Carpokes Porsche Forum

I dug into my bench-test speedometer recently, in order to add a trip reset button -- as part of the on-going testing of my 3D-printed odometer gear. Thought it might be useful to document what I found, because I wasn't able to find anything on the internet...

I am also going to post a video in the next day or two showing how the reset mechanism works, mechanically, and why I think it's something of an internet myth that resetting while moving will strip the gear -- more on that soon.

The trip reset is actuated by a solenoid. The coil for it is part of the cluster housing, and the plunger is part of the speedometer. The coil always has positive voltage and when you press the button to reset it, you are supplying ground, which creates an electromagnet that pulls the plunger down to trigger the reset mechanism.



The solenoid coil is just a simple wire winding. Note the two black wires.....


The solenoid coil wires come out the back of the cluster and have little ring connectors that get screwed to the circuit board. I checked several coils and they all have 3 ohms of resistance between the two ring connectors -- so it draws quite a bit of current (over 4 amps at typical battery voltage). I tried to test the solenoid with my bench voltage supply and it didn't have enough current to power the solenoid. I had to use an old-school 50amp battery charger to make it work. If you suspect the solenoid is bad, check for 3 ohms between those two ring connectors. If it's open or shorted, you found the issue.

Just to further help with diagnostics, the upper ring connector is the positive side of the coil. It gets its power from the bottom connector pin that plugs into the back of the speedometer. Whether the speedometer delivers raw battery voltage (my guess) or conditions it somehow, I don't know. But if you want to bench test the cluster, you can supply 12 volts to the upper ring connector and ground to the bottom ring connector (briefly -- like a button would), and it should actuate the solenoid. In the picture, these two points have direct continuity (zero ohms).

The other ring connector goes to pin 17 in the middle edge connector (brown with yellow stripe). That is the wire that goes to the button on the dash. Then you pus the reset button, it grounds this wire, which powers the coil and pulls the solenoid plunger down to reset the trip meter. I'll show how that works mechanically in a video soon.....

I think that anyone who's actually repaired this already knows that the gear wasn't broken because of pressing it while not moving. When you can't touch the broken gear without teeth continuing to fall off it and crumble in your hands, it's hard to maintain that somehow it was going to last if only you hadn't reset your odometer.

I think the myth just comes from a higher chance of the moment it breaks being when the most force is exerted on it, even if it's only a tiny bit more than normal driving. When you're at least moving, it's _slightly_ easier for the gears to mesh when you reset the odometer, so I'm guessing there's just a slightly better chance of success without failure.
-Neil

neilschelly wrote: Sun Aug 28, 2022 7:50 pm I think that anyone who's actually repaired this already knows that the gear wasn't broken because of pressing it while not moving. When you can't touch the broken gear without teeth continuing to fall off it and crumble in your hands, it's hard to maintain that somehow it was going to last if only you hadn't reset your odometer.

I think the myth just comes from a higher chance of the moment it breaks being when the most force is exerted on it, even if it's only a tiny bit more than normal driving. When you're at least moving, it's _slightly_ easier for the gears to mesh when you reset the odometer, so I'm guessing there's just a slightly better chance of success without failure.
-Neil

I did a video showing exactly when happens when the trip meter resets, and will post that soon. The story I've heard over and over is that the gear is more likely to strip if you reset it while moving -- people advise to come to a full stop, etc. Whatever people believe, without doubt the real issue is that the factory amber-colored gear turns to a sticky crumbling mess when old -- and THAT is the reason it fails -- as you said. Resetting the trip meter actually reduces the pressure on the odometer gear, although when the trip meter starts back up, there 'might' be a nanogram of extra resistance. If a gear breaks during that process, it's equally likely to break over a pot hole -- the gear is already a goner, just a matter of when it crumbles to pieces...

I did a video on how the reset mechanism works, and why resetting the trip meter (when moving or stationary) doesn't strip or damage the odometer gear....

Hey @Tom , excellent, excellent video. Nothing like data or a practical demonstration about how something works to help investigate things like this. This odometer reset thing is a very interesting piece of 944 dogma; thanks for putting in the calories to shed some light on the situation.

The only other thing that occurs to me, and is in line with your conclusion about the gear being already on the way out: Is there a kind of shock that gets transmitted to the gear through the white plastic "case"? It sort of seemed like the solenoid was a little less than gentle on actuation and release. Since it looks like everything is connected maybe that force is enough to send that gear over the edge. Like you said, a pothole might do the same thing at that point.

Cheers

Zirconocene wrote: Wed Aug 31, 2022 9:55 am Hey @Tom , excellent, excellent video. Nothing like data or a practical demonstration about how something works to help investigate things like this. This odometer reset thing is a very interesting piece of 944 dogma; thanks for putting in the calories to shed some light on the situation.

The only other thing that occurs to me, and is in line with your conclusion about the gear being already on the way out: Is there a kind of shock that gets transmitted to the gear through the white plastic "case"? It sort of seemed like the solenoid was a little less than gentle on actuation and release. Since it looks like everything is connected maybe that force is enough to send that gear over the edge. Like you said, a pothole might do the same thing at that point.

Cheers

Yeah, the solenoid is pretty violent, so something probably gets transmitted through the case. But hard to believe that -- or any part of the mechanism -- puts more strain on that gear than just the bouncing and vibrations of driving the car. That gear is the furthest away (most insulated) from the reset mechanism, yet is always the one to break. You might argue the worm gear puts more strain on that gear, and that contributes to it's failure, but the worm gear turns whenever the car is moving, so not related to the trip reset. For whatever reason, VDO used a different material on that particular gear -- whether to better self-lubricate against the worm gear, or to dampen the noise, or whatever -- and the bottom line is that particular material starts to break down and turn into a gummy crumbly mess after a few decades on most cars.

Somewhere out in the middle of Kansas there was a 100-year-old barn that was just ready to fall over. One day, a butterfly landed on it and the whole thing fell over. Do butterflies destroy barns?

Same thing happens on lots of VDO odometers from the same era (with no connection to the reset function). Here's the volvo version....

https://forums.turbobricks.com/showthread.php?t=352648

Hi @Tom - Thank you for the write up and video. On my 1986 944 NA, both the trip and main odometers work fine, but pressing the reset button has no effect. I’ve pulled the cluster and verified I have continuity between each ring connector to the two points you referenced in your original post (back of the cluster & pin 17). Do you have any suggestions for testing the Trip Odometer Reset Button Switch (944.641.321.00) or testing the wiring from the switch back to the cluster?

Welcome to Carpokes DJOTron!

Do you hear the solenoid activating when you push the button? If so, that would suggest a mechanical issue with the reset mechanism (as shown in that video).

If you don't hear anything, one common issue is when the switch button isn't aligned with the plunger in the dash vent trim. With the cluster all hooked up and the ignition on, does it work if you press the round plastic button in the switch with your finger -- i.e., where the red arrow points below? If so, then you just need to make sure the black button in the vent piece is aligned over the button (and not broken off or something) so that it activates the button when you push on the outer vent button.


If none of that helps, pull the button/switch out of its connector and try quickly shorting the connector's 2 pins (simulating a button push) with the ignition on and the cluster all hooked up -- just wear gloves or otherwise be careful about sparks as it carries quite a bit of current. If the trip meter resets when you quickly short the connector pins, the button is probably bad. You can test the switch itself by confirming the circuit is open between the switch's two pins normally, and closed (zero ohms) when the button is pushed.

If none of the above is the issue, then I'd check the resistance between those two ring connectors on the back of the cluster to see if you have about 3 ohms. If that's an open circuit, or a direct short, across those ring connectors, then the solenoid windings are probably bad (although the flexible circuit board in theory could be the issue too). The solenoid actually has a separate part number, 92864192100, and appears to be still available. If you have 3 ohms there, I'd take the cluster out and quickly apply 12 volts to the upper ring connector and ground to the lower - this should activate the solenoid. When applying the 12volts, but sure to use a source with plenty of current.

Not sure how much of that makes sense, but if you need more explanation on any of that, let me know. Chances are those tests will point you to the problem, but report back either way with your findings and I'm sure we can get to the bottom of it...

Thank you for the welcome, quick reply and for providing the solenoid part number. I should have clarified that I do not hear the solenoid engage, absolutely nothing happens when I manually simulate a reset by pressing the black reset switch button when the engine is running (so it’s not a pin alignment issue). Once I have the cluster back in the car, I’ll try shorting the switch connector pins and testing resistance between the two ring connectors with the engine running.

That said, I just tested continuity between the two ring connectors with the cluster out of the car (no power) and I have continuity. Does this indicate a faulty solenoid, or should continuity/resistance between the ring connectors only be tested with the engine running?

The solenoid has around 3 ohms, and many multimeters (like mine) will beep and proclaim continuity with such low resistance. So if you tested by checking with the multimeter set to the continuity setting, try setting it to the ohms setting and measure exactly how many ohms you see between the ring connectors. If you then see zero or very close to zero, the solenoid or circuit board may have a short. My guess/hope is you see about 3.

Since you have the cluster out now, this is prime time to apply power to the ring connectors to see if the solenoid and mechanism actually work or not. You need to ground the bottom ring connector and then just tap the top one with 12 volts momentarily like you were pushing the button. The solenoid pulls over 4 amps when activated, so the only challenge is finding a 12 volt supply that can deliver that much power. An old school battery charger is a good option if you have one (but beware most modern chargers have 'smart' features that won't work for this purpose). Worst case, you can run a couple of hook-up wires to the battery posts and use those to test the solenoid. Just be super careful not to short the wires together or sparks will fly and wires will burn. Best to put an inline fuse in one of the wires really. Or you could tap into the cigarette lighter socket to get the same power already fused... Or use a 12volt, 5amp wall wart with the connector cut off... However you source the power, this is probably the single most useful test to run right now -- it will tell you if the mechanism works or not. If it works, then it's just a matter of tracing back through the circuit to see where the power is interrupted when you push the button. If it doesn't work, or make any sound at all, then you most likely need a new solenoid.