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#4115952 - 07/22/21 11:17 AM 1993 P71 erratic jumping speedometer
93p71 Offline

Registered: 05/23/20
Posts: 2
Loc: New England
1993 P71.

The analog 140mph speedometer needle periodically jumps around erratically. It does it sometimes while driving and it also does it while the key/ignition is turned to accessory mode with the engine off. There are also times when it works completely normal and reads accurate speed as should. Transmission and car seems to operate normal when it does it.

Wondering what the cause is and how to fix?
Speed sensor on transmission?
Bad speedometer gauge cluster?
Is is bad for the car/transmission to drive it like this?

#4115980 - 07/22/21 05:33 PM Re: 1993 P71 erratic jumping speedometer [Re: 93p71]
Bellwestern80 Online   content

CVN Discord: Edsel
Over the Hill

Registered: 07/01/11
Posts: 4064
Loc: Evans, Georgia
I’d check two things right off.

The first would be the body ground off the battery. There should be a heavy connection that goes down to the engine or frame, and a smaller connection that goes to the fender. If the small one isn’t hooked up, some electrical components like the instrument cluster may behave funny.

Second would be to inspect the VSS and wiring harness coming off the transmission. Ensure the wiring harness is not chafed and rubbing on any surfaces.

Since the speedometer is 100% electric, there should not be any risk to operating the car with it doing that, however it would be reasonable to determine why it’s doing that.

My Cars:
-1964 Mercury Comet 202 (116K Miles | 170 I6) - Restoration Project
-1986 Dodge D-150 Royale SE (112K Miles | 360 V8) - Waiting on a TorqueFlite
-1987 Mercury Colony Park LS (330K Miles | H.O. 302) - March 2017, November 2019, & September 2021 POTM Winner
-1997 Mercury Grand Marquis LS (230K Miles | nPI 4.6) - The Daily Workhorse & April 2013 POTM Winner


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