I have a '92 F150 w/guages and am experiencing an odd problem. When the vehicle runs the tach jumps up to read about 3000 rpm and stays there pretty much unmoving until the engine rpms approach cruising speed - 55-60mph - about 2000 rpm, at which time it does begin to move a bit according to throttle pressure. It had been working normally before this and since it began there was one instance where it did return to functioning properly but only for a brief time - a few minutes.
Is this tach signal read right from the dist. or does it go to the computer first? I have not been able to locate a diagram showing the guage wiring in my Haynes manual. A look at the pinouts of the ECU does not seem to show any connection to the tach that I can see but maybe I've overlooked something. All the other guages work correctly.
Does anyone have any thoughts as to what might be causing this? I'd appreciate any suggestions or help.
More than likely a bad tachometer.
The signal comes from a splice at the Ignition suppression resistor on a white wire with a pink stripe (648). The splice is fed from a tan wire with a yellow stripe (11) from the Ignition coil, tachometer side.
The location of the Ignition suppression resistor for a 1992 F-series is on the LH side of engine compartment, taped to harness, near C101.
I will look into that. Maybe there is a voltage source nearby or bare spot in a wire.
These meter movements are electromagnetic field in nature, if I remember correctly. Maybe I can find a source of induced voltage that is causing the meter to read too high.
I hope so. The replacement tachs at LMC are between 200 and 300 bucks and I don't even see the '92 listed, only '93 and up. And a junkyard replacement for that year would be exactly that, an old junkyard replacement. Not good for electrical stuff.
Thanks for identifying the circuits for me. I'll check it out. It may even be possible to repair the tach itself.
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