Engine/Transmission (1994 - 1998) Continued erratic tachometer and ammeter

[DSThompson]

I continue to have and erratic tach and ammeter readings after the truck has been driven long enough to get to normal operating temperatue. Whenever it is started cold, all systems work perfectly. when it is running perfectly, the ammeter always shows just over 14 volts. After driving a while, the tach becomes erratic and eventually reads zero and at the same time the ammeter shows mostly high voltages (as high as 18) and sometimes the voltage drops below 14. When it drops below 14 the tach is usually zero. The ac quits immediately when the tach and ammeter are erratic and the cruise control and overdrive operate intermittently.



I searched for solutions to the problem on the 12 valve forum and so far

I have replaced the engine speed sensor (also called the crankshaft position sensor in gas engines) with a new one from Cummins (they also confirm the 12 valve doesnt have a crankshaft position sensor) and adjusted the clearance on the harmonic balancer to the factory specs with a non-magnetic feeler gauge.

In addition I checked the connector just above the engine speed sensor and tightened the connectors there.



those items didn’t change anything, so I grounded the PCM to the battery ground on the left inner fenderwell. That didn’t change anything either.



I found that some folks have added an external voltage regulator to control the high voltages. Apparently the PCM also functions as the voltage regulator. Is the PCM adversely affected causing these problems and will adding an external voltage regulator control the high voltages? What about when the voltages go below 14?



The moderator suggested checking for diagnostic codes and the result was:



34-open or short in the speed control solenoid circuits or speed control switch voltage too high or too low.



62-SPI miles not being updated as vehicle travels.



53-Internal PCM circuity or communication failure.



Fortunately, the truck runs well even when there is no ac, cruise or OD. Is driving it like this potentially causing problems?



Any suggestions on where to go from here would be greatly appreciated.

 

[kaffine krazed]

Try cleaning all the connectors on the PCM battery and alternator.

 

[DSThompson]

Ok, I will try it and hope for the best... . thanks

 

[GCrowl]

I am having the exact same problems... tach failure, max voltage, or low voltage, and no 4th gear or lockup when the problem is occuring. I am relatively sure that this 265,000 mile previously faithful truck need a new PCM.



The problem absolutely does not occur in the morning when it is cold. However, if I drive it for over 1/2 hour, or let it sit out in the Palm Springs sun all day, the problem pops right up. To help isolate the problem, I placed the PCM in the refrigerator for 1/2 hour and then plugged it in and tested the function. Result: fully functional. Next, I used my heat gun to warm up the area around the PCM while my wife watched the dashboard. Result: After 2 minutes of heating the system shut down again. I repeated the experiment again to rule out randow error. Same result. So... does anyone know of a cheaper way to replace the PCM, other than the dealer? And, Is there any other systems that I should check that could have contributed to the PCM failure. I would hate to install the $600. 00 unit, only to have it fail again due to some other faulty component. ??????

 

[Howard Durand]

GCrowl,



It appears you have pretty well isolated the PCM as the failing component.

The PCM modules used in these trucks have to be pretty robust electronic devices in order to withstand the extreme temperature variations they are often subjected to but they can fail.

I doubt if anything else in the vehicle has contributed to it's failure except possibly excessive heat.

You should be able to replace it and not have to worrry about premature failure of the new one.

I have never had one out so don't know whether or not a heat sink is involved where it is mounted to the firewall.



DSThompson,



It appears you may have the same problem, a failing PCM.



The danger of continuing to drive the truck with it doing as you describe would be overcharging the batteries with the 18 volts you mention.



It is a long shot but one thing you might check is the common ground for all of the engine related sensors. They all come together and are grounded by a bolt at the front of the engine on the driver's side next to the #1 cylinder valve cover.

 

[Missouri Mule]

The engine speed sensor and connector were a good choice to check out since it is involved with most of your problems accept cruise control (a bad or broken ground wire to the engine speed senosr has been known to knock out ac).



PCM would seem to be the problem most electrical items seem to mess up when they warm up as they are on their way out.



But check for loose grounds and broken wires(engine speed senors and related senors to your problems).



You mite also check out if the sensors to related components are receiving power (usually 4 volts).



But check out the first code that came out before doing any thing else. Cruise servo is under the left battery make sure the wires/connections are OK (Battery temp sensor can be checked out at same time).

Usually if you take care of the first code the other problems will be fixed too.

 

[dennyhare]

Check out the last posting on the above thread "96 charging system problems" ( 2nd part) This is how I solved the same problem you have after fighting it for 2 years. Good Luck.

 

[Rescue Ram]

GCrowl and DSThompson,

Please keep us posted on your suituation. I was suffering erratic charging, heat grid operation and flickering lights. All occured under the same conditions you mentioned. I just replaced the PCM on my '96 5sp, and the problem seems to be remidied. Look at the thread titled "'96 charging system problems", some of the suggestions from other members may be helpful to you. Good luck resolving your troubles.



Brian ZOo.

 

[DSThompson]

After trying many remedies, I also replaced the PCM and it resolved all of my problems. I only experienced problems when the truck got to running temperature, so it was temperature sensitive. It would seem that inside the cab would be a much better place for the PCM than the engine compartment. My truck was originally from Phoenix, so hopefully up here in the northwest it will last longer.