All true. However, we were talking earlier about the possibility of adding a sensor outside the diff, just fabbing a bracket and fabbing some sort of notched disc to serve as a trigger wheel. Someone even suggested just notching the U-joint cover on the axle.
That would be the difficult solution. Simple would be put in correct speedo 
If it wonāt work after I change bad transducer, Iām sure I could get $100 for this used newer 89+ speedo, and buy a used older 88- speedo for $100? Instrument cluster hook up is identical.
Donāt forget the possibility of a stripped speedo drive gear. Itās what Iām betting on here. Far more common, and better fits your symptoms, than a failed transducer.
What makes you think thereās anything wrong with your original speedo?
Very possible, yes.
If I might be allowed a stroll down memory lane.
Many years ago, mid-1980s, I was at a retirement party for a GM Field Service Engineer. A well-versed, always-congenial, and very humorous fellow named Frank Muraglia.
When time came for him to āsay a few wordsā he did a lot of reminiscing over the many trials and tribulations of GM engineering over the decades.
As an amusing example he mentioned the never-ending problem of stripped and āapple coringā speedometer gears. As proof he produced a slew of TSBs-- Technical Service Bulletins ā describing the issue. One was dated [something like] 1949. The next, 1953. Then 1956, 1959, 1963, 1969. Then 1975, 1980, and 1985. All containing advisories about stripped/apple-cored speedometer gears on various GM products !
Cheers
DD
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The original speedo was working (for a few weeks), itās the odometer that never worked. I guess I could bring it to a repair place and see if they can fix it for under $100.
Regarding the plastic gear, when I remove the huge nut holding the transducer, will I have access to the gear?
One more step: Undo one bolt to allow the speedometer drive housing to slide out. Note: Unless you jack the car up on one side, tranny fluid might leak out. Thereās a seal inside the housing to keep tranny fluid from leaking past the speedo gear, and itās a cheap POS; there are vastly superior quality seals available.
Thanks, Iāll take a look soon. Iāll probably jack up car from side with two jackstands on drivers side, as itās right in the middle of the car, so quite tight to crawl under there any other way. These are times I wish I had a lift!
Luckily this being a GM TH400, pretty cheap and easy to get seal and gear. For the seal, are you talking the big oring?
No. You obviously need a new O-ring. But thereās an actual lip seal that runs against the plastic surface of the speedo gear itself. The OEM is a simple rubber thing, looks kinda like a valve stem seal. However, with a bit of looking you can find a really nice one complete with the spring around the back side of the lip.
Hereās a cheap one:
Hereās a good one:
Thanks Kirby! Like owning my 72 Chevy Camaro again, ordering gm trans parts
Is that summit seal an example, or exact replacement?
Google ā1240382 GM Sealā and youāll get lots of hits
Amazing that I remember that part number over the decades
Cheers
DD
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Iād go with the upgraded one that Kirby linked to !
Cheers
DD
Thanks! And I simply remove speedo housing to access? No need to remove driveshaft?
Nah. Itās easy, other than the possibility of spilling some tranny fluid.
New development! Iām going to return new instrument cluster.
While I had old one out (original in my 88), i removed that electric pot looking thing in the speedometer that has blue plastic worm screw which turns odometer gears, squirted some silicone spray into it, and gave it a lot of turns back and forth by hand. Put old cluster back in today, speedo instantly works and now odometer works too!
So obviously the transmission transducer does not work with speedo for diff transducer. Another incompatibility Iām now noticing, when the tach moves up and down, so does the fuel gauge!!!
I can cancel transmission transducer order, but Iām going to go ahead and replace for preventative measures. Do these things tend to fail eventually? My old speedo was intermittent, although me taking it out and putting it back in may have fixed up a bad contactā¦I have removed trip computer and cruise control, so canāt verify with those unfortunately.
And thanks for all the info guys, itās been a huge help! :)
I fixed my intermittent transducerā¦ pried it open and re-soldered the joints. Works perfect since.
Aristides
So I decided to fix my current cluster best i can until an excellent condition comes up for my year. Good news, speedo/odometer work great now!
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Nice! Perhaps overkill, but it certainly should be reliable!
Greg,
Nice job. Iām glad it got your speedometer/odometer working again.
I am not a fan of the crimp connectors after having random problems with their reliability over the years. I solder all my wiring connectors because I hate fixing the same thing twice.
Paul
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Wanted to follow up - after my last post, the speedo started going intermittent again. Symptoms were it would work for a few minutes, then not for a few minutes, and then again, repeatā¦
So over the weekend, I finally got around to changing speedometer transducer in the transmission. I also pulled the body - the gear looked excellent (brown with 39 teethā¦mine is a 1988 Vin 141xxx).
Only test drove it 15 miles so far, but itās working 100% now, touch wood.
A few notes for others -
- You can tuck the wires/plug up into the foam below the body so they stay away from exhaust.
- Do NOT over-tighten that big giant nut! I over-tightened, which made the two-tang fastener come out. Let me tell you, trying to remove that big nut without those two tangs holding the body from turning is not easy. But I finally did, removed the two-tang fastener and bent it into shape better (so the two arms are bending out a bit for more force). Put it all back together, finger tightened the big nut, and then gave it just a 60 degree turn with wrench.
- Speedometer in the cluster has a little adjustment thingie, if you move it one way or the other, you can change your speedo +/- 5 mph!