Troubleshoot Gauges

[dibbons]

I would like to learn how to calibrate the gauges. For example, my coolant temp gauge seems to read just a hair too low. However, in all my searches online, it must be a big secret, because no one is willing to share that information. Same thing with a speedo that has the correct pinion gear but the speedo is not accurate, no one will share the little trick to make it right.

During my searches, I did find a brief video on how to troubleshoot gauges that don't work at all and I would like to share it here (Ford Mustang guy, but I believe his explanation applies to Mopars as well). The video title is wrong, it does not show how to "calibrate" only how to troubleshoot:

how to calibrate classic car gauges? - Bing video

In closing, if anyone knows how to make minor adjustments (that is, really calibrate them) to gauges that work, but are just a little off one way or another (i.e. coolant reading a little too low, speedo reading a little too high with correct pinion). feel free to share with us. I will try not to get my hopes up too much.

 

[my68barracuda]

I would like to learn how to calibrate the gauges. For example, my coolant temp gauge seems to read just a hair too low. However, in all my searches online, it must be a big secret, because no one is willing to share that information. Same thing with a speedo that has the correct pinion gear but the speedo is not accurate, no one will share the little trick to make it right.

During my searches, I did find a brief video on how to troubleshoot gauges that don't work at all and I would like to share it here (Ford Mustang guy, but I believe his explanation applies to Mopars as well). The video title is wrong, it does not show how to "calibrate" only how to troubleshoot:

how to calibrate classic car gauges? - Bing video

In closing, if anyone knows how to make minor adjustments (that is, really calibrate them) to gauges that work, but are just a little off one way or another (i.e. coolant reading a little too low, speedo reading a little too high with correct pinion). feel free to share with us. I will try not to get my hopes up too much.

I think the best you can do is make sure that all of the electrical connections from the sensor to the gauge are as good as possible, with as little as possible additional resistance added. You can also verify that the dash IVR is functioning properly. Concerning the water temp. Be sure you are checking the temp exactly where the sensor is and OE speedometers were probably + or - a few mph when new.

 

[dibbons]

My speedo is off around 10 mph. Paid $669.00 for gauge restoration. I will definitely need to pull the transmission speedo pinion gear out to verify color/teeth and go from there, but I have absolutely no reason to believe the installed gear would be incorrect for my vehicle.

This looks like fun:
how to calibrate classic car gauges? - Bing video

 

[65 Dartman]

If your speedo is off that much it could be that your tire diameter does not match what it’s supposed to be based on whatever gear you have

 

[RedFish]

The factory thermal gauges are very basic range indicators. Your temp gauge not reading quite as high as you would like doesn't indicate its calibration is wrong.
I've restored and recalibrated a crowd of them. I do this calibration with the same solid state 5 volt supply that will be in the car. They are calibrated to 3 electric ohms resistance points, not fluid temperatures or pressures.
If someone calibrated your gauges with a solid state regulator that puts out 4.9 to 5.4 volts at 1 to 1.5 amps, and you install them with a OEM mechanical voltage limiter the gauges will read a hair higher. The opposite applies, calibrate with a mechanical limiter that is closer to a 6 volt and much higher amperage supply, then installed with a solid state regulator, the gauges will read a hair lower. Because they are simple thermal range indicators, slight variations are expected.
As for speedometers, they are calibrated to the drive RPM at the workbench. It would be hard to get that wrong. The drive cable rpm is changed with the cable drive gear ratio. The actual MPH changes with axle gear ratio and tire diameter. Cable drive gear change brings MPH and cable RPM together.

 

[Garry in AZ]

OEM electric gauges were never designed for a high degree of accuracy, more as a simple indication of a potential problem. I totally agree with RedFish's comments above as well.

 

[Frnknsteen]

My experience is with the 68-70 rally dashes, but may apply to other gauges as well....

Temp, Fuel, and Oil Pressure gauges work with a 5 volt power source from the voltage regulator on the gauge assembly, and a variable resistance from the sending unit. There is a bi-metal strip that heats and expands or contracts as resistance changes, and moves the needle.

On the back of the gauge body, at least on the rally gauges, there are two little windows on the back of the gauge body to give access to adjust the gauge for accuracy. Inside those holes are some teeth that allow you to adjust the gauge. Basically, you can tweak the upper and lower limit by moving those teeth with a small screwdriver (See attached image). Keep in mind that they are tied together by that bi-metal strip though, so as you tweak one, the other may change too, so you have to go back and forth tweaking it to balance it out best you can

How I tested mine was to use a DC power supply set to 5 volts DC. I put the hot lead to one side of the gauge and connect the other lead to a resistor, then from the resistor to the other side of the gauge. Our gauges use a 10-80 ohm scale range. Standard resisters used to test are 75 ohm, 23 ohm, and 10 ohm. The 75 ohm resistor should make your gauge read empty, the 23 ohm should put the needle somewhere in the middle of the gauge range, and the 10 ohm should put the needle on full.

I used the 75 ohm resistor first and adjusted the prongs in the low window to put the needle just under the empty mark. I then used the 10 ohm resistor to test the high end of the gauge and adjust. Once I felt I had it reading where I wanted to, I used the 23 ohm resistor to confirm the needle ran approximately in the middle.

The attached picture shows the inside of the gauge body and I circled the adjusting teeth I am referring to. I also circled the bi-metal strip that controls the movement of the needle.

Speedometers are another story. They are hard-geared. If they are off, it is most commonly affected by rear tire size, gear ratio, and having the right amount of teeth on the speedometer gear in the transmission. If you call Brewer's Performance and tell them what transmission, differential gears, and tire size you have, they can tell you which gear you should be running, and can sell you the correct one to use with your setup.

 

[dibbons]

The question with regard to the speedo is this: When the proper speedometer pinion gear is installed in the transmission assuming it is calculated correctly using data with the tire diameter, rear end ratio, and transmission gearing and the speedo is STILL off, how does one correct the discrepancy?

In the above case, isn't there a magnet and/or spring inside the speedo that are the culprits and would need repair? And if so, who is able to share the procedure working with the spring and magnet to get things right again?

 

[RedFish]

The proper adjust point is the needle clock spring right behind the screen.
You might get lucky and simply rotate the needle on its post but the instrument would need to be exactly the same MPH wrong throughout the range for that to work.
It is much less likely to work when the instrument reads slow. Reposition the needle that direction and it might not return to '0'.

 

[dibbons]

Now that is beginning to make sense, especially since I just found a complete 71-74 BBody gauge cluster in a box (see fotos 1 & 2), along with an EXTRA speedo minus the needle (see foto 3), besides the cluster already installed in the BBody.

I don't see any way to "calibrate" the speedometer. Looks like Model A technology, just the aluminum drum that moves the needle when the two arms spin around it and provide some current/magnetic field. Kind of like a torque convertor where the two parts don't actually touch.

 

[dibbons]

If you look at the dead center of the foto, you can make out the tiny return spring for the speedo needle.

 

[dibbons]

Here is a better foto of the drum and one of the two arms that spin around it, thanks to the speedo cable. Obviously, the odometer is a strictly mechanical connection.

 

[dibbons]

There looked to be a slight build-up of crud on the two arms on the inside where they just barely clear the drum as they spin around. I sprayed some WD-40 in there and then slid a paper towel in the tiny space, trying to see if that crap would come loose.

 

[dibbons]

The company that sells the solid state voltage limiters (that reduce voltage down to 5 volts from 12 volts) included a resistor that can be used to check calibration of electric coolant temperature, oil pressure, and fuel gauges. With the resistor wired in temporarily in place of the sending unit, the gauge needle should move to the maximum reading.

 

[Garrett Ellison]

Once you install a set of quality aftermarket gauges in any of these cars (unless it has to be a correct restoration, of course), you won't even give the factory gauges any more thought other than they are just another part of the car best updated.

 

[Mike69cuda]

I made a test box for the gauges. I use it to substitute for the sensors. Since they all work the same the box can be used on any of them. It fits in a small flat rate box. If you want to borrow it I will send it to you.

 

[dibbons]

That is a pretty cool rig, only wished it worked on the speedo!

 

[Mike69cuda]

Yeah, speedometers are a pain

 

[RedFish]

The company that sells the solid state voltage limiters for to reduce voltage down to 5 volts from 12 volts included a resistor that can be used to check calibration of electric coolant temperature and oil pressure gauges. With the resistor wired in temporarily in place of the sending unit, the gauge needle should move to the maximum reading.

That's a bit of half assed test. Many of these thermal gauges have mechanical needle stops. So a ten ohm resistor that takes the needle up against the stop doesn't mean the needle would rest there if the stop wasn't present. The needle could be trying to go further. If the needle stops short of maximum reading you could adjust to it I suppose but... All too often a layer of crud has built up on the winding restricting beam bow/movement ( like a ace bandage on your arm for example ). So moving the beam to get needle at max doesn't dictate that it will be right at half or at min.

 

[Mike69cuda]

I’m surprised that low watt resistor doesn’t burn up.

 

[Garrett Ellison]

I know it's not for everyone, but thanks to the good folks at new vintage autogauge, I calibrate my speedometer with a push button and a measured mile now but can use the hop-pulse count if I want to do the math.

 

[RedFish]

I’m surprised that low watt resistor doesn’t burn up.

You need to realize the gauge itself is a resistor. It does get hot. Depending which gauge... Either approx' 13 inches of tiny 1.5 ohm per inch nichrome wire or approx' 13 inches of 1 ohm per inch of nichrome wire. Aint a whole hell of a lot of current going to get to the next resistor in the path.

 

[dibbons]

I’m surprised that low watt resistor doesn’t burn up.

You use the resistor while running the gauges thru the voltage limiter. Would the pulsing 5 volts burn it out?

 

[Mike69cuda]

Looks like an 1/8 watt resistor max. 13 inches at 1 ohms is about 13 ohms. 13+ 10 = 23 ohms. 5V / 23 ohms = .22amps. 10 ohms x .22 amps = 2.2 volts across resistor. 2.2 v x .22 amps = .48 watts. Over wattage pretty significantly if that is an 1/8 watt. Probably get pretty hot if left on very long.

That’s why I used high wattage resistors in my box. Left on very long to small of a resistor will likely get hot and change value.

 

[krazykuda]

My speedo is off.

Not as off as this guy looks in a speedo....