Oil guage spike

[Toastyyosty]

So I've had my challenger for a few months and I'm starting to go threw the dash gauges and try and fix the ones that aren't working. Some ill pass on due to price at the moment but the one that I'm looking at right now is the oil guage. Following the schematic that I've got it's pretty simple. But when I start the car I get a spike in it. (gos all the way to the top) normally I'd think that's a short to ground problem but it shows on the schematic that it grounds itself to the motor? So unless it's a cut in the power wire that's grounding out before the sender. Shouldn't it work? Or does the sender have its own power and ground supplied? and the wire going off the top a signal wire and that being cut be the reason it's spiking..... Help!?!?

[Bullitt-]

The spike could be caused by the voltage limiter for the instruments.  The power is regulated to 6V from 12V, if the OE limiter is not working properly your gauges will read high. If not corrected they can be damaged.

Solid state limiters are available  http://rt-eng.com/rte/index.php/RTE_limiter

[72cudamaan]

And what happens when you disconnect the wire?

[Giveitawack]

Well...it's 5 volts, actually

[dodj]

What do you mean by a spike? My oil pressure is quite high initially until the motor warms up and the relief opens. Then it settles down lower.

[Giveitawack]

What do you mean by a spike? My oil pressure is quite high initially until the motor warms up and the relief opens. Then it settles down lower.

As I have understood it the voltage regulator is designed this way. Jump start the gauge. I have the RT-Eng/DashWorx solid state regulator. That too is designed to heat up the gauge initially.
If the regulator did not have this function, the gauge would probably be sluggish. Show lower oil pressure at first until the bimetal strap has heated up. That would not be correct either.
But if the needle goes all the way...sounds like an error

[MEK-Dangerfield]

If the regulator did not have this function, the gauge would probably be sluggish. Show lower oil pressure at first until the bimetal strap has heated up. That would not be correct either.

That's how mine works.

I turn the key and the gas gauge will shoot up, then start the car, and the oil pressure gauge comes up a couple of seconds later. It will hover about mid-way after that.

[dodj]

The gauge doesn't work on heat or need to be heated up. It works on current flow through a coil which deflects the needle. The amount of current flow is mostly dependent on the oil pressure sender on the motor which is really just a variable resistor to ground (engine block). The reason the fuel gauge instantly reads is because the fuel level variable resistor is already at 3/4 full or whatever you have in the tank. The oil pressure has to build after the engine starts. Then when the sending unit 'feels' the pressure, it's resistance drops allowing more current flow. That increased current flow deflects the oil gauge needle. How high your gauge reads depends on the oil pump. I'm sure there are all sorts of pressures and relief springs out there.
Stock gauges are designed to be sluggish. Otherwise the voltage regulator would have to be a higher quality unit than that bi-metal strip. 

[MEK-Dangerfield]

Thanks for the explanation.

I guess I'm normal then...Or at least my car is normal then.   

[Toastyyosty]

When I disconnect the wire ontop the sender the gauge gos to low. No signal I'd imagine. I had though originally that it could be the voltage limiter but all the other gauges work fine except for the clock and tachometer. I had a bad fuel level sender go bad a few months back and though it was thar limiter but as far as I can tell its working properly.  So if the sender is susuppose to ground itself and isn't wouldn't it stay high no matter how warm the engine is?? I believe the oil pump is stock. Might not be but the previous owner is the one who did most of the engine work.

[Giveitawack]

The gas, oil and temp gauges DOES work on heat. I did an effort to repair my temp and oil gauges many years ago. Took them all apart.
On modern gauges there is a coil that moves in a magnetic field. Less resistance from the sender means stronger magnetic field = higher reading on the needle. (The amp-meter works on magnetic field)
But on these gauges there is a bimetal strap. Bimetal is two different metals fused together, forming a seemingly solid piece of sheet metal. The two metals are chosen because they expand differently when heated. The strap is then wrapped in a thin wire that has a little resistance built in to it. When a current is run through the wire it will heat up. It heats the bimetal that bends. More heat = more bending. This is what moves the needle.
So how did my repair work out? Well, it didn't. Ran with VDO gauges. But then I got an idea. Maybe I didn't have the correct resistivity in the wire i had wrapped on. Built a variable voltage regulator. A kit, just soldered together. 5 volts wasn't enough. Turned the voltage up a bit and behold, the gauges worked! Extremely important to get the correct wire. I later replaced them with professionally repaired gauges.

[dodj]

The gas, oil and temp gauges DOES work on heat.

I stand corrected. Giveitawack is correct. Sorry for the mis-information. I made an assumption that I shouldn't have (obviously)
While the principles are the same, I had always thought the gauges worked on current causing magnetism, not current causing heat.
My bad
  giveitawack!

[ShelbyDogg]

You should be able to use a 10 ohm resistor between the sending unit wire and ground and get a reading at the line "H" or "F". If not, you bend the needle a little to get it there. This will tell you if your gauge is normal, then you can move to your sending unit or oil pressure spring.

[Giveitawack]

giveitawack!

No hard feelings, dodj!
Friends?

[dodj]

Friends?

Of course!