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  #1  
Old 04-27-2020, 10:08 PM
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722.303 electronic speedometer, bad sensor?

So after I fixed my odometer in my 1981 300SD with new gears about 3 years ago, I noticed the odometer would register slowly or even stop recording mileage at speeds over 60 mph. Sometimes, it would record mileage accurately at those speeds with no problem. I tried the o-ring fix on the end of the magnet and that didn't work. I sent the cluster out to JamesDean59 earlier this month for a refresh and it bench tested fine, recording miles perfectly at 85 mph. I got it back and installed it and it's working better, but still slowing down a little bit above 60 mph though not as much. If it's bench testing well, I'm thinking the fault lies in the signal coming from the transmission to the odometer. Yet, the speedometer is accurate against GPS and cruise control works (thanks Kris!), so I'm not sure why it's just the odometer slowing down bc if I understand correctly, the speedometer and odometer use the same signal. The transmission was rebuilt 3 years ago so I'm thinking maybe something didn't get put together right. Where do I start?

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'81 MB 300SD, '82 MB 300D Turbo (sold/RIP), '04 Lincoln Town Car Ultimate

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  #2  
Old 04-27-2020, 10:55 PM
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I battled this issue in my SDL. The O-ring helped to an extent, but what solved it definitively was slathering the gears (and the O-ring on the magnet end) with some silicone grease. My only explanation is that perhaps there's enough lash in the gear train that the stepper motor doesn't always step positively and can alternate back and forth instead. The grease seems to add enough "stick" that I never had another issue with that car. I did the same in the 350SD and have never had an issue with it either. The original gears had grease on them, I figured it must have been there for a reason.
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Old 04-27-2020, 11:36 PM
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Quote:
Originally Posted by Diseasel300 View Post
I battled this issue in my SDL. The O-ring helped to an extent, but what solved it definitively was slathering the gears (and the O-ring on the magnet end) with some silicone grease. My only explanation is that perhaps there's enough lash in the gear train that the stepper motor doesn't always step positively and can alternate back and forth instead. The grease seems to add enough "stick" that I never had another issue with that car. I did the same in the 350SD and have never had an issue with it either. The original gears had grease on them, I figured it must have been there for a reason.
I had Kris put a little grease on the gears to quiet things down, but that still wouldn't explain why it bench tested fine.
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Old 04-28-2020, 12:10 PM
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More speculation here - Perhaps the signal on the bench is "cleaner" than what the car provides in practice? I'd expect dropouts on the speed sender to affect the speedometer too since they share a common input.

Another possibility is a poor ground connection on the cluster which can cause a noisy signal or potentially a week impulse to the stepper motor. I haven't torn into an early car, but on the later cars there's a metal strap across the speedometer that bridges the tachometer and speedo to the gauge cluster. Everything grounds through the gauge cluster and the speedo/tach are bonded through that strap. If it is heavily oxidized or corroded, you can have ground problems there. Worth a look if nothing else.
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Old 04-28-2020, 02:24 PM
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Originally Posted by Diseasel300 View Post
More speculation here - Perhaps the signal on the bench is "cleaner" than what the car provides in practice? I'd expect dropouts on the speed sender to affect the speedometer too since they share a common input.

Another possibility is a poor ground connection on the cluster which can cause a noisy signal or potentially a week impulse to the stepper motor. I haven't torn into an early car, but on the later cars there's a metal strap across the speedometer that bridges the tachometer and speedo to the gauge cluster. Everything grounds through the gauge cluster and the speedo/tach are bonded through that strap. If it is heavily oxidized or corroded, you can have ground problems there. Worth a look if nothing else.
I cleaned the grounds about a year ago including the big ground behind the instrument cluster and changed the fuses out to brass tipped ceramic fuses and that fixed the grounding issues I had but not the odometer.
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Old 04-28-2020, 09:25 PM
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Look at your odometer gears. There were significant changes starting in 85 but both my 84 and 85 SDs have plastic gears that break, lose teeth etc. The speedo is electric and works but I've seen several odometers that needed the gears replaced.
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  #7  
Old 04-28-2020, 09:48 PM
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Originally Posted by Junkman View Post
Look at your odometer gears. There were significant changes starting in 85 but both my 84 and 85 SDs have plastic gears that break, lose teeth etc. The speedo is electric and works but I've seen several odometers that needed the gears replaced.
The gears are brand new.
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-German Narrator in a MB Promotion Film about the then brand new W123.
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  #8  
Old 04-29-2020, 12:02 PM
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More rampant out-loud thinking here - Have the capacitors been replaced on the speedo module?
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Old 04-29-2020, 02:33 PM
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Originally Posted by Diseasel300 View Post
More rampant out-loud thinking here - Have the capacitors been replaced on the speedo module?
They were replaced by JamesDean59 as part of the instrument cluster refresh.
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  #10  
Old 05-03-2020, 08:30 PM
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Went out on the highway today to test my odometer against GPS. Here's what I found.

Odometer starts reading slowly around 60 mph. With cruise control set at 65, odometer showed about 4 miles covered when I'd actually traveled 7.

Now here's the odd thing.

Above 73 mph, the odometer is spot on. So between 60 and 73 mph, it slows down for some reason. If I'm driving below 60 mph or above 73 mph, it's accurate. What in the world?
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'81 MB 300SD, '82 MB 300D Turbo (sold/RIP), '04 Lincoln Town Car Ultimate

Sooner or later every car falls apart, ours does it later!
-German Narrator in a MB Promotion Film about the then brand new W123.
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  #11  
Old 05-03-2020, 08:50 PM
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That is exactly the same behavior I had in the SDL. The cure in my case was friction on the gear train to reduce lash. If you keep the interior quiet, you should be able to hear the stepper motor "click" every couple seconds (sort of like the noise a quartz clock makes) as you move and will happen faster the quicker you're travelling. If it is "clicking" while you're moving at the speeds you're getting misreported odometer readings at, I'll bet you dollars to donuts it's slop in the gear train causing it.
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Old 05-03-2020, 11:42 PM
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Quote:
Originally Posted by Diseasel300 View Post
That is exactly the same behavior I had in the SDL. The cure in my case was friction on the gear train to reduce lash. If you keep the interior quiet, you should be able to hear the stepper motor "click" every couple seconds (sort of like the noise a quartz clock makes) as you move and will happen faster the quicker you're travelling. If it is "clicking" while you're moving at the speeds you're getting misreported odometer readings at, I'll bet you dollars to donuts it's slop in the gear train causing it.
It's hard to keep at 300SD interior quiet at speed. The engine drowned out any noise from the cluster at above 40 or so. Is this the noise? He applied grease to the gears to quiet it down and I haven't heard it anymore. Would locktite on the shaft of the smallest gear reduce lash?
https://jamesdean59.smugmug.com/Instrument-Clusters/W126-300SD-81-JM/i-mDJMSP6/A

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-German Narrator in a MB Promotion Film about the then brand new W123.
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