What is the single most concerning dash gauge reading?

[Dirtdiggler]

For me, it's this!


3 hours into a three day road trip and the volt meter started bouncing around. A fuel gauge also jumping a little. I've had an alternator leave me stranded before in this rig, so I jumped out, popped the cab, grabbed the multimeter, and started diagnostics.

1) lights on the alternator all good.
2) power from the alternator at ~28v and 14v
3) batteries at 27.8v
4) power at relay panel good on both

Is this a real problem or am I dealing with dash issues only? Cause if I am, I'm going to duct tape over that volt meter because it's about to give me a heart attack!

 

[Dirtdiggler]

And also, my temp gauge isn't working, but that's not as recent. Looking for advice on both!

 

[Ronmar]

Well 14 and 28 is a good sign out of the alt... what do you see at the power panel test points on the left side when you open the dash power panel?) a bad high resistance connection resists current flow(like a kink in a garden hose) so with no current flow you should see the same voltage at the battery as is seen at the alt. The fact that it is losing .3v from alt to batt indicates that what should be fully charged batteries are pulling enough(minimal) current to the batteries that it can incur a voltage drop from the current restriction in the path(ground is part of the current path)...

You will see a .5 volt drop between batteries/alt and dash. The PPD has diodes that knock off .5v to whatever voltage it is being fed...

These truck ar notorious for bad connections and grounds, like at the polarity protection device(PPD) between spare and air filter(lower spare to access) have you cleaned the 4 terminals on the PPD? Also clean the shunt terminals(below PPD) while you are in there the shunt is in the cable between the battery ground and the ground point on the front of the starter motor... These are out in the weather and are a regular issue...

The volt meter simply samples the dash power, so if you have bad connections up to the cab, your volt meter is simply reading the decreased voltage in the cab due to the loads in the cab drawing current thru the bad connections... It's still in the green

All the grounds need overhauled starter to frame, alt to frame, pax frame to cab behind grill, cab to dash(under dash) dash to power panel ground test point(under power panel)...

The temp sensor is located on a T on the rear of the thermostat housing(above turbo), front right on the engine. you should be able to unplug it there and put in a jumper on the harness plug. get back into the cab and turn on the ignition switch(only long enough to determine if the needle moves or not then turn it off). The jumper/low resistance should cause the meter to go to max temp. if it does, the gauge and wiring are ok and the sensor has failed. if the temp meter does not sweep to high, or even move, you have a wiring or gauge issue...

Good Luck!