2nd Gen Non-Engine/Transmission Tach bouncing and warning lights flash

[SRadke]

I'm working on a 95 model. It has a random event in which the tach will bounce up to redline and all the indicator lights will turn on (water in fuel, wait to start, oil pressure, etc). There doesn't seem to be any pattern to it, happens warm or cold, all weather and multiple times daily. Only lasts a second then back to normal. The owner replaced the alternator with no sucess. Any ideas? Thanks



-Scott

 

[Mhannink]

id be looking at the pcm

 

[Silver_Ram]

Check grounds also. Are the battery terminals corroded?

 

[DavidC]

The last two early 2nd gen problems similar to this led to wiring in the auto trans. One was a pinched wire after servicing and the other led to wire insulation failure in the trans. Don't know if your dealing with an auto or not but thought I would throw this in.



Dave

 

[SRadke]

Truck has a 5 speed so transmission wireing is out. Battery terminals look fair. They're old but not acid corroded. The truck has not had charging or starting issues, seems to work normally in that reguard.



What does the PCM do in this model and where is it located? I'm assuming the "CM" is control module but help me out with the "P"?



-Scott

 

[DavidC]

Powertrain. Passenger side firewall in engine bay. I would assume same place on 95



Dave

 

[DavidC]

Have you checked the crank speed sensor on the front of the engine and it's associated wiring?. Check where it plugs in as you could have an intermittent short pegging the tach.



Dave

 

[SRadke]

Honestly I was hopeing for and resonse like the crank sensor. That was where my logical mind was taking me. I'll start there and work toward the PCM. What inthe powertrain does the PCM control? Far as I can tell it's all mechanical with the exception of the link between it and the gauges.



-Scott

 

[DavidC]

Honestly I was hopeing for and resonse like the crank sensor. That was where my logical mind was taking me. I'll start there and work toward the PCM. What inthe powertrain does the PCM control? Far as I can tell it's all mechanical with the exception of the link between it and the gauges.



-Scott

Alternator, gauges,warning lights etc. Although it may not control some items it also monitors alot of things. With out going thru a book I could not list everything the PCM does honestly. An intermittent crank speed sensor will make the tach freek out and a momentary loss of signal will chime warning lights. Could be the sensor, could be wiring to the sensor, could be none of the above but I would be looking at the sensor wiring first as there has been issues with that part on several vehicles. And it is the cheapest place to start.



Dave