Well my find at Carlisle is not working out as well as expected. I have the sender unit in but the Oil pressure gauge is heating up. I just have the gauges out in the open to try them out. The oil pressure gauge is going down to zero when everything is hooked up but it is quickly heating up and starts a little smoke. The resister is sagging a little but it is not broken and is still showing resistance. I don't know if the gauge is shot or just the resister is shot. Can you still get just the white resister that goes between the terminals? The 74 gauge is rare but I can get one. I'm doing this to remove the mechanical oil pressure gauge and go with an electric one. I got what I thought was a good one at Carlisle and all the other gauges seem to be working fine. Only the one I'm interested in is acting up. Any suggestions from electrical wizards?

 

The resistor is a calibration shunt. It is wired in parallel to the gauge. The gauge should share the current flow with the resistor. Take the resistor off and measure the gauge resistance. It should measure close to the resistor, I believe.

 

Big_G said:
"The resistor is a calibration shunt. It is wired in parallel to the gauge. The gauge should share the current flow with the resistor. Take the resistor off and measure the gauge resistance. It should measure close to the resistor, I believe."

The gauge is basically two coils with one end of each coil joined at one of the 3 gauge connections. The resistor is in parallel to one of the coils.(Lets say points A & B)

Remove the resistor and measure its resistance
Measure the resistance across the terminals A & B.
They should measure approx the same at 120-140 ohms.

There should be a third terminal to which the resistor was not connected (C). Measure the resistance between that terminal, and poits A & B You should find one reading at 120-140 ohms and the other at 240-28o ohms. If you get these readings, the gauge is probably working


Was the pressure sender and gauge out of the same working car?
My guess is that you don't have the proper pressure sender for the gauge.
If the resistance of the sender is too low, or you have pressure switch instead of a resistive sending unit, It would ground the oil pressure signal which could overheat the gauge and resistor.

Kilroy1024

 

Thanks for the info guys. The gauge was from a 74 car. I went through a lot to get the correct new sender unit. I have worked with the parts store manager many times on small projects like this and he has always got me the right part. I will try the resistor checks today. This is the info I was looking for.

 

I'm no electrician here but here is my readings. With my multimeter set at 200 on the ohm scale I get 82 when measuring across the resistor. Across the ignition and sender terminals I get 86. The sender teminal to ground terminal I get 100. The ignition to ground terminal I get 185. Does this mean anything? Looks good to me as there is none of them tha tis way out of range.

 

Make sure the sending unit is for a gauge, and not an "idiot light". I just checked a 74 we have in the shop, the sending unit measures approx. 64 ohms with no oil pressure.(un-plugged)

 

I specified that it was for gauges. I just went and measured mine and I'm only at 1 ohm with no oil pressure unplugged. Maybe it is for idiot lights. They are usually smaller, this looks like a big bell shapped unit. I'll have to fire it up and see if it goes up in ohms or if it just opens the circuit.

Just tried it running. It started at 1 ohm not running and running went from about 75 to around 100, jumped around a little then the lead fell off. So it looks like it is for gauges because it varies in ohms.

 

Your gauge coil and resistor values look ok.
I'm a bit concerned about the 1 ohm resistance on the sender.

If I recall correctly, the resistor is connected between 12V and the sender. The 82 ohm resistor will dissipate 2W when the sender is 1 ohm. Considering the size of the resistor, it will get pretty warm.

NOTE: The resistor is a special shape, but its just a resistor, you can replace it with any 82 ohm 3-5 watt resistor. Just make sure the leads are dressed so there is no chance of them shorting on something.

Kilroy1024

 

I'm working on a guages tech page -

Does this info help?

 

I just got home and will need a little time to digest this.

 

I went out and checked the pressure sender on my car. (76)
Without the engine running, the gauge reads 0 PSI and the sender is approx 10 ohms.

With the engine running and reved up, the gauge reads 80 psi and the sender is approx 80 ohms. Therefore resistance rises with pressure.

This is good! The bias resistor will only get hot when ignition key is in the RUN position and the engine is not running. The gauges do not get power in the accessory or start position.

So this is really only a problem if your planning on sitting in the RUN position for long periods without the engine running. The resistor shouldn't get that hot in normal operating conditions.


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Interestingly enough, when I pulled the wire off the sender, the gauge went way past the 80 PSI point. Blowing a hole in my statement that you can't get more than 90 degrees sweep. I'll have to chew on that.

 

That is pretty close to what I am getting. Ignition on not running I'm at 1.1 ohm. With it running and about 60 psi it reads about 75 ohms. I did not run it long for fear of it heating up. According to your post this should not be aproblem. I'll have to try it when I get a chance. I did send you a picture of the back of the gauge panel. Let me know if it is OK

 

Just had another thought here!
The gauge came out of a working 12V car so you know that the gauge and bias resistor will work together, right?

Were trying to determine the correct sender resistance that goes with them. Go out and get a handful of resistors that you can put together in combos of 10 to 200 ohms or so.

Start at 200 - Got 80 PSI? Increase or decrease the resistance until you figure out how much it takes to get 80 psi.

Short the sender to the block. Is that 0 PSI?
How about shorted to the block through 10 ohms?
Still close enough to 0 PSI? Keep incrreasing until you find the highest value that you would consider an acceptable 0 reading.

There you go, you got the required resistance range for your gauge.

 

Yes that all makes sense to me now. I'll keep playing with it and some resistors to see what I get.