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Topic: '87 Coupe speedometer picking up tach pulses/noise? (Read 684 times) previous topic - next topic

'87 Coupe speedometer picking up tach pulses/noise?

87 Turbo Coupe with electronic speedometer that seems to pick up electrical noise or tach pulses.  Come to a stop and it revs up to 30 mph.  rev the engine and it follows the tach.  Kookie.  Anyone have this problem and what was the fix?  With the vehicle speed sensor also connected to the computer, it ups the idle at a stop since I believe the computer thinks the car is going like 30 MPH...  I disco's the VSS line to the computer to stop that (which disables de-acceleration controls, etc.) but it is very anoying and would love to get this fixed.  Maybe a cable operated speedo but that seems like a lot of work.

'87 Coupe speedometer picking up tach pulses/noise?

Reply #1 –
I've read over at NATO that a twitchy speedometer can be a sign of an insufficient charging system. Perhaps your alternator is starting to go south.
This may be a long shot in your particular case, but I thought it may be worth mentioning.
[SIGPIC][/SIGPIC]

'87 Coupe speedometer picking up tach pulses/noise?

Reply #2 –
Appreciate the info.  This thing is a 4cy turbo coupe converted to a 5.0 H.O. that is now twin turbo'd.  I put in a mustang wiring harness with an A9P computer.  The alternator wires had to be extended since on a 5.0 it is on the passenger side.  Maybe I have something hosed up there but the charging system in general is working fine and the alternator is relatively new as well...

'87 Coupe speedometer picking up tach pulses/noise?

Reply #3 –
I had one that would read mph just idling... Turned out to be a poor connection at the quick disconnect plug on the back of the cluster...

Use a pencil eraser to clean the contacts on the plug and check the screws that hold the speedo, they are also the electrical...