The procedure for setting the rays in their proper positions on the shafts is different for each indicator. But one thing is common - they all are set with power on. When doing the initial positioning, just gently press the ray onto the shaft. They can be pressed on fully later.
The oil pressure indicator is easy. When the ignition switch is in the RUN position, the ray should set so that it points at the 0 psi mark.
On the other hand, to be set with any degree of accuracy, the other two indicators (for the coolant temp and system voltage) really need to be set by a method called "in-line verification". This is just an engineering term that means using a known good instrument to check the accuracy of another sensor/indicator - one that is installed and in use.
The voltmeter is simple to calibrate. With your voltmeter turned on and the probes connected to the studs on the back of the meter (or any other convenient hot wire and ground near the meter), turn the ignition switch to RUN. Note the displayed voltage and set the ray on the stem so that it points to the corresponding mark on the indicator face.
Two down. Now comes the calibration of the coolant temperature indicator - that procedure is more involved, but it's still not that hard:
Because the temperature indicator has a "dead zone" on the low end (where the coolant temp is below the measurable range) it can't be calibrated like the oil pressure indicator.
When the engine is not running, the oil pressure is known to be 0 psi. So, with that in mind, the indicator's needle can be set to the 0 psi mark and it will be safe to assume sufficient accuracy throughout the entire readable range (provided the indicator has not been altered).
But with the coolant temperature indicator this method doesn't work well. You can't simply supply power to the indicator, and then set the ray to some point on the scale - one that approximates the ambient temperature in your garage. Doing this will almost certainly result in an inaccurately calibrated coolant temp indicator.
Here are a couple of ways to get it accurately calibrated. Both require that the coolant temperature sender be pulled from the engine but remain connected to the sensing leg lead - that makes the procedures a bit cumbersome. It is best to calibrate an indicator as close to the midpoint of it's range as possible. So you will be heating the sender to 210 degrees F:
1. You can use a heat lamp, a coffee can and a good quality oven thermometer. Put the sender (with it's associated lead attached) into the coffee can. Cover it with some kind of insulation and insert the probe of the oven thermometer through the insulation. Do this in such a way that the tip of the probe is either touching or very close to the sender's tip. The dial of the thermometer should be outside the area covered by the insulation - this is so the temperature can be read without removing the insulation.
Set up a heat lamp or a contractor's flood light so that it is under the bottom of the coffee can - turn the lamp on and wait.
When the oven thermometer reads around 210 F, go ahead and set the needle to the same temp mark on the indicator. All done the calibration. Break down the calibration apparatus, screw the sender back into the cylinder head and reconnect the lead.
2. The second method relies on this fact; No matter how much heat you apply to a open container of water, once it hits 212 F and starts boiling, it will never exceed that temperature. This way is more of a pain, but the good thing about it is that you don't need an oven thermometer. Boiling water at atmospheric pressure is always 212 F - period.
Just set the sender (with the lead attached) in a container of water- a coffee can works well for this method too. Only the tip of the sender needs to be submerged.
Use a propane torch, a hot plate, a heat gun (or whatever you have on hand that can provide sufficient heat), bring the water to a full boil. The tip of the sender will at that point be at 212 degrees F.
Go ahead and set your needle to the 210 F mark on the indicator (key in RUN position).
All done. Break down and reassemble.
In case you are concerned about how accurate the sender's response is, use the chart below to check the resistance across it at various temps. Just connect one of your VOM leads to the normal terminal and the other to the part of the tip that is in the coolant stream.
Temperature/resistance chart for air and coolant senders:
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