I have a 2001 Ram 2500 diesel and just recently the check engine gauges light comes on at highway speeds and the voltage reading is at 18 volts. When I slow down to get off the highway the check engine light turns off and the voltage goes back to normal. It seems to happen when I get above 65 mph and goes away when I get around 50 mph. everything else is working fine. Is this my alternator or is it something else?

 

Alternator, or voltage regulator. Unfortunately, regulator is part of the PCM........ Not sure why it would be speed sensitive though..... If you rev the engine to where it would be at freeway speeds, does it do the same thing?

 

The last two times mine spiked like that it was dirty battery posts but mine wasn't speed sensitive, I was going pretty slow when it did it.

 

Today I did check the truck while idling and I raised the idle to above 2800 rpm and the check engine light came on, I then checked the battery with my Fluke and it showed 14.3 volts not 18+ like the gauge stated. The spike did not last long since it went back to idle shortly after I went out to check the voltage.

 

Today I ran my truck while parked and revved the engine to 2800 RPM and the check gauges light came on I then checked the voltage with my meter and it showed 14.42 volts. The instrument gauge continued to show 18+ volts until I let off the gas. Could this possibly be an instrument cluster malfunction or is it still possible it's the PCM?

 

Need to see what voltage the PCM thinks it is seeing.....

 

Sorry, how do I check the voltage on the PCM?

 

Plug in with a scanner that can read live data. Go for a drive.