Difference in tach signal

[DerekTheGreat]

Is your truck a TBI?

I had a bouncy tach for awhile in my '89 with the unicorn '91 cluster swap. Thought it was the stepper motor until research told me to look at either the ignition control module, the plugs or wires. Turns out I had a cracked plug and the boot for that plug was burned out. Replaced all the plugs & wires, no more bounce. I had swapped another cluster in to rule out the stepper motor and it bounced too. It still bounces a bit at idle when first started, but it has always done that. Aftermarket dizzy & ICM so I figure that's why..

 

[VenomousSVT]

im running a 99 burb dash and cluster in my 91 and even with the resistor mod to "excite" the tach it is so inaccurately low its sad. It reads like 1500rpm at 5k and 800 at idle. it sucks.

 

[laidbackbigun]

My factory cluster works fine but any newer than a 95 and i get this crazy tach.