Oil gauge goes straight to H when I start truck!!!

[Onehairysalad]

Help!

My oil gauge goes straight to H when I start the truck, it never moves. I have an oil filter that I'm waiting on so that I can do an oil change. I'm hoping it's a clogged filter. Anybody else got any tips or experience with this here. I have a 79 d50 sport

[geezer101]

Is there a sender unit or an oil pressure switch it's connected to? If it's only an oil pressure switch it'll push full voltage to the gauge...

[tortron]

Does it do it when the key is on and the engine is not running?
If so its the wire damaged and grounding out somewhere.
If it's at zero and goes to full at idle I doubt its reading correct and possibly an issue with the sender, maybe a wrong one for the gauge (there was issues on this forum getting an idiot light sensor instead of a gauge sender)

I doubt it would be an issue with the pressure valve in the oil system as the pump would in no way make that much at idle.aybe if the system is badly blocked

[SubGothius]

To confirm and clarify, the gauge pegging high suggests a short to ground somewhere on the wire between the gauge and the sender, if not in the sender itself. The sender works as a ground path to the block, with oil pressure applying a variable resistance between the gauge wire and block/ground. The idiot light switch (technically not a "sender", as it's either on or off, not sending a variable voltage signal) also works by closing a path to the block/ground; oil pressure holds this switch open, keeping the light off.

If the sender is faulty, it could be shorting straight to ground, pegging the gauge. The wire to the sender may also be disconnected and hanging loose somewhere it happens to be touching a ground, or the wire itself may have melted/abraded insulation somewhere it's touching a ground along its length. If you've swapped the wires to the gauge sender and light switch, or replaced a bad sender with a new switch, that would typically result in a pegged gauge when the engine's not running (w/ key on) and a dead gauge when it is.

Some setups may trigger the light when the gauge is pegged and/or peg the gauge when the light goes on, but I'm not sure offhand if this arrangement may apply to our trucks or not.

[Onehairysalad]

Yup Sender unit. I have a whole other level of headache, after replacing my spark plugs twice and spraying wire connectors and inside of distro with we 40, truck will now idle violently & then die. I'm still waiting on the sender replacement, and I also did an oil change today. Things are going pretty rough right now.