This is what I have

77
F-250 Hiboy
390
NP205
Custom fuel tank in the bed

I installed a brand new painless wiriing harness last year, right after that I installed custom Speedhut Legacy gauges. Ever since I installed the wiring harness I haven't had a stable fuel gauge, my tach bounces a lot, and I think my oil and coolant gauges are a little high (oil's at about 90psi, and coolant's at about 230). BUT, my speedometer (which is electric) doesn't jump around and seems to be accurate. I've replaced the coil to see if that helps the tach but that wasn't helpful, except for having a new coil. lol.

So my question is this... would this all be cause by a bad alternator or Voltage regulator? And how do I test my Voltage Regulator? If I have to get a new alternator do I need one without a voltage regulator built in or does it not matter? I have a brand new Red top Optima battery too BTW.

Thanks in advance!
Duffy

 

The voltage regulator, in theory, could cause erratic gauge readings. Do your headlights flicker? You could easily test this by connecting a multimeter (set to volts) across the battery terminals with the engine running, and making sure the reading doesn't move around significantly.