Moparts Tech Archive
Recalibrating factory gauges- can I do it or should I send 'em in?
The title says it all In doing in the break-in and tuneup (actually it's still ongoing) of my recently built 383, I found out that both my water temp and oil pressure gauges were off by a fair amount. To check this, I temporarily put in a middle-of-the-road oil and water temp gauge, both mechanical (and to tell the truth, the water temp gauge isn't actually installed- turns out that the sender is too large to fit where the factory sender goes so the best I can do is leave the radiator cap off and shove the probe into the radiator towards the inlet). The factory oil pressure reads around 35psi while the aftermarket reaads 55psi, factory water temp says 250 when aftermarket says 220. Both readings are at idle and yes the water temp is higher than I'd like to see it too but that's another thread....
Can the original gauges be cleaned and recalibrated so they read at least reasonably close to what they should? I had replaced both the oil pressure and water temp senders and while it helped, it didn't help much. And if they can be recalibrated, can I do it or should I send them in to have it done?
And how did you arrive at the 20ohm=220* reading? Just measuring the sender resistance and compare it with a known accurate temperature gauge?===============================
You can make a gauge checker. The dealers had them. All the sending units /guages used the same resistance values well up into the eighties and still might be the same but I am not sure if they have changed in the later years or not.
You need a 3 position switch, and 3 resistors 1/8 or 1/4 watt, available at any Radio Short errr i mean Shack. The resistor values are 78.5 ohms for the low reading, 21 for the mid reading, and 10 for high. Assemble the three resistors so that each is on one leg of the switch with the other being tied to the common. Attach two test leads to the output of the switch. One lead goes to a good ground, the other will attach to the sending unit wire. Turn the key to the on position and wait for about 3 to 5 minutes. The guages are thermally activated by the current flow and respond very slowly. Move the switch to the next position and repeat. Every time you switch positions you have to wait 3 to 5 minutes for the guage to respond. This will verify the guage and the wiring but it will not tell you which of the two is at fault. If there is a problem and you verify it's the guage you can remove it and set it up on the bench using a battery with the voltage limiter in line. If you have the patience you can recalibrate the guage by adjusting the two adjusters in the gauge. However moving one affects the other so it is a tedious process. I did the one in my 68 Charger and it took quite awhile (4 to 6 hours). Also the calibrating needs to be done in a non drafty area as this will affect the readings. I made one of these 10 years ago by borrowing one from a friend who worked for Chrysler and copying it, I threw it in my tune up cabinet and it has come in handy more than once.
hope this helps,