Engine/Transmission (1994 - 1998) Erratic tach operation

[MT12V]

My 96 Cummins 2500 (~209,000 miles)has developed a "mild Illness (hopefully)". The tach has started showing erratic rpm's and at night the hi-beams seem to sync with the rpm drops. This seems most prevalent when minimal accelerator used) i. e. downhill. The engine and speed don't seem affected. Could this be a generator problem, or wiring or rpm sensor/connect or tach instrument failing. The batteries are less than a year old (Mopar). Any ideas? The dlr svc mgr thought it might be a bad battery. ??? Longer term question, could continued operation till I have the $$$ to solve this hurt Ole Blue?

 

[ewcmr2]

The crank position sensor feeds the rpm signal to the tach and tells the alternator to turn on, so it could cause both your tach and high beam problems.

 

[MT12V]

Thx ewcmr2,, I'll take it in to the local dlr @ have it checked.

 

[GAmes]

Easy enough to check it out yourself (and a lot cheaper). The sensor is located on the upper right (passenger side) quadrant of the engine damper. It has two wires coming out of it. One is hot and the other goes to ground. I'm pretty sure that if you clean the grounding terminal your problems will go away. BTW, the gap between the damper and the sensor should be . 050 inch.

 

[Mhannink]

yeah check the gap and the wiring ill bet your problem lies there. more specific i think its in the gapping, i think its probably too close. does it jump up or drop out?

 

[MT12V]

Easy enough to check it out yourself (and a lot cheaper). The sensor is located on the upper right (passenger side) quadrant of the engine damper. It has two wires coming out of it. One is hot and the other goes to ground. I'm pretty sure that if you clean the grounding terminal your problems will go away. BTW, the gap between the damper and the sensor should be . 050 inch.

Hello,

I'll check the ground wire connect as soon as I can locate it. Perhaps U can point me to it or I can scan the shop manual. Re the gap, I'll see if I have a . 05 gap feeler. MT12V

 

[MT12V]

yeah check the gap and the wiring ill bet your problem lies there. more specific i think its in the gapping, i think its probably too close. does it jump up or drop out?

Hello, I'll check the ground connect,, not sure what U mean by "jump up or drop out"? MT12V

 

[Missouri Mule]

There are actually three wires going to the engine speed sensor, hot wire, ground wire and signal wire.

Clean connector for engine speed sensor and check for loose or bent pins.

Sounds like a gound but the engine speed sensor gets it ground from the PCM.

Looking at the FSM the PCM and headlights have different grounds.





You can set the gap of engine speed sensor using 4 to 5 business cards.

 

[GAmes]

Yup, three wires. Memory failure there. Right next to the #1 valve cover is a bolt with grounding wires. The housing for the sensor might be a ground also. I'd clean up both areas first.

 

[MT12V]

12/22 update. I had the crank sensor replaced. Problem still appears when a) low rpm's (drive to down is mostly downgrade) and rig warming up. Else problem is less frequent and no effect on hi-beams now. On freeway drive, everything is A-OK. Could this intermittant problem cause any others - long term, like ECM? Motor runs fine, thoughout.

 

[GAmes]

Saying you had the sensor replaced indicates to me that you didn't do it yourself. Check for the proper gap (. 050 in) and use a gauge that can not be lifted by a magnet. The engine won't care if the pcm, alternator or tach doesn't work. Nice thing about the 12 valve, it is all mechanical. You don't have an ECM.

 

[MT12V]

Thx for the response,, yes, had the local dealer replace the sensor ( I don't have a heated garage & weather has been around zero here for a while... ), they claimed they gapped it right...

 

[GAmes]

It takes 5 minutes to check their work.