1978 308 GTS US After a very spirited drive with the local FCA chapter (pictures at Facebook "SoCal 3x8 enthusiasts" group) I noticed the speedometer was reading slow by about 20mph. When standing still it reads below zero where it used to read 10 mph. I verified it with GPS and its off about 20 mph throughout the speed range. (speedo says 40, GPS says 60) The needle action is smooth and consistent, just reads low. When stopped it jitters a little like its trying to go even further below zero mph. When I turn the ignition on the needle jumps to about 10 mph then returns to below zero. The odometer and trip meter seem to be working OK and match the GPS data. I've reviewed the threads I could find and couldn't find one with this problem. Is it possible the needle moved (doubtful) or the potentiometer has failed or needs to be cleaned? Is there an adjustment other than the potentiometer to set needle location? I don't think its the sending unit as everything works, the speedometer just reads low. I've attached pictures of the speedometer now and before. Thanks in advance for any help. I'm trying to avoid going down the proverbial rabbit hole again. Barry Image Unavailable, Please Login Image Unavailable, Please Login
I wonder the same thing. It just seems so odd. I ordered a cheap square wave generator so I can test it. Seems like it has to be something to do with the needle/needle drive mechanism since its static (power off) position changed. My plan is to test it with the SWG at the sender location. I expect it won't change anything. Then pull it out and bench test it. Again I don't expect anything to change. Then disassemble it and hope its either a simple fix to the needle mechanism or cleaning and adjusting the potentiometer.
Pretty sure the needle socket/axle connection slipped if the ODO/trip are still fine. Open the speedo (remove front bezel and rear three screws), remove the internals and have a look from the side where you can see the coil and magnets foe the needle driver. Gently hold the moving coil mechanism in place with a screwdriver or pen while rotating the needle back to the original position. Then, with the rear of a ball pen hit the needle above the axle one or two times to set it back into place.
