Tach problem

[F150Gearhead]

Recently my tach started behaving oddly. When started it immediately jumps up to about 2,000 rpm and stays there until the engine rpm's approach that speed. At which time it does start to move along with the throttle. It varies like it should but reads about 1000 rpm higher than the actual engine rpm. At cruise speed where it should be reading about 2k it reads 3k and if you slow down it will drop back to 2k but go no further. I know that the drivetrain is functioning properly. I have had this truck for a quite a while and know how it sounds and feels at various speeds.

These tachs are pretty expensive and I was wondering if it might be possible to repair it. I know that these meter movements are electromagnetic in design and are probably not that complicated. Does anyone know of a schematic that might be useful in a repair attempt or perhaps be able to point to a component that may have failed?

I looked a bit for a possible source of induced voltage as the wiring passes very close to the coil but didn't see anything that looked abnormal.

Any thoughts, anyone?

Gearhead

 

[debe]

May not be that easy to repair. Had a simmilar problem with a Nissan 4WD tacho & it was a faulty Chip that drove the tacho, Not avalable as they were custom made chips. Was cheaper to get a second hand one from the wreckers.

 

[F150Gearhead]

Yeah, I was just googling around for info, and there is some data out there about the various types of meter movements used in cars. Some troubleshooting tips, too. I'll start there with the external stuff before I tear my dash apart again. There are actually some circuit diagrams out there for electronic tachs, which this probably is, being a '92. Maybe I'll get lucky. It's already scewing up so I guess I can't hurt it much more trying to fix it. As you say, It may not be repairable. New replacements are from 200-300 bucks, but they are out there. LMC shows one for the '93 that will work, I'm told.

Thanks,

Gearhead

 

[alec_t]

If you can't get hold of a published schematic can you reverse-engineer it and sketch out the circuit (including any part numbers/markings)? That might help us to decide if it's repairable.

 

[debe]

Pictures of your instument may be of some help. Heres some pics of a Ford sedan 1995 made by VDO. There is extensive use of Chips & they drive Cross field instruments, the one dismantled is the Tacho.

 

[F150Gearhead]

Thanks guys,

I can't get to it this weekend, but Monday looks ok. I gotta spend some time with my wife. We don't see each other much during the week nowadays. Your pics are excellent.

G