When I turn the key on to cycle the glow plugs on my 1990 250 7.3l IDI the glow plugs cycle but when the wait to start light goes off the tachometer and battery voltage gauges go up n down synchronized for close to 45 seconds even if it is running or if I just keep the key turned in the warmup position..

 

The glow plugs draw a lot of power, and will continue to cycle for a short time even once the engine has fired.

 

Okay so it's normal for my tachometer and voltage gauge needles to go up n down while they cycle? With vehicle not running the tachometer should move?

 

I have noticed my 93 7.3 doing that after it's started. Tach and volt gauges moving pulsing at same time. I haven't paid attention to it not running, will check it out tonight

 

My truck does the exact same thing, so I assume it's normal. It'll also bounce around a little bit after the truck is started, when the GP's click on and off. Once the GP's stop cycling, the tach and voltmeter go back to normal.