It's surprise after surprise with this car lately. 93 240, LH2.4, originally auto converted to M47, 200,000+ miles.

Today the wife tells me that she noticed the speedo was reading 38mph while she was on the highway, assuredly doing more than 38. I take it for a quick drive and sure enough, up to about 37/38mph the speedo seems to work fine, but then it just stays pegged there regardless of how much faster you go. No other noticable issues with the car, it accelerates beyond the point where the speedo stops, and everything feels fine. When you deccelerate it starts reading the correct speed again as you slow past 38mmph A firm smack on the dash doesn't change anything.

Any idea what could cause this? I probably have a spare somewhere, but I'd much prefer finding and fixing the problem over throwing parts at it. Especially because this problem could be outside of the gauge cluster.

In a 740 or 940 this symptom would have an electronic cause, but in a 240's electronic speedometer the limit is mechanical. In previous examples the cause is a tiny bit of steel found its way into the gap in the magnet where the moving coil driving the indicator needle fits. In every case, the cluster or speedometer was previously disassembled on a workbench where metalworking has left filings from sawing, grinding, etc. The meter magnet is powerful, and the movement within it is delicate and precision. https://en.wikipedia.org/wiki/Galvanometer
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Art Benstein near Baltimore

"A harmonica plays better too, once you learn to keep the slobber out!" -machine man

This one was working correctly until recently. I removed it while troubleshooting another recent issue (which I mistakenly thought might be related to the ignition switch), but at no time was the cluster disassembled, nor did it leave the car. I just put it on the rear seat. Of course that doesn't mean that something from the past ~3 decades didn't get in there and come dislodged when I moved the cluster around.

I this something I can repair if I want to, or am I better off just tossing one of my spares in? I'd live to avoid having to manhandle the replacement odo to the correct mileage, though it sounds like maybe that's the easier option.

Cluster in the back seat doesn't fit the syndrome. Speedometer apart to change the odo gear on the garage workbench does, especially when someone recommends using the air nozzle to "blow out" broken gear pieces.

So, I don't know. What I would do is swap clusters to be sure. For a '93, it has to be a '92 or '93 to replace it. Then, if the speedometer is to blame, you'd want to carefully pull it from the cluster and see if you can feel an obstruction as you gently move the needle by hand past the 38 stopping point. Going after the obstruction sounds like some kind of combination of skill and luck would be required for success. Can't tell you any success stories. Nor can I offer a quick and easy solution to retaining your proud mileage record using a substitute meter.
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Art Benstein near Baltimore

"Middle age is when a narrow waist and a broad mind begin to change places." -E. Joseph Cossman

Will any K40168 speedo/cluster work here, or is there something more specific I need to look for? The car is a sedan, with ABS.

I've found a spare K40168 on my shelf, in unknown condition (likely pulled from a junkyard when I was there for something else). I'll pop it in, and assuming the same issue doesn't recur I'll be digging into the malfunctioning unit. I'm generally not very good about taking pics as I work on things, but if I find anything worth actually looking at I'll try to take pics, in case you'd like to add them to your site.

As always, your assistance is very much appreciated.

Yes, your replacement meter needs to be K40168 or K39200 (or K24964 if for kilometers). ABS is part of the story, but not this story, since a 91 ABS speedometer won't work. The 40168 is for the sedan, whereas the 39200 is for a wagon. Either will work, just substituting a sedan for a wagon meter will put the calibration off a few miles.

If you see the obstruction, it is likely to be smaller than a pinhead and held fast to the magnet. I wouldn't worry about getting a photo, just be careful about blowing unless you're very sure the surrounding area is very clean.
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Art Benstein near Baltimore

"I don't want to retire. I'm not that good at crossword puzzles." -Norman Mailer

Hi Art,

There is sure a lot of information in the Wikipedia link on meter builds. Thanks!

I read the posts about the needle sticking and agree that approximately thirty years of dust and grime can “sift in” at any time.
I read on a thread where I guy pulled his cluster to find all kinds of white powder all over the dash components. His first thought was it might be a busted bundle of dope!
But after studying it for a moment he decided the previous owner must have worked near some mining operation.

I’m going to suggest that if he pulls the back of the cluster off to “not touch the needle by hand” but to use just a “breathe” of air to push it.
I know it hard to puff that much in the right place.
I’ll suggest to use a hair dryer, blowing cold, on a low speed setting would work best.
If brought over towards the needle from a distance and then moving closer as the needle move up and around to the 38 mph, it may show the same effects, as if it was in operation.

Using the tactile touch of a finger push just might “bust” right on through the tight spot and not feel a thing!
The deposit of a particle could just move elsewhere without ever knowing if it was still there!
This test method can be used further, a few times, to test more of the swinging range if that happens.

Like the galvanometers it is a delicate instrument but not that much so because it’s in a car and not a laboratory.
Just be gentle with some diligence in diagnosing, will probably fix it.
TLC always goes a long ways as it’s a simple mechanism as it’s only one axis of movement involved.
Maybe just a drop of denatured alcohol on each pivot point will remove a crusty spot.
It would work as a rinsing agent.
Lubricants are probably not used in the factory as they attract things later.
A tiny blast of “dry silicone” and with it’s thin carrier agent, it might be the best solution too!

As far as the coil gap I’m thinking if something is in there, a gentle enough rinse might be good.
I agree that a blast of air can ruin hair wound springs and dislodge who know what!

I hope our discussion gets him fixed up with the right thinking tools for this small challenge.

Phil

I appreciate the suggestion of the hair dryer (which I don't have, but I do have a heat gun that can probably be set to no heat. In 10 years of marriage I can't once remember my wife actually using a hair dryer, but if the heat gun doesn't move enough air I can ask if she has one squirreled away somewhere). I can always step up to pushing it with my finger, in the event that blowing air doesn't accomplish anything.

I also wonder if I can secure a funnel or something to a small shop vac such that I can apply some directed suction to clean things out, rather than positive pressure. But that's predicated on actually knowing where I should be focusing that effort. Hopefully this weekend I'll have some time to dig in.