I let my mother in law borrow my truck today and she calls me around 5pm and tells me shes having problems with the truck. so she tells me that while she was driving on the highway all of a sudden the tach goes out and she can't get it above 30 mph it drove great earlier today. so she pulls off the side of the road and shuts it down to see what happens. when she goes to start it back up the tach goes to 3000 rpms when she clicks the key to the on position (before it cranks over) and then doesn't work at all when the truck is running. she also noticed that the "wait to start" light never came on. so she turns on the hazard lights and limps it back down to my house (hazard lights are dim she said). could this just be the batteries or another electrical problem somewhere. unfortunately I'm at work and won't be able to look at the truck until sunday afternoon. The truck is a 95 2500 with an auto (stock tranny) the only mods i have are #6 plate, 4" MBRP exhaust, AFE air intake. any suggestions

 

i had an almost similar problem. on the freeway my speedo and tach went out and my truck wouldnt go into overdrive, also the tach would jump all over every once in a while. took it the shop and it ended up being the crank position sensor, a wiring module?, and a relay.

you might have the same problem? just an input

 

oh by the way the cost me 850 bucks

 

Check the engine speed sensor, it's gap and wiring if it's ok you might have to replace the sensor, any autoparts have them. Replacement is easy.
Figures 3 and 8 here might help http://www.tstproducts.com/DowelPina.pdf

If the ESS doesn't see rpms it thinks the engine isn't running, nothing electronic will work.

Sounds like someone was throwing parts at it, likely the $30 sensor was the only problem.

 

thanks guys, i'll definetly look at that tomorrow morning when i get home from work.