All types of problems

[Bagged90]

Not sure where to start. First off the fuel gauge doesn't work. I knew of that issue when I bought it. When you turn on a turn signal all the gauges bounces with the flasher. If you turn on the headlights the oil pressure gauges pegs at high and voltage drops to almost nothing. Also I noticed tonight the headlights are super dim. Any idea or suggestions on where to start?

 

[martin]

Start by checking that the battery and alternator are good and go from there.

 

[RonD]

What engine is in that '90 Ranger?


Yes, get a volt meter and check the battery voltage first.
12.5v-12.8v means battery is OK, 12.4v and lower means it is at the end of it's life, look for a sale price

Remove and clean terminals, wire brush works good.
Follow each cable connected to the battery and check it's other end, remove and clean.
The black ground cables are VERY important, clean and tight.

Hook it all back up and start engine
With engine at idle battery voltage should be 13.6v-13.9v(1.1v above engine off battery voltage), it can go as high as 14.8v just after starting but should come down into the 13.6v-13.9v range in a few minutes.
If it's under 13.5v raise RPMs and make sure it's going above 13.6v.

With engine idling turn on all the lights and the heater fan on High, radio on, cigarette lighter in, then check voltage, should still be 13.6v or higher, if it's below 13.3v then alternator has at least 1 bad "field" maybe 2.
Head lights dimming at idle usually means at least a 1 volt difference from higher RPM to idle RPM.
In simple terms an alternator has 3 "fields" that generate voltage, they are independent units inside the alternator, if you have a 90amp alternator then each field would produce 30amps.
As fields fail then the 90 amps is down to 60amps, then down to 30amps, and that's at high RPM, at idle it might be 15amps, so voltage drops down, and lights dim.


A common issue when people work on an engine is to disconnect the ground strap going from the engine head to the firewall, and then not hooking it back up or retighting it, that's the main ground for the instrument panel, if this is not hooked up the gauges will do all sorts of odd things.

With the exception of the fuel gauge it reads like you have a ground that is loose or disconnected.

Fuel gauge could be fixed by finding that loose ground, but often it is a problem in the gauge itself(anti-slosh circuit) or a problem with the sending unit in the fuel tank.
I would find and fix the other issue first then tackle fuel gauge.