5 Tips For Perfectly Working Classic Instruments Gauges

In a perfect world, everyone’s hot rod would be just the way they want it, and nothing would ever need to be changed. Every car would be a perfect representation of the owner, and no blood or sweat would be expelled to get to the end result.

Luckily, our imperfect world affords us the time to do a little cussing, sweating while we tinker in our respective garages to build our dream car. The reason for this respite-from-life time in the garage is because we, as enthusiasts, enjoy our time in the shop. We like making upgrades to our cars, and companies like Classic Instruments are here to help us accomplish our goals.

Classic Instruments gauges can be designed to have a near-factory appearance or a customized look, like these Impala gauges.

Classic Instruments is recognized as a go-to company when enthusiasts are looking for OE-appearing and custom-gauge clusters for hot rods and muscle cars. In fact, Classic Instruments manufactures a myriad of gauge styles and certainly has one to fit your taste and style. At the company’s shop, every gauge is assembled from hand-selected components at the personal workbench of one of the company’s skilled craftsmen.

Not only does Classic Instruments offer a varied assortment of gauges in a wide variety of configurations, but the people at the facility can even design and build custom instruments to fit almost any application. But, designing, manufacturing, and selling gauges is just the beginning of the journey. Once a consumer has the new gauges in their hands, the install takes place.

Many enthusiasts are comfortable ordering aftermarket gauges, but sometimes, installing them the correct way can raise questions. Installation is not difficult, but there are times when an enthusiast might have a couple of questions and could use a little help.

classic Instruments

One of the leading causes of improperly functioning gauges can usually be traced back to a bad ground. Always make sure to scrape away any paint or rust where the wire will be attached.

The folks manning the tech lines at Classic Instruments get a lot of calls each day, and most of the time, something simple is the cause of the headache. In fact, many of those calls seem to be repetitive, so with the help of Classic Instruments’ Devin Butterbrodt, we thought it would be a good idea to assemble some of the more frequently asked questions in one location.

Having Trouble Calibrating Your New Speedometer?

The first thing to do before calibrating a speedometer is to make sure it is working and getting a signal. Begin by taking a short test drive to make sure the speedometer is at least showing a speed (it probably won’t be accurate). If you try to calibrate a speedometer, and for some reason, you don’t have a good signal, the speedometer will still not work correctly and may require you to reset the speedometer to the factory defaults before you fix the signal and try calibrating again.

The new speedometers can be calibrated in one of three ways: “instant calibration” (in which you will need to drive exactly 30mph), “measured mile” (in which you drive a known mile at any speed), and “real-time” (in which you adjust your speed up and down while driving to match a known speed, like with a GPS). The easiest way is the measured-mile method. You can easily find a known mile by driving another vehicle with a working odometer, using a GPS, or by using mile markers on a highway. The speed at which you drive doesn’t matter, which will also make it easier to perform, especially on your own.

Speedometer Is Not Working At All?

If the speedometer is not working at all, first, make sure the speedometer is wired correctly and getting power. When you first put power to the speedometer (turning on the key), the pointer will do a 1/2-sweep initialization. If this doesn’t happen, then the speedometer may not be getting power, or power isn’t being shut off (as it should) when the key is turned off. Next, try resetting the speedometer by locating the calibration settings of the speedometer, and then see if the speedometer starts working. If this fails, check to make sure you have a signal.

Classic Instruments’ SN16 pulse generators attach to transmissions with a speedometer gear drive and produce a 5-volt square-wave signal. They should have an output of 2.5 volts at the signal wire (white wire) when generating a signal. This is also the same for signals coming from most transmission-control computers. The signal voltage should change from either 0- or 5-volt to 2.5-volt when the SN16’s shaft starts spinning. Using a drill to rotate the shaft makes testing easier. If you don’t get 2.5 volts when the SN16 is spinning, make sure the SN16 is getting 5-volts between the red and black wires of its harness.

Pulse Signal Generators (SN16 on left and SN96 on right) are available with a choice of fittings to connect directly to most metric, GMC, Ford (from the early torque-tube type speedometer drives to the latest electronic automatic overdrives), and some Chrysler transmissions.

If you are using a vehicle speed sensor (VSS) that has two wires, measure AC voltage between the two signal wires. One of the two wires should be connected to the speedometer’s speed-signal wire (purple), and the other should be connected to the same point where the speedometer is grounded (the speedometer’s ground wire and the other signal wires should be touching to ensure a strong signal). This voltage should increase as the vehicle speed increases.

To check square-wave signals from a computer or SN16, measure DC voltage between the signal wire and ground. Voltage should change when you begin seeing a signal but will remain constant – no matter the speed (frequency) of the signal. To check sine-wave signals from a two‐wire VSS, measure AC voltage between the signal wire and ground. The voltage will start at zero when not moving and will gradually increase with speed. The faster you go, the higher the AC voltage should measure.

Your Fuel Gauge Doesn’t Work Correctly?

Fuel gauges are made to work with a sending unit with a specific Ohm-resistance range (from empty to full). If the sending unit has a resistance range that is different from the fuel gauge, the gauge will not indicate the correct level. For example, if the gauge is set for a 240-ohm (empty) to 33ohm (full) and the sending unit resistance range is 0-ohm (empty) to 30-ohm (full), the fuel gauge will read full all the time because the resistance coming from the sender will always be less than 33 ohms.

You should also make sure the fuel gauge is wired correctly and not defective. An easy way to do this is to apply power to the gauge (turn the key on) and remove the signal wire. The gauge, no matter what resistance range it was made for, should either go past full or past empty. Next, ground the signal of the gauge. It should peg in the opposite direction. If it registered full without a signal, it should go empty. If it was empty without signal, it should go to full.

Classic Instruments has fuel sending units in either a tube or traditional swing-arm style.

You should also make sure the fuel gauge is properly grounded. Since the gauge light uses the same ground as the gauge, make sure it lights (without the signal-wire connected to the gauge). The gauge will ground through the signal or dash-light power wire if it doesn’t have a good ground to its ground connection. This will make it appear to be working, but not correctly.

If the problem is a mismatched-resistance range between gauge and sender, Classic Instruments has a device called Fuel Link that will control the fuel gauge using a sending unit with any resistance range. If you are not able to change the gauge or sending unit, this is the most convenient way to get them to work properly together.

Your Temperature Gauge Reads Too High Or Too Low?

If the temperature gauge reads too high, make sure the sending unit is installed directly into the engine and is not using any bushings/adaptors to convert the threads to fit the engine. If necessary, a sending unit with the correct thread size is available. Using a bushing can cause a vapor pocket to form where the sending unit’s probe is located. The vapor is not moving through the radiator, so the temperature gets much warmer than the actual coolant. This is the temperature that the gauge is showing.

Also, make sure the gauge is properly grounded. If the ground is bad, the gauge will partially ground through the sending unit, which will cause the gauge to read high (if you don’t have any ground to the gauge, it will peg rather than just read high).

Classic Instruments

If you use a bushing, it can cause an air pocket to form where the sending unit’s probe is located. As you can see, the probe does not protrude into the coolant passage with the bushing installed.

If the temperature gauge reads too low, make sure the sending-unit body is getting a good ground. Since the sending unit doesn’t have a ground wire connection, it relies on the contact with the threads on the engine for its ground. If you have a lot of sealant (Teflon tape) on the threads, this can cause a bad ground connection and make the gauge see more resistance. The gauge reads lower with more resistance. That means a bad ground at the sending unit will make the gauge read lower than it should. If you need sealant to keep the sending unit from leaking, use a small amount of liquid Teflon or equivalent.

A good tool to determine if your gauge is working correctly is an Infrared heat gun. Point it at a spot next to the gauge sending unit (not on the sending unit). A heat gun’s laser point represents the center of the area being measured. The actual area measured is cone-shaped (the cone gets larger the further the gun is from the target). For this reason, hold the gun within a foot of the sending unit, so the gun measurement isn’t affected by other heat sources (i.e., exhaust). The reading from the heat gun should be about 10‐15 degrees cooler than the gauge temperature. This is because the heat gun is reading the surface temperature, and the gauge is getting core temperature from the sending unit.

What Senders Do You Need For Your LS Engine?

Classic Instruments oil pressure and temperature senders need to be used with Classic Instruments gauges — even on LS engines that already have these sensors for the computer. The SN12mm temperature sender (12mm x 1.5 threads) is available to use. It can be mounted at the rear passenger side of the engine. There is a plug that needs to be removed using an 8mm Allen wrench or Torx bit.

You need to use sealant on the SN12mm threads since they are not pipe threads. The SN12mm also comes with a copper crush washer that will help ground the sender. The oil-pressure sender can be installed behind the intake where the original factory oil-pressure sender was, but it requires a 16mm x 1.5 metric to 1/8-inch NPT adapter. Since this location is hard to access and may still have the factory sender mounted there as required by the computer, it is recommended to mount the sender in an alternate location (i.e., above the oil filter).

There is an oil-bypass adapter above the oil filter that can be removed, and then drilled and tapped for the Classic Instruments oil-pressure sender. If you prefer not to drill and tap this, there are pre-drilled aftermarket adapters available for purchase.

Hopefully, your Classic Instruments install will go flawlessly, and you’ll not need any help from the tech line. If the answers to the questions we have given here will help you with your gauge install, we’re glad we could be of service. But, if you do have questions, you can be sure the folks at Classic Instruments will give you the answers you need to help with your situation.

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About the author

Randy Bolig

Randy Bolig has been working on cars and has been involved in the hobby ever since he bought his first car when he was only 14 years old. His passion for performance got him noticed by many locals, and he began helping them modify their vehicles.
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