The other day my tach stopped working. I looked up how to pull the cluster and in a short while it was out. The tach gets the rpm signal from a circuit board attached to the speedometer. On the back of the tach is three mounting screws. These screws also serve as electrical junctions. I was able to test my tach using a 5 volt power supply and a sine wave function generator. An oscilloscope was used to verify sine wave amplitude.

The three screws function as follows

Looking from the rear:

Top is ground
Left is 5v +
right receives a sine wave @ 1v peak to peak in the 100- 400 Hz range

By adjusting the frequency of the function generator, the tach would respond. Linear response was noted from 100 Hz to ~400 Hz (can't remember exactly how high I turned it up).
Assuming the cluster is getting rpm information from the ecu, the problem stems from the circuit board attached to speedometer. I did notice some blown transistors on this circuit board, so there you go.

 

Nice post! The other day I was reading a thread for a grounding system and how important it is that all systems receive a proper ground. A power line is only as good as its ground. Running through the fsm the schematic shows that the ground for the cluster instruments is the one that bolts to the UIM near the firewall. I personally feel the block itself is an improper ground as the sissy lil ground wires coming off it from the factory are just sad. I believe a supplementary ground off that top tach screw can be helpful to the longevity and performance of our clusters. That or a real ground system done properly. Excellent job with demystifying the tach as so many people tend to have issues with the cluster tach/speedo system

 

I just did the ground behind the cluster yesterday, and figured to redo/add grounds so im going to go on ebay (which sell for 1.30-1.40 per foot) to order some 4g wire. i figure about 5-6 feet should be good enough..

i saw a ground on the bottom of the block on the driver side, which looks worn out which really made me want to redo them.

 

I've read several guides about these issues. I'm planning on running some new grounds in the engine bay as well as from the cluster. Hopefully it will help. Please post any pics you have of these grounds especially new grounds from behind the cluster thanks!

 

Update: The chip in the tach receives 12v from the speedo board. At 12v the tach chip is more responsive to inputs.
Transistor 8 "tr8 i think" on the circuit board had failed. Transistor 8 is actually a simple IC made by Rohm. Lookup the datasheet for DTC144es to build an equivalent circuit. Happy to report my tach works now