This is kinda a follow up question to one I previously asked about the gauges for my Chrysler not working, I'm down to the speedo, which I'm still more or less stumped on, I've tried a few different things without success, and here's what I'm down to:
1. The speedo itself simply is broken - Before I took the gauge cluster apart the speedo itself was making an attempt to work (which was the ticking noise I heard coming from it), but had been thickly oiled up by someone previously to where you couldn't even spin it manually. But after cleaning the sludge out of it, it no longer makes an attempt to work, even after switching out the cluster cable with one that for sure works, I can now spin the speedo manually with my hand and it moves so the gears aren't broken, and I know that it has power going to it, yet nothing happens. My theory is when I was cleaning it I messed the speedo up in some way that prevents it from functioning. My solution to this would to take the old speedo from the old cluster that did work and swap it into the current one, hopefully solving the problem, though the old speedo doesn't have a working odometer (it's missing the gears for that).
2. The speedometer cable going to the transmission somehow became disconnected around the same time I took the gauge cluster apart - This is possible but not likely, though I don't think it would hurt to check this, of course the solution to this would be either to reconnect the cable or buy a new one.
Anything else I could be missing?
Send it to a dedicated service shop for old speedometers. They're extremely delicate machines. And must be calibrated if it came from a different car, if not outright repaired. It's almost 60 years old!
There is absolutely nothing in the speedometer unit that requires power to operate- there are no wheel speed sensors, computers, etc. The speedometer works whether or not power goes to the unit, you just won't see it at night. It reads the speed of the output shaft from mechanically- springs and magnets are attached to a metal cable in the housing. The other end of the cable is driven by a gear that meshes with the gear at the output shaft of the transmission.
Check the output shaft gear, they're usually plastic and can wear out. Take the instrument cluster out and drive slowly on a not busy road and observe the end of the cable. It should rotate freely. If it doesn't, the cable is binding up, it's broken, or the teeth are stripped on the drive gear.
If it moves steadily, the issue is in the speedometer head. You can't rebuild them yourself, they require a pro to do it -even when that car was new, the speedometer wasn't serviced by any ol' mechanic at the dealer. Chrysler approved and certified shops around the country where people took them for service.
You'll need to cough up a few hundred bucks to fix it, or just ignore it, and get a speedometer app on your phone and attach the phone to the windshield.
