Engine/Transmission (1994 - 1998) Crazy Tach

[JustScott]

Does any one know why my tach is flipping around? It seems to do it worse when the engine is cool. (98 12v auto) When the Tach drops to zero my check engine light comes on, and so does the wait to start light. When that happens the transmission shifts are kind of erratic when you let off the accelerator.



Also when this started I began to smell coolant in the cab of the truck. It seems to be coming through the vents. There seems to be a coolant leak somewhere around the firewall on the passenger side of the truck, but I'm having trouble locating it.



Are these problems related or was it just a coincidence that they started at the same time.

 

[WadePatton]

CO-inkydink as Kelly Bundy would say.



Sounds like the heater core and CPS crapped out near the same time.



Not sure where the 12v gets its tach signal.



Heater core is a major pain. The one on my 95 was replaced years ago.

 

[Howard Durand]

Tach signal comes from the engine speed sensor mounted just above the vibration balancer. If it is not working properly, you will have loss of tach, erratic shifting, loss of cruise control and other problems as well.

Follow the wires from the sensor up to the top of the engine where they connect into the harness. Make sure the connection is good and clean.



As to the coolant leak, the PCM is on the passenger side. If coolant is getting into the connector somehow it may be causing electrical problems as well.



Coincidences do happen but usually they are related in some way.