HELP How do you set needles on gauges correctly?

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Supa.Slo.SS

Apprentice
Aug 7, 2020
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So I made the mistake of not paying attention on the orientation of the needle before removing them, all the gauge needles for the entire cluster, i.e., Speedo, RPM, Fuel, Oil Pressure, Temp & Voltage were removed & replace.

I snapped the new retro-fitted needles (self-made) without worrying about the orientation but now I don't know if the readings are correct.

Current readings at idle, Speedometer - 4 MPH, Fuel - Past Full, Voltage - 16, Oil Pressure - 60, Temp - 100, RPM 300-400.

Anyone know how to set them up? I thought about having the ignition switch to the on position (2 clicks) & set all the needles at 0, except for the fuel & Voltage.

The fuel gauge previously did not give the correct reading because of a faulty sending unit that needs to be replace, so I guess I will have to address that one later, but once fixed I would imagine filling up the tank & set the needle at full.

The voltage I thought about doing the same, turn the key and check the voltage at the amp? with volt meter then set the needle at that number?

Thanks in advance.

84 Monte Carlos SS
 

ELCAM

Royal Smart Person
Jun 19, 2021
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You are on the right track with the voltage. Measure with a DMM and place the needle to match.

Oil pressure put a tee on the sender with a mechanical gauge and place the needle to match.

Fuel gauge put a 91 ohm (white brown black) resistor between the sender wire and ground and place the needle to full.

Tach, use a dwell tach meter and set idle to 1000RPM and place the needle to 1000 RPM.

Water is a crapshoot unless you pull the sender and put it in a pan of water with a thermometer. You will have to run a ground wire to the sending unit case. Boil the water and set it up as it cools to 200 or 180 what ever marks you have on the gauge so it is easier to place the needle accurately.
 
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Supa.Slo.SS

Apprentice
Aug 7, 2020
57
26
18
CA
You are on the right track with the voltage. Measure with a DMM and place the needle to match.

Oil pressure put a tee on the sender with a mechanical gauge and place the needle to match.

Fuel gauge put a 91 ohm (white brown black) resistor between the sender wire and ground and place the needle to full.

Tach, use a dwell tach meter and set idle to 1000RPM and place the needle to 1000 RPM.

Water is a crapshoot unless you pull the sender and put it in a pan of water with a thermometer. You will have to run a ground wire to the sending unit case. Boil the water and set it up as it cools to 200 or 180 what ever marks you have on the gauge so it is easier to place the needle accurately.
Thanks for the info ELCAM!

For water, how accurate would it be if I run the engine for 10-15 minutes then take a temp reading with a Spot IR Thermo gun right at the temp sensor?

Also, for the fuel, does it matter if my current sending unit is faulty? Do I need to fix that first?
 

ELCAM

Royal Smart Person
Jun 19, 2021
1,114
1,248
113
My IR thermo gun reads about 10 degrees low, might want to see how accurate yours is.

You can do that but keep in mind you are reading metal temperature on the outside not water temperature on the inside so it will be low. There will need to be a correction factor, perhaps add 10 degrees or something like that.

On the fuel gauge remove the sending unit wire and replace with the 91 ohm resistor to ground, then the bad sending unit does not matter. The sending unit for these cars is 0 to 90 ohms 0 being empty and 90 being full. 91 ohms is the nearest commonly available fixed value resistor available. just disconnecting the sender and grounding the terminal on the gauge will set it to empty this will work but I prefer to set it to full I think it is more accurate.
 
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L92 OLDS

Comic Book Super Hero
Mar 30, 2012
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For water, how accurate would it be if I run the engine for 10-15 minutes then take a temp reading with a Spot IR Thermo gun right at the temp sensor?
IR guns are usually very inaccurate. I have a Raytech gun that allows me to adjust the emissivity and it is still off compared to a surface contact probe that uses a conventional J type thermocouple. Use a surface probe and you’ll be OK.

 
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Supa.Slo.SS

Apprentice
Aug 7, 2020
57
26
18
CA
You are on the right track with the voltage. Measure with a DMM and place the needle to match.

Oil pressure put a tee on the sender with a mechanical gauge and place the needle to match.

Fuel gauge put a 91 ohm (white brown black) resistor between the sender wire and ground and place the needle to full.

Tach, use a dwell tach meter and set idle to 1000RPM and place the needle to 1000 RPM.

Water is a crapshoot unless you pull the sender and put it in a pan of water with a thermometer. You will have to run a ground wire to the sending unit case. Boil the water and set it up as it cools to 200 or 180 what ever marks you have on the gauge so it is easier to place the needle accurately.

IR guns are usually very inaccurate. I have a Raytech gun that allows me to adjust the emissivity and it is still off compared to a surface contact probe that uses a conventional J type thermocouple. Use a surface probe and you’ll be OK.

Thanks L92 OLDS.

I have a Flir, I have never messed with the settings but I believe it also allows the emissivity to be adjusted. I don't have a surface probe unfortunately. What was the difference in Temp between the IR and surface contact probe?

Follow up question, if I were to find a cheap digital (or analog) temp gauge, couldn't I just plug it directly to the temp sensor on the top of thermostat housing? Or do those wires go through some sort of resistors that would give a wrong reading &/or burn up the temp gauge. There are a few new ~$20 digital gauges on ebay/amazon, some even come with the sensor, don't know how accurate they would be, or if the thread on sensor is the same but just trying to understand if that's an option.
 
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