Please help! I have a 1997 2500 with an automatic transmission that has tach and cruise control trouble. When driving (and very rarely at idle) the tach will "fall" erratically. For example, if I am driving at 1800 rpm the tach will fall and bounce around 1200 or even lower. The cruise control has also been cutting out on its own. Both problems started to occur at the same time. Other than these issues the truck starts and runs fine. The truck does have tilt. I have had the steering column apart to look for broken wires but can up empty handed. What might be causing my problem? Any help is sincerely appreciated.

Thanks,
Chris

 

Does the cruise cut out when you hit a bump? My brake light switch was out of adjustment and I found that hitting a decent bump would flash the brakelights/shut the cruise off. That may be the source of your cruise issue.

John

 

The cruise will cut out even on flat and smooth highway.

 

OK. I'd say to give that a look anyway. If you go out at night and look behind the truck you'll be able to see the brakelights come on. Mine would just about come on if you looked hard at the pedal . Its probably not the problem but it'd be worth a look. If you have a service manual see if there is a common power source or ground shared by the two systems, that may lead you in the right direction.

John

 

Check out the engine (crank) speed sensor. Wires might be corroded, or it may not be grounded good to the block. It should have .050 clearance to the damper checked with a non-magnetic feeler gauge.

 

dido look at it and see if its loose or out of spec. might need a new one

 

Chris, following this lead further, does your charge voltage act up too? The reason I say this, the crank sensor tells the PCM "engine is running" which then enables (or disables) the things you are mentioning including the alternator regulator ckt in the PCM.. There have been problems with excessive gap, bad wiring, bad sensor, etc that shut down the Alternator... Probably quite a few alternators that got replaced for the wrong reason...


K.

 

Is this the sensor on the front of the engine above the balancer?

 

Yes, about 3/4 inch wide.

 

Thanks for all the input everyone. I removed the crank speed sensor and cleaned it along with the dampner. No help though. I checked the alternator to see if it is charging properly. It looks to be 100%. I did notice that when the tach falls so does the volt meter on the dash. I am starting to get pretty frustrated with this whole thing. Is it true that the ECM has a lifetime waranty? I am afraid to think what a new one would cost, but I think that this MIGHT be the problem. I think I am going to look into how much a new crank speed sensor will cost as well. If anyone has any other ideas please put them forward.

Thanks,
Chris

 

If the volts drop too, then the PCM is likely being told(or is interpreting) that the engine is NOT running and disables all those systems..
Plug a voltmeter into the lighter socket or across the batteries and watch the volts and see if the voltage is TRULY dropping from normal charge voltages when the thing acts up... If it is, the PCM is not seeing an Engine Running signal from the Crank Sensor circuit.( also provides the RPM signal )
I can't remember if the dash voltmeter is independant of the PCM(simple voltmeter) or is just another slave dash gauge driven by that darned computer...
Did you check the wiring plug just inches away from that sensor? It'll be buried in that oily muck on the front of the engine close to the waterpump/gearbox area.
Beyond this, a OBD-II(?)scantool may be needed to see inside the computer's mind and see what IT is (or is not) seeing.

My bet is still on the crank sensor circuit..

K.

 

I maybe getting one step closer to the root of my trouble. I was under the hood today and was checking various electrical connections looking for signs of trouble. I did notice that when I changed the fuel filter a while back that my tach problem seemed to cure itself for a little while (I know that this sounds very unusual, but follow along). Beside the connector for the "water in fuel" sensor there is a larger 3 or 4 wire plug that goes to the injector pump. I checked this harness out closely at the plug and at the pump. Upon fire up the tach worked 100%. I went for a small drive and everything seemed ok. What ARE these wires for? They go to the rear of the injection pump to a part that looks similar to the solinoid on a GM starter. Could this be a possible source of my problems? Thanks for the help.

Chris