Recently I’ve dealt with several speedometer heads – some I’d fixed before; some new to me – that worked fine but either

i) quit and won’t restart with a dash-tap; or
ii) suddenly started quitting – usually on bumps; but restart with a dash tap.

The puzzling part is restarting with a tap on the dash above the cluster.

Recap of the problems I've seen and some solutions (1986-93 240s only):

1. I’m excluding speedo/odometers that didn’t work when I got them eg. someone buys a 240 with a non-working speedo and/or odo.. This can be a wire failure at the axle; bad circuit board; looseness at the L-shaped clip; bad gear; cracked solder joints – all discussed in threads here.

2. When the speedo and odo quit together and restart by themselves – tap on dash doesn’t help – this is often frayed wires at the rear axle and/or a bad plug.

3. If the odo quits and restarts – sometimes on bumps or with a dash tap – I’ve found too much solder on reflowed joints – the gear hits it and sticks. You can test for this by assembling the unit and turning the odo-gear motor with a needle. Sometimes a piece of debris has worked it’s way into the odo-gear mechanism.

4. If the odo and speedo quit and restart on bumps or a dash tap, this can be a poor connection where the L-shaped wire clip attaches to the back of the speedo board: a) The board contacts may be worn/ dirty/ corroded; b) the prongs in the clip may be loose and sometimes can be re-bent to offer more pressure on the board strips - or the clip can be replaced.

I’ve had to replace boards and clips.

All this done – board contacts cleaned, L-clip prongs re-tensioned, joints re-flowed - I still have 2 speedo-odo units that cut out on bumps and restart with dash taps.

I drove one 1990 this week. Speedo cut out/on rough back roads; then worked as it should for ~50 miles; then cut out when I hit bump pulling into a parking lot.

I believe this has to be a problem where the L-clip connects to the board – a loose prong, bad contact surface.


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240 drivers / parts cars - JH, Ohio

"All this done – board contacts cleaned, L-clip prongs re-tensioned, joints re-flowed - I still have 2 speedo-odo units that cut out on bumps and restart with dash taps."

Trial and error can be frustrating. Take it out, look at it, tighten stuff, reflow stuff, clean stuff, put it back in, drive it, didn't fix it.

Worse yet, jacking a wheel up to test it.

Could be easier to troubleshoot if you could run it with the guts in your hands, like this: Notes on Speedometer Diagnosis

If it is only the odometer, my canned response to those who replace the gear and have trouble is the result of many, many help threads here on this forum and elsewhere:

1. Find the broken teeth and get them out of the ring gear.
2. Be sure the rotor's thrust bearing is not jammed.
3. Reflow the three motor pin terminal solder connections.
4. Check to see you didn't plug the tach wire on the cruise control tabs.
5. Read this thread 92 240 Speedo good, odometer inoperative

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Art Benstein near Baltimore

"Never trust the work of the last guy, even if you're the last guy"

Back to this car, I

1. re-flowed the 3 solder joints
2. triple-checked for gear pieces (even though the odo and speedo were cutting out at the same time).
3. squeezed the wires in the L-clip.

The odometer didn't work at 1st; ~1/2 mile down a rough road it started coming on and quitting; after 2-3 miles it came on and stayed on. I drove ~30 miles w/ no prob.

The owner has ~200 miles on it and says it hasn't quit once.

(n a different speedometer head, I noticed that the aftermarket big gear was tight on it's pin. I enlarged the hole with a drill to make it a hair larger. It spins w/ less resistance on the bench.
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240 drivers / parts cars - JH, Ohio

You're right. None of my gear tips apply because the speedometer cuts out. That's not the usual form of trouble folks have after replacing the gear.

There are more places for trouble to occur than just the L-shaped plug. Solder cracks can happen on the light components too, and in the case where neither odo nor speedo work, those are all that remain. And of course, the trouble could be external to the gauge. The same type (although larger versions) of connectors used in that temp faker board are used to pass and fuse power to the gauge. It is obviously a loose connection. Reference to "bad" electronics that acts up with mechanical influence is a synonym for "loose connection" always.

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Art Benstein near Baltimore

"Keep in mind that random processes can fluctuate in such a way as to mimic a signal when no signal exists." --Ransom Stephens

- The car went ~700 miles with no probs - the speedo worked fine.

- Today he drove ~2 miles on a rough road and parked.

- When he restarted, no speedo or odo.

- He drove a few miles tapping the dash. The speedo needle would jump and the odo 10ths would advance with each tap. He stopped the tapping.

- A couple of miles later the speedo and odo started working; then quit; then worked but the needle would bounce between 50 and 30 MPH; then it worked fine.

I'm going to check the wiring on the differential again although I don't see how it could be that if tapping makes it work sometimes.
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240 drivers / parts cars - JH, Ohio

Thanks Art, I hadn't see either of those threads.

Your "Notes on Speedometer Diagnosis" is nice. I'll look into getting a 9V power adapter. I have a 12V unit now that I use to check bulbs on the bench.

I do sometimes put the rear wheels up and test speed-/ odometers plugged into the car harness but sans cluster.

The start-stop aspect on bumps / hitting the dash remains a mystery. I've seen so many cars start working after a few miles drive and work fine from then on

Dave Barton (as I'm sure you've seen) discusses this http://www.davebarton.com/index.html

'While I have not directly experienced this myself, I was told by a speedo repair professional that on a number of occasions they had experienced a "dead" odometer after replacing a broken gear in one of these VDO electronic speedos. The speedo function worked, but the odometer just wouldn't turn. On each of these occasions, the odometer would mysteriously start working after a period of driving . . . sometimes after an hour. Sometimes after a week. So far, no one has been able to say precisely why this happens, but it is generally thought that there must be some sort of circuit breaker protection designed into the speedometer circuit board that finally resets after a period of time. After I posted this information here, I began receiving identical reports from a number of customers who have installed new gears in VDO electronic speedos. So if this happens to you, you might give it some time and see if it comes back to life. If you have a similar experience, feel free to write me about it.

"Sure enough it started working after about an hour or so of driving. Specifically, I drove for about an hour without any action from the odometer, then shut it down for about 5-6 hours, then started another drive and it was working immediately upon starting out." C.S., Raynham, MA

Thanks go to Martin Uden of Australia for submitting the below info:
"Read your page regarding odo repairs and I have good experience in repairing most electronic VDO units of this type that use the 25t gear. They are found on Nissan Ford and Holden (GM) cars here in Australia and I also get the situation occasionally where the odo won’t work after changing the gear. What I do to fix it (99% of the time this works) is to reassemble the instruments into the car without putting on the plastic front so that I can get to the odo digits. I have a 3/16th inch thick length of plastic (so as not to harm the dials) that I have sharpened as you would a pencil and whilst the vehicle is moving, I physically “attack” the odo 1st number on its gear cog (odo, not the trip), gently pushing it back and forth until it begins working. Then of course you can remove the instrument housing and put it back together properly. Usual disclaimer of course regarding fiddling whilst you drive is not legal!! Hoping that may help the more impatient Volvo owner."

And one customer found his odometer suddenly begain working again 15,000 miles after his gear replacement!

Latest mystery fix email:
"Speedo now works after advice from others on making sure the 4-prong connector on the upper right was seated properly with the pins going into the 4 little boxes. I then followed your tip and physically pushed the odometer numbers with a nylon bolt sharpened at the end while running it and after about 6 tries of wiggling the digits back and forth the odo and trip sprang to life! You should really add the info on the 4 prong connector. Thanks."

And sure enough, there are occasionally some odometers that don't seem to return to life as one would hope. Some of these might be a result of bad electronics, such as a bad motor or other component on a circuit board. I know of a few that were found to be caused by a loose connection of the small electrical contacts inside the plug on the back of the speedo. After tweaking the contacts to get a better grip, things began working again. And a few others were thought to be very small cracks in solder joints (there are three) for the odometer motor on the circuit board. If you suspect a cracked solder joint, you might try re-soldering those joints if you have the tools for that. Otherwise, I'm afraid at this point I seems to have run out of answers, except what I might do if unable to make things work . . . such as searching for another used speedometer.



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240 drivers / parts cars - JH, Ohio

Just a note on that power adapter. It is truly an AC power adapter, of the type that provides an AC output, not an AC adapter to provide DC as tends to be the common form of wall wart in the current era. It is simply a transformer.

Some have given me feedback on this, saying it is hard to find one. I'm afraid my junk collection is older than most. And others have success using a DC power supply that has no filtering, such as one to charge small batteries. The nine-volt voltage isn't critical, it is an AC signal above one volt needed. The input circuit of the gauge does a pretty good job of limiting higher input voltages, but that doesn't mean you want to put 30+ VAC in.

The advantage of injecting the signal back by the diff is you can troubleshoot the connector, ground, and power path provided in the car.

Better yet is using the tone generator app as described in the other link. I think this is more accessible to the 21st generation. And you can use your 12V supply to power the gauge:






PS, yes I've read Dave's remarks and some of the feedback from his customers. He has a nicely organized web presence.
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Art Benstein near Baltimore

What's the fastest liquid on earth? Milk, because it's pasteurized before you see it.

Art: I found several ac-ac adapters and tried a few. They ranged from 4 10 volts.

None worked - moved the speedo needle for me.

Of course it wouldn't work on the unit with the bad grooves where the L-shaped connector attaches.

But it didn't work on other units.

Operator error I'm sure. I'll try again as bench-testing odo-speedo heads would be a great tool to have.
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240 drivers / parts cars - JH, Ohio

Jon, I don't know what to say. Maybe if you wrote more specifics, or included some pictures/drawings...
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Art Benstein near Baltimore

Always keep your words soft and sweet, just in case you have to eat them.

I scrounged around in our basement and found 5-6 AC-AC adapters w/ output ranging from 6V-30V. I'm going to try this today.

If I understand, on the L-clip wires you use 1 AC 20V+, 1 DC 12V+, 1 DC 12V ground.

My tests last night w/ car, rear wheels off ground - 3 speedo-odo clusters, L-prong clips bent in to make a firm connection

1. The unit from the car in question does quit if you wiggle the L-clip but it can't be turned back on by more wiggling of the wires. The only consistency I found is that it would sometimes restart if I set it down on the plastic edge behind the steering wheel.

2. 2nd unit worked and would not quit for any kind of jiggling of the L-clip

3. 3rd unit same as above

I infer from this that the L-clip in the car wiring is OK but the board it attaches to on unit 1 has a problem.

Visually, there seems to be less solder where the L-clip attaches to unit 1 than on units 2 and 3 - you can see through the solder strips on unit 1.
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240 drivers / parts cars - JH, Ohio

If chosen just for voltage, I'd choose the 6V AC adapter. Plenty of voltage.

You know, I agree about your inference. As for the solder on the circuit board fingers where the connector mates, there are a lot of internet suggestions to build that up so there's more thickness for the connector. I disagree with that, knowing the connector will eventually squeeze out the solder, making the connection even worse. Anyone who did extensive electronic repair back in the 70's and 80's with card edge connectors has that same disappointing experience to report.

A good magnifier may help you find a crack in one of the solder connections. An engineer who used to post on this board years ago would suggest reflowing all the connections on the circuit board when the unit became intermittent -- sort of a shotgun technique, but you know how those often do the trick.
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Art Benstein near Baltimore

“Never underestimate the difficulty of changing false beliefs by facts.” --- Henry Rosovsky

Henry Rosovsky was a longtime Harvard professor and administrator. An expert in Japanese economic history, the elder Rosovsky was chair of the economics department from 1969 to 1972 and served as dean of the Faculty of Arts and Sciences from 1973 to 1984 and again from 1990 to 1991.

Art: I went through two odo-speedo head units carefully: reflowing joints, new gears, blowing out bits, squeezing the prongs on the L-shaped connector prongs, ....

One now works (500+ miles); the other remains intermittent - quits and takes a dash tap - sometimes many taps - to restart.

Re the unit that quits intermittently:

Under a magnifying glass you can see that the face on the board where the L-shaped connector prongs make contact is scored. More-so than 4 other units I examined for comparison.

1. the prongs were loose and moved around as the car vibrated on the road and they eventually wore through

2. Or, someone punched the prongs and they cut through as the L-shaped connector was pushed on.

I thought about trying to widen the face of the L-shaped connector prongs so that they made contact on each side of the groove. I also thought about shifting the L-shaped connector prongs either left or right.

I may try to replace that board with another unit to see if it makes a difference.
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240 drivers / parts cars - JH, Ohio

Thanks, I did misread as AC->9-volts DC. I'm not sure I have anything like that. Old phone-company stuff that I have is 24-volts.

I was sure you'd seen Mr. Barton's page and comments. I'd read it too but had missed his take on why the odo is dead at 1st and then starts working.
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240 drivers / parts cars - JH, Ohio