Engine/Transmission (1994 - 1998) Erratic tach - pcm problem??

[DSThompson]

I started having an erratic tachometer, which immediately stopped the ac from working. In addition the cruise control and overdrive worked only sporadically after the tach became erratic. I researched here on the web site and started with checking the contacts for the engine speed sensor on the front of the engine. They were fine, so I put on a new one with no change. Then I grounded the pcm to the fender where the battery ground is located. No change! I have been driving it during this time with no other problems. It always starts and runs perfectly with a good tachometer reading when the engine is cold. After a period of time driving, the tach gets erratic and eventually the tach goes to zero. When it hits zero, neither the tach, ac nor overdrive work. I have noticed that the ammeter has wide swings from below 14 volts to almost 18 volts. It is during this time that the tach starts getting erratic. I assume there is some heat component to this problem.



I realize that the pcm has the voltage regulator in it, so is that the problem?



I have been slow to take it to a dealer because of the discussion I have seen from other members of this site. Is there someone competent in dealing with these problems in the Seattle area?



Truck Profile:

'95 3500 SLT 4X4 AUTO 3. 54 REG DIF 8 FT BED TST#6 PLATE POWER VISIONS WESTACH GAUGES

 

[illflem]

http://www.thepowershop.com in Enumclaw does good work.



Have you tried the turning the key off three times trick, you should get a trouble code. Click this http://dodgeram.org/tech/dsl/troubleshooting/94_DTC_codes.htm

 

[Bob4x4]

If the engine speed sensor is not working,either due to poor connection or sensor failure,the pcm will not turn on many functions. Check there first.



Bob

 

[K5IP]

intermittant tach

The same conditions occurred on my 95 and it turned out to be the sensor connector which it located just above the hall effect sensor. The connector is not restrained very well and became intermittant, causing the problems you describe. Check it out.

K5IP

 

[DSThompson]

I checked for codes, but there were none. I had disconnected the batteries when I grounded the pcm, so they were probably erased?



I can't find the hall effect sensor in my maintenance manual..... does it go by any other name?



Thanks for the ideas.

 

[illflem]

Crank position sensor