Hello All,

I have a 1980 C3 with TH350. The speedometer has always bounced and when I recently replaced the gauges with Dakota Digital speedometer that uses pule module I was surprises to see the bounce is still there. Since I no longer have a speedometer cable I assumed the driven gear was bad but I pulled it out and it look perfect. Only thing left if drive gear but before I pull it apart for replacment I wanted to see if anyone else has seen this. Very light bounce at low speed with larger bounce coming in after 30MPH.

TIA

Jose

 

perhaps cable drive gears INSIDE trans tailshaft housing were faulty ... and still are ?
IIRC, the replaceable plastic gear has a centered, square 0.104" hole that cable (and now DD adapter) slips into. If that hole is wallowed out, the input to cable or module will be jerky/jumpy.

 

I run a extra Dakota digital speedo just under my highrise hood. through the windshield view. I have a hall effect sensor mounted off the tailshshaft of the trans reading the U joint as it spins past. reading anything magnetic that moves in relation to the vehicle can be dialed in with most electronic speedo's. pulling apart your auto tranny sounds like a nightmare. perhaps consider mounting a hall effect sensor somewhere else. I put this in something like 14 years ago. Still works great reading the bumps of the U joint spinning past. And super accurate as it can be dialed in.

 

that’s a great idea. Can you provide details on what you used?

 

Details? well let me try. I've put electric speedo's on a number of old Harley's that came with cable drive speedo's . but due to age replacement parts are hard to come by and the electronic conversions can be dialed in to perfection.
Dakota digital sells a universal hall effect sensor that is threaded about an inch or so up it's body to make it easy to adjust on a universal bracket. I have used this universal sensor many times with homemade brackets reading the teeth on the trans sprockets on Harley's with great success.
When moving to Australia 14 or so years ago I wanted a speedo that read out in K's that was easy to read. I put a small rectangular Dakota digital speedo on my wiper tray cover. easily read through the windshield so I don't need to look down so low to read the small numbers on my factory speedo. used a gen. Harley speedo pickup that would normally mount in the trans case and read the spinning gear teeth. I made up a bracket and mounted it to the tail shaft of my trans using nothing more than screw type hose clamps. as the U joint passes by the sensor it reads the "bumps" or pulse's. then it's just a matter of how many "bumps" per mile. easy to set up. you could count the ribs in a brake rotor, a rear half shaft U joint. Whatever moves magnetically with vehicle speed and then just calibrate the speedo.
contact Dakota digital for universal sensors and directions on how to adjust your particular speedo. make up a simple bracket to mount the sensor so it is close but just misses the moving part.
This stuff works great.

 

You would want an inductive proximity sensor. Fairly cheap, should be able to get one in your hands for <$50, possible <$20. Get one that works over a wide wide voltage range like 10-30V. They have 3 wires - power, ground and signal - and with a wide voltage range you can just connect it directly. The datasheets will also say how close it has to be to the metal passing by. Very generally speaking, the larger the diameter the bigger the air gaps have to be but the further away it will work.

Make sure the bracket will never allow contact or else with a bit of movement it will hit and then start bouncing smashing itself apart rather quickly.

Generally, these get differential mounted because with the slip yoke at the transmission the u-joint and transmission can move relative to each other which could mess up the sensing or cause the yoke to crash it.