intermittent tachometer problem please help !!!!

[97psddrw]

my tach on my 97 obs only works half the time seems like theres a loose connection or maybe a bad ground , im pretty sure when its not working its throwing off my shift points

i heard it could be cam sensor or pcm connection , but i dont have a clue as to where these are to check the conectors , is there any common causes of this etc.. anyone else have this issue if so what was the problem

ps im new the electronically controlled diesels so please help with location of anything applicable

[Patrick Feeley]

First suggestion: If you plan to keep or work on the truck, get an Electrical and Vacuum Troubleshooting Manual (EVTM) from Helm Inc (about $22), best money you'll ever spend.

The tachometer gets it's signal directly from the Powertrain Control Module (PCM). The PCM is low on the driver's side firewall inside the engine compartment (NOT the one below the driver's side hood hinge ... that's the Injector Driver Module or IDM). It will be a White wire with a Pink trace (circuit 648) from PCM connector (C1027) pin 50 (for a 49-state truck) or pin 19 (for a California truck). It will go from the PCM connector to the big firewall feedthrough connector (C202) pin 25. From C202, it will go to the instrument cluster connector (C250) pin 9. The ground is from C250 pin 3 (circuit 57, Black wire ... there is another ground on pin 7 and they are spliced together). Cheers!

[97psddrw]

Originally Posted by Patrick Feeley

First suggestion: If you plan to keep or work on the truck, get an Electrical and Vacuum Troubleshooting Manual (EVTM) from Helm Inc (about $22), best money you'll ever spend.

The tachometer gets it's signal directly from the Powertrain Control Module (PCM). The PCM is low on the driver's side firewall inside the engine compartment (NOT the one below the driver's side hood hinge ... that's the Injector Driver Module or IDM). It will be a White wire with a Pink trace (circuit 648) from PCM connector (C1027) pin 50 (for a 49-state truck) or pin 19 (for a California truck). It will go from the PCM connector to the big firewall feedthrough connector (C202) pin 25. From C202, it will go to the instrument cluster connector (C250) pin 9. The ground is from C250 pin 3 (circuit 57, Black wire ... there is another ground on pin 7 and they are spliced together). Cheers!

thanks i appricate the info i reset the connector and checked the ground , still same thing , so ill start tracing the wire tommorow