuit will no longer function as intended. The remedy? Replace the cap and hope that nothing else was damaged in the process. You will have to unsolder about five places to pull the board from the body of the speedometer but not hard. There is only one like it so it isn't hard to miss once you get the thing apart.

IC failure. This does happen sometimes. I replaced the cap on the board and still had no functionality. So I checked the output of the IC on the two pins that drive the stepper motor that runs the odometer. Square wave on one, not on the other. I wondered if the IC was bad, so I swapped it out from the other speedometer head. Still no luck. In my case the IC did turn out to be good, but is not unheard of to fail.

Stepper motor failure. Again - not real common however my take on it is that it can happen sometimes. This motor is what actually turns the odometer.

Coil wire failure. This may not be real common, however it did happen on the one speedometer head I had that I also replaced the cap on it. I don't think there was a correlation between the two, but don't quote me on that. After finding no output on the other pin that drives the stepper motor even after replacing the IC, further investigation into the circuit revealed that one of the tiny wires leading into the coil was broken from the post it was supposed to be soldered to. I re-soldered it, and the odometer now works again. From what I can tell, the IC that was used is fairly robust that when a load is not connected on the output, it did not appear to damage the output. That is good. Or maybe I was lucky, I dunno.


I will say I have a preference o the MotoMeter units for some things. The circuit board appears to be better as far as manufacturing quality. Also, the MotoMeter unit was far easier to try to calibrate as they were good enough to include a trim pot on the board. Just input the square wave at the correct frequency for the speed and adjust - this does take patience though. I now have the needle about 2 MPH off at the most all the way to 140. I gave up on the VDO unit for now - later I may investigate switching one of the resistors to a trim pot and try to solve it. The values need to be very small increments and I didn't have the best assortment, leading to large jumps in accuracy. I feel the gears may be made out of a better material on the VDO units, but still not as good as they could be.

As far as interchanging the units - if you have an understanding of them it isn't that difficult. My E30 is a 318is, and the other cluster I had was from an M3. Both have K=7784 stamped on them, as well as on the part number sticker. What this means is the number of pulses per mile the speedometer is calibrated to. Swapping to/from the M3 and 318is is easy as they use the same constant K. Going between others would likely require changing the divider in the IC which is done by pins on the board. I haven't looked at the numbers yet, but just my guess that it can be done to re-calibrate to another vehicle, or, as I have been wondering, the possibility to re-calibrate for different tire sizes...

That is what to look for. I can now repair my speedometer heads after many hours of tinkering and developing an understanding of how they work. Quite a fun venture to undertake. And. The odometer can be changed, it just takes patience but is not hard. The reason I changed one is so I could put it in the car with the exact same miles on it as the other I was taking out but then I decided I didn't need the extra 10 MPH anyway compared to having a more accurate speedometer. So I just swapped the needles over until I tinker with the circuit on the M3 speedometer to see if I can get it calibrated a little better.

I am looking into the tach functionality. The SI boards use the same part numbers, so that means everything is in the coding plug and the tach sweep. I think I can adjust the output from the frequency to voltage converter on the SI board to compensate though. That is for another day.

Next I want to tackle the OBC and cluster coding plug. I can duplicate OBC coding plugs I believe but I have to build on to replicate it. Then figure out what all the contents are. The cluster coding plug is a little more difficult as I have to build a test circuit to read the plug contents and I m not sure which protocol it is using yet, but might have narrowed it down to two with 9 combinations of connections.

So that is it for this weekend. Until next time, keep the shiny side up.

Last edited by 03IRM3; 09-24-2007 at 12:21 AM.. Reason: speeling... I hate laptops keyboards...
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Old 09-24-2007, 02:22 PM   #2
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Clusters

Very well thought out,, keep us updated , thanks
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Old 09-24-2007, 02:59 PM   #3
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this should be in the FAQ section.
you got any pictures of the Motor Meter adjustment??
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Old 09-24-2007, 11:58 PM   #4
03IRM3
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Thanks - as soon as I find out more about the electronics I'll put info out. I am trying to finish up my diagnostics interface at the moment but once I get that done I'll get back to this.

For pics - I don't at the moment, but I'll pull out the camera tomorrow evening and get a picture of it before I start putting it back in.
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Old 09-25-2007, 09:42 PM   #5
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Okay - I attached the image of the MotoMeter unit. The arrow is pointing to the potentiometer used to adjust the calibration of the speedometer. This does not change the odometer reading itself - just the needle position in relation to input frequency generated by the speed sensor.

There are four contacts on the opposite end of the board as follows:

+ - Power (+12V)
A - Signal output
31b - Signal input
31 - Ground

To adjust, connect a power supply to ground (31) and the +12V input (+) to power up the unit. Connect a signal generator to the signal input (31b) with a square wave of about +10V. The frequency in Hz to input is based on the constant K divided by 3600 times speed in MPH:

Hz = (K / 3600) * MPH

So in my case, I would input a value of (7784 / 3600) * MPH to check the indicated speed. I played around with trying to input a frequency in 10 MPH increments and adjust but found this to be fairly difficult. So I ended up setting the input frequency to 140 MPH, adjust the pot, and then check the indicated speed across the sweep. This provided a much closer overall difference between the indicated speed and what the actual speed should be.

It may be because of the circuit design itself does not provide an exact linear change with input to the IC the pot is connected across opposed to the value of resistance it sees. Plus they are using a somewhat high resistance that makes it difficult to adjust with a pot. Just remember after making the adjustment, turn the pot back the opposite direction just a touch and let the circuit settle for a few seconds before making another change.

Anyway, that should help. I don't know that all the MotoMeter units are set up in this manner, mine was which made my work easy.
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Old 03-03-2008, 05:15 PM   #6
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So a M3 cluster into a 318is is a direct swap or does the code plug still need to swaped out.
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Old 12-07-2010, 02:00 PM   #7
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Fantastic writeup.

My odometer stopped working recently. I did a quick search and found lots of threads about damaged gear inside the speed.
So i thought easiest fix would be to go and buy another instrument cluster, which i did and.... odo also doesnt work. I took the original dash apart and i see 1 tooth missing of the infamous odo drive gear, but if i manually spin the shaft is still keeps turning the other gears therefore issue must be electrical...

I'll have to do more investigating... and take apart the 'new' instrument cluster and see what's broken there....

I'll report my findings
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Old 12-08-2010, 12:35 PM   #8
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Ok, i have pulled apart both instrument clusters last evening. one is vdo other one motometer.

on the vdo there's 1 tooth missing from the 12 tooth drive gear (the smallest one). I was able to bench test it and speedo and odo is working fine - all though odo stopped working in the car. Perhaps the fact i took it apart and put it back together have fixed it.

the motometer one.... the small 12 tooth cog is complelty stripped of it's teeth. also the gear that drives it (bigger one) is also missing few teeth, so it will also have to be replaced.

I'll go to jaycar today and see if the have those replacement cogs.
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Old 12-09-2010, 05:23 PM   #9
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Great thread!
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Old 12-11-2010, 02:13 AM   #10
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I managed to repair the VDO unit by replacing gears with the set found in jaycar electronics as per the guide on e30tech.

I was unable to find replacement gear for the motometer unit - particularly the uncommon 44x17 gear... I would prefer to use motometer unit because a) it is in metric units, which is my preference and b) because it is possible to calibrate it via the trimpot provided.
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Old 02-08-2011, 11:12 AM   #11
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potentiometer?

Hey I was recently replacing an odometer gear on my girlfriend's E30, and saw this little adjustment on the speedo, and of course my curiosity was all like "hey it moves!". I thought I put it back in the right spot, but now the speedo reads funny... I haven't had a chance to test for accuracy, all I know is that when you get rolling the speedo doesn't move much and then moves what feels like very slowly compared to how it's supposed to.

Is there an easy way to fix this or do I need to know the stuff that 03IRM3 was explaining? I don't think I can easily get ahold of a signal generator.
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Old 02-08-2011, 11:21 AM   #12
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Quote:
Originally Posted by Helion42 View Post
Is there an easy way to fix this or do I need to know the stuff that 03IRM3 was explaining? I don't think I can easily get ahold of a signal generator.
Since you know how to take the speedo apart now... Drive down the road with a gps and check the difference. If there is a difference, turn the pot a little bit in one direction. check again. If the difference increases, turn it the other way. adjust till you're close enough for your liking.

OR

Borrow a signal generator.
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Old 02-08-2011, 12:22 PM   #13
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Quote:
Originally Posted by Helion42 View Post

Is there an easy way to fix this or do I need to know the stuff that 03IRM3 was explaining? I don't think I can easily get ahold of a signal generator.
I was able to successfully trigger e30 speedo by "PC signal generator", i.e. you put the cable into "speaker out" on your computer then you play sinne or square wave. There are programs that will allow you to generate different waveforms from your PC.
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Old 10-11-2011, 06:46 PM   #14
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in 03irm3's description of calibrating a motometer speedo, does anyone know which way to turn the pot to decrease what the speedo reads? any idea as to the sensitivity? (i.e. 1/4 turn equals 10mph)

noticed that my newly acquired eta speedo reads 10% high accross the range.

thanks!
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Old 02-08-2012, 09:50 PM   #15
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Quote:
Originally Posted by 03IRM3 View Post
There are four contacts on the opposite end of the board as follows:

+ - Power (+12V)
A - Signal output
31b - Signal input
31 - Ground
Is it possible to run the motometer speedometer with only the
+ - Power (+12V)
31b - Signal input
31 - Ground
wired correctly? Will the gauge function with only those three connected?
I am trying to ask what is the A signal output for? Where does that wire connect to? What is the output of the speedo used for?
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