E30 - Speedometer has failed

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'91 318is, 120k miles
Speedometer has been quitting intermitently, but has finally quit altogether, with the oddometer failing at the same time (I presume this is
obvious.. but ...). Checked the connection at the erar differential and all seems clean and in order. Do I need to disassemble my dash, or is there something else to check first? TIA
Matt
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Nice :-)

It's probably the diff sensor. maybe the wiring is corroded. Try remaking the connections.
--
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Email: snipped-for-privacy@unixnerd.demon.co.uk, John G.Burns B.Eng, Bonny Scotland
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M Warren wrote:

No, it is *not* obvious ... but it is a very valuable diagnostic. If *only* your speedometer had failed, it would indicate the plastic gears in the dash were broken. However, if both have failed simultaneously, it is almost undoubtedly the sensor, or the wiring to it, in the rear, since both instruments (and your OBC, 'economy meter', and cruise control, if so equipped) share that signal.

Back under with you! If the wiring is all *good* (do a continuity check - especially if they have ever been 'baked' by an exhaust leak back there), then replace your sensor unit. That will probably be it. -- C.R. Krieger (Been there; broke that)
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While the wires can certainly be a problem, I'd suspect the simple mechanics of the sensor's gears meshing with those inside the diff. My guess is the diff is fine, and the speed sensor itself is toast. It could be the wires, but I think other wires in the area would be similarly aflicted, and if the wires LOOK okay, they probably are.
My money is on the sensor itself, it's either failed or the mount screw is loose and it moved.

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J Strickland wrote:

OK; forgive me for being an E28 Guy®, but does the E30 *not* share the same type of speed sensor as the E28? If so, there *are* no gears, as such, back there; it's a magnetic sensor reading a toothed wheel and there's no mechanical contact at all. They just fail. You spend the $10 or so for a new one and stick it in.
That said, the excellent BMW tech who educated me about these told me it's quite common for the wires to get 'baked' by a now-forgotten exhaust leak. They are now more likely to break inside, so a continuity check, at least in any area where it's close to the exhaust system, is usually a good idea. -- C.R. Krieger
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J Strickland wrote:

OK; forgive me for being an E28 Guy®, but does the E30 *not* share the same type of speed sensor as the E28? If so, there *are* no gears, as such, back there; it's a magnetic sensor reading a toothed wheel and there's no mechanical contact at all. They just fail. You spend the $10 or so for a new one and stick it in.
That said, the excellent BMW tech who educated me about these told me it's quite common for the wires to get 'baked' by a now-forgotten exhaust leak. They are now more likely to break inside, so a continuity check, at least in any area where it's close to the exhaust system, is usually a good idea.
Well, okay then. I would suspect a mechanical failure before a wiring problem, but I also assume that while one is unplugging the wires to look at the mechanicals, one would notice the baked wires and change course in mid stream. I haven't had the speed sensor out of my BMW, but I have had several speed sensors out, and operating by mechanical means -- gears -- or by magnetic pick up presents pretty much the same repair experience, except that gears are admittedly easier to look at and identify a problem.
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the
wires,
at
several
Okay, so it's back underneath tomorrow. I will do a continuity check on the wires and pull the sesor if that checks out. Thanks to all
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I would take a look at the speed sensor in the diff before I took the dashboard apart.
The speed sensor gear is typically mounted on an eccentric to accomodate installing different gear sizes to account for changes in the ring and pinion gears. If your car had 3.07:1 gears, and you wanted to change to 4.10:1 gears, then you would need a new speed sensor gear so your speedometer and odometer remained in proper calibration. Since this would require speed sensor gears with different tooth counts, then the speed sensor gears would necessarily be different sizes, and would need to be mounted differently in order to maintian the proper gear mesh.
The eccentric is generally held in with a single screw that clamps the edge and prevents it from moving. If the screw is a bit lose, then the gears on the speed sensor might fail to mesh properly, and the speedo would stop working, or might not work smoothly. It's possible that the speedometer would work fine until the teeth (plastic -- nylon really) wear off, then suffer a catastrophic failure all at once.
In any case, I'd pull the speed sensor from the diff and give it an inspection. You may find worn teeth, or you might find that the device grinds and growls while you turn it with your fingers. Either way, you should find the problem. When you re-install the sensor, you will want to spin it in the mounting hole to mesh the gears tightly. Since the teeth are plastic, you'll want to get a good mesh without mashing the gears together too tightly.

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Yes pull the sensor, and like Krieger said, you will find no gears on it what-so-ever. It is a proximity sensor and there is nothing mechanical about it. It counts the reluctor wheel teeth and sends the tooth count as pulses to the ECU and dash cluster.
In the old days some companies used rotary pulse generators that were driven by gears, but they proved to be unreliable in vibration enviroments. So what was adopted was a toothed wheel and a proximity sensor. As a tooth was under the sensor the signal would rise (on), and when a tooth was NOT under the sensor the signal would fall (off). The on/off pulses are then sent to the speedometer and there they would get translated into a "speed".
When you change gear ratios in a BMW the speedometer would need something to tell it what the new ratio is, and that something is a "Key" or a color-coded plug you snap into a socket. This plug calibrates the speedometer to your new differential ratio.
Cheers, DieInterim
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Thanks for that. I have a Jeep, and regearing requires a new speedometer gear that screws into the transfer case on the output shaft. This is the same as getting the information from the differential, but it's at the other end of the drive shaft.
When we need a new gear, we have to count the teeth, but having the gears be different colors would help alot. Maybe the gears are different colors, hmmm I'll have to check on that.
In any case, a magnetic pick up has to accomodate changes in the ratio in some manner that resembles teeth on a gear. It would have to do it by varying the pulse width somehow to create more or fewer pulses, as the case may be.
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I thought the whole idea behind BMW's taking the speedo input from the differential ring gear was to make it independant of the upstream ratios.. It's actually measuring the wheel rpm's.

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If you think about it, the same thing happens when you take the data from the output shaft of the transmission or the tcase (in the case of 4WD). I suspect there is a certain degree of concern for the parts involved in a 4WD if those parts are in the diff, obviously those concerns are not present in a street-only vehicle.
I suppose that if the speed is picked up from the outside of the ring gear, then it wouldn't matter which gear set one was using because as you point out, the outside of the ring gear can be designed to always be in the same place without regard to the actual tooth count on the gear. In this case, the same speed sensor could be used no matter which gear set was selected.
Back to the OP's issues, I would start my search with the speed sensor and its wire harness, then look to the speedo head later.

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Jack wrote:

The output shaft of the transmission is the same.
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Actually, Fred, he's right. If the gear set in the diff is changed, the ouput shaft will change its speed accordingly. But, if the speed sensor looked at the Ring Gear to gather the speed data, and all of the possible combinations of gear sets were designed to provide the speed data, then all gear sets could use the same speed sensor. But as you and I know, the speed sensor mounted on the trans would need to be modified as the gear sets change.
I have no clue how BMW accomplishes this, but in theory it is entirely possible that the speed data comes from the ring gear, and this would allow the same sensor with all ring gears.

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Yes the encoder ring does not change. I did not mean to carelessly leave an open end here.
A better example: Changing tire sizes. In older cars you had dip switches to flip to compensate for tire size increase/decrease. Later VDO began using color coded plugs that were nothing more than jumpers that replaced the dip switches. Those plugs I think are no longer used today but can be found on 80's BMW's. Without those plugs making changes to the car could mean you needed an interface to correct the speedometer reading. In this manner VDO could use the same guage on many different models of cars.
BD
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ratios..
The output shaft of the transmission is related to the wheel speed by the ratio of the diff. Change the diff, change the ratio.
Only on the other side of that gearing, can you measure wheel speed reliably and independantly of driveline component changes. Not before. (at least not without other trickery)
-Russ.
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I had a similar situation develop on my '84 318i and was able to keep it going for another year just by giving the instrument cluster a sharp rap with my knuckle whenever the speedo quit. This will also let you know if the problem is in the dash or elsewhere. All of the instruments in the cluster are connected to a backplane by plug on connectors which can become intermittent due to oxidation. You can obtain a more permanent fix by disassembling the cluster and cleaning the connectors. I have also heard people say that they repaired theirs by resoldering connections on the backplane. Removing the cluster is not really a big deal. My bentley manual said that the steering wheel had to be removed in order to remove the cluster but I found that this was not true. Remove two screws at the top of the cluster that are common to the glare shield. Remove two thumbwheel type nuts from the backside of the bottom of the cluster that are accessable by removing the trim panel that is under the dash. Rotate the cluster aft and down to gain access to two wiring harness connectors and you're done. I had mine out several times as different gauges failed and could get it out in 15 minutes after some practice. Another thing you should consider is that's it's probably time to replace the batteries on your service interval circuit board as long as you have the cluster out. You can get the solder tab batteries at any radio shack. Instructions for doing this are available on the E30 website. Don't forget to disconnect the battery before you start.

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- snip
I've got one of the *tap to use* speedo's..... had the instuments out a few times, cleaned all the contacts etc.. still comes back after a few months. I can live with it... :)
keith
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wrote:

Tried the 'tappit' method, and then the more persuasive *smack* - nothing. was very dissapointed it wasn't so nice and convienient.
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Hello,
Whilst these other gents sort out their differences let us take a minute to think this through.
Have any other gauges acted funny.? What about your service lights, are they on constantly? If so then I would say chasing the sensor might not be what you need to do. I think you might have the dreaded SI board failure caused by leaking batteries.
The batteries leak acid down onto the board and eats the electrical traces off the board. I have repaired many and re-soldered new batteries with good results. Others simply buy a new board.
Here is a link for you to ponder... http://e30m3performance.com/maintenance/SI_Board/SI_Board.htm .
Best of Luck! DieInterim
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