Customer Service 1-844-3-Gauges (844-342-8437)


Speedometer Calibration – 12 Dip Switch Model

• Where are you getting signal?  (SN16, VSS, ECM, MAG Sensor)
• Are you using a SN74?
• In what position are the speedometer’s dip switches set?
• Are you sure power is off to the speedometer when you make a change to the dip switches?

If using SN16, make sure you are using the 16000ppm calibration chart and dip switches 5,6,7,8 are OPEN.  If you are using a SN74 along with the SN16, make sure the dip switches are also set at the default (5 6 7 8 OPEN).

If you have a VSS signal from a transmission or any 2-wire sender (i.e SN96, SN95), make sure you are using the SN74 with switches 1-4 OFF (2 may be ON if you are experiencing interference) and have the speedometer dip switches set to 5 6 7 8 OPEN. 

If you have an ECM signal, set the SN74 switches to 1,2 ON with the speedometer dip switches set to 5 6 7 8 OPEN.

Make sure the dip switches on the speedometer are not set backwards.  Open is pushed in away from the numbers.

Sometimes a dip switch will look like it is pressed in the correct position but it hasn’t “clicked” and is not making contact in the correct direction.  Make sure they are all “clicked” in the correct position by toggling every switch once and then setting the default again.

If no change is seen after you have made dip switch changes, verify that power was off when the changes were made.  Check the voltage to the speedometer with a meter.  If you don’t have a meter, disconnect the power wire from speedometer or positive wire from the battery to make absolutely sure the power is off.


Speedometer Calibration – Ultimate Technology

• Is the pushbutton connected properly?  One wire to ground and one to brown harness wire.
• Are you pressing the pushbutton until the engine is started?  Necessary when calibrating speedometer.
• Is the speedometer pointer at 30mph or 45mph when doing a marked mile calibration?

Sometimes the pushbutton is not connected to a ground.  Check for continuity through pushbutton (should have continuity only when button is being pressed)

The pushbutton does not move very much when pressed making it hard to tell if you are actually getting the pushbutton to work.  If you are having troubles calibrating, make sure to press the pushbutton extra hard to insure it is making contact.

A common mistake is a customer will enter calibration mode to set the speedometer and then start the engine.  When this happens, power is cycled and the speedometer is no longer in calibration mode.  (It is possible to calibrate the tachometer without engine running)  The pushbutton must be pressed until the engine is started (if not a few seconds after).  Once the engine is started, the pushbutton can be released.

If, during a measured mile speed calibration, the speedometer pointer is pointing at 30mph, no signal is getting to the speedometer.  Check the signal.  If it is at 45mph and you are NOT moving, it is getting interference from somewhere. 

The use of the pushbutton is a common problem.  The pushbutton needs to be pushed and HELD for about 4 seconds to enter (or exit) a specific calibration mode (i.e.  tach cylinder setup, speedometer marked mile calibration, speedometer real time calibration…)  Once in that calibration mode, the pushbutton can be released.  To cycle through options, the pushbutton needs to be tapped.  A lot of the time the customer will hold the button too long and not be able to cycle through the options (tach at 1, 2, 3, 4 or 8).
It is not necessary to hold the pushbutton while calibrating the speedometer during the marked mile method.  The pushbutton can be released after the speedometer pointer moves to the 30mph position (if vehicle is not moving, 45mph if vehicle is moving),


Speedometer Calibration – 8 Dip Switch Model

• Where are you getting signal?  (SN96, VSS, ECM, MAG. Sensor)
• Are you using SN74? 
• What position are the speedometer dip switches set?
o Default should be 2,6,7,8 OPEN
• Are you sure power is off to the speedometer when you make a change to the dip switches?

SN96 signal usually doesn’t require an interface box.  Just calibrate according to 8-pulse calibration chart.

A mag sensor signal may or may not require an interface box, depending on how many pulses per mile it is producing.  If the signal is less than 6000 or more than 10000 pulses per mile, they will need a SN74

An ECM signal usually requires a SN74.  (Some ECM signals are 8000ppm and can be connected directly to speedometer.)  Set the SN74 dip switch 1-3 ON, 4 OFF.

A VSS signal from a Ford transmission (except Tremec) is 8000ppm and most likely doesn’t require a SN74.  Any other VSS signal will require a SN74. 


Speedometer Not Working

• Does speedometer pointer come off stop when power is applied?
o Pointer of 16 pulse and ultimate speedometers should move up with 12V.
• Does odometer work?  (if odometer works, signal is probably OK)
• Is speedometer on a 5” speedtachular or “black box” style Bel-era or 1957 Chevy?  (possible problem with phone type cables)
• Where is the speedometer signal coming from?  (pulse generator, VSS, ECM, mag sensor)

Especially with 5” speedtachulars and “black box” style Belera and 1957 Chevy clusters, customers only wire +12V to one of the two IGN posts on the black box.  Make sure power and ground are run to both boards is the box.  If odometer is working but not the speedometer pointer, it could mean a bad small phone cable.  If speedometer is working but odometer isn’t, it could me a bad large phone cable (which is the same as a computer Ethernet cable)

If signal is coming from VSS, a SN74 is required and could be the cause of the speedometer not working.

If a SN74 is being used, is the green light next to the dip on?  If it is not on, then the SN74 is not getting power and won’t send a signal to speedometer.  If the green light is on but doesn’t blink when the vehicle is moving, the SN74 is not getting a signal and you should check the signal source (VSS or ECM).  If the SN74 green light is blinking and the vehicle is not moving, it could be interference.  Try turning the SN74 switch #2 ON to filter the noise out of the signal. 

If SN16 is being used, test by turning shaft with a drill to see if speedometer responds.  The signal can be monitored by connecting a multi-meter set to DC volts between the signal post and ground post of the speedometer.  With power to the speedometer ON, slowly turn the shaft of the SN16.  The voltage should alternate from battery voltage and zero volts.  If it does not alternate, then the SN16 is bad.

If a SN96 or SN95 is being used, test by turning shaft with a drill to see if speedometer responds.  To test the signal, connect a multi-meter set to AC voltage between the two wires of the pulse generator.  Spin the shaft with a drill.  The AC voltage should start at zero when the shaft is not spinning and gradually increase the faster the shaft is spun.

Insure the speedometer has a dedicated chassis ground (not touching other ground wires) and a dedicated +12VDC.  The power should not be run off the same fuse as high current devices such as radios, AC, electric fans, etc…
Temperature / Oil Pressure Reading Too High

• Where is the sending unit mounted?
• Are you using a Classic Instruments sending unit?
• Do you have anything other than the gauge connected to the sender?
• Is the gauge grounded well?

If sender is mounted in the engine head, it will read 15 – 20 degrees higher than actual because of heat from the exhaust.  Relocate the sender to the intake manifold if possible.  (Exception:  LS engines should have temp sender installed in the head and are not affected by exhaust heat.)

If the gauge has a bad ground, it will make the oil pressure or temperature gauge read high (or even peg if there is no ground).

Connecting more than one device to the sender will cause the gauge to read high.  Only the gauge should be connected to the sender (NO fans, other gauges, warning lights, etc…).

Make sure you are using a Classic Instruments sender.  Temperature senders should not be in any bushing (size adapter), which will cause high readings because of air pocket forming under the sender. 

OEM senders and other aftermarket senders are not compatible with Classic Instruments gauges.

Hot temperature readings can be caused by air pockets in the system.  When the vapor in the air pocket passes the sending unit, the temperature will increase higher than the actual coolant temperature.  It may be necessary to “burp” the coolant system to remove any air pockets. 

Make sure you have the proper coolant level for the cooling system.

Temperature / Oil Pressure Reading Too Low

• Where is temperature sender located?
• Does the sender have any Teflon tape or other sealant on the threads?
• Are you using a Classic Instruments sending unit?

Teflon tape or other sealants on the threads of the sender will cause a bad ground which increases the resistance seen at the gauge and causes the gauge to read lower than actual.

Connecting more than one gauge or electric fan, etc… will cause the gauge to read incorrectly.

OEM senders and other aftermarket senders are not compatible with Classic Instruments gauges.


Fuel Gauge Not Working / Incorrectly

• Did the fuel gauge come in a set that included a sending unit?
o Sets with included sender have a 240-33 ohm gauge (SN35 sender).
• What is the ohm range of the fuel sender (if OEM sender being used)?
o GM before 1964 = 0-30
o GM after 1966 = 0-90
o Ford before 1986 = 10-75
o Ford after 1986 = 16-158
o Dodge/Chrysler before 1989 = 75-10
• What is the fuel gauge ohm range?  (Is there a sticker on the side of the gauge indicating ohm range?  If not, it is a 240-33 gauge)
• Does the pointer move when the gauge is powered?

Make sure the ohm range of the gauge matches the ohm range of the sender. 

If the sender doesn’t have a ground or has a bad ground, the gauge will not read accurately.  0-30, 0-90 and 16-168 gauges with the corresponding sender will read full or high if the sender has a bad ground.  75-10 and 240-33 gauges with corresponding senders will read empty or low if the sender has a bad ground.

The most common problem with a 240-33 ohm gauge and a SN35 sender is the gauge reading Empty or not reading all the way to full.  This is mostly caused by a bad ground on the sending unit.

Sometimes the float arm of the sender is not able to move freely inside the tank (hitting side of tank or baffle).  Take it out, attach a ground wire to the mounting plate and move the arm manually to make sure the gauge responds as it should.  Also, make sure the float arm falls on its own to empty when you let go of it.  (Insure it is not sticking)


Erratic Speedometer

• Is the pointer fluctuation more or less than 10mph?
• Where is the speed signal coming from?  (VSS, ECM or pulse generator)
• What transmission is on the vehicle?  (turbo350, 700R4, etc…)
• What speedometer are you using? (8-pulse, 16-pulse, ultimate)

Speedometers with a pointer fluctuation under 10mph are usually caused by the transmission using a speedometer gear.  (i.e. turbo350 and 700R4).  This can be fixed by a new plastic speedometer driven gear or by building up the size of the pulse signal generator shaft by adding a piece of 1/8” diameter shrink tube to it. 

Speedometers with pointer fluctuations over 10mph are usually caused by ignition interference.  Usually, shielding the speedometer takes care of this problem. 

If you are using a SN74, make sure that dip switch #2 is ON.  This will filter noise that may be on the signal going to the speedometer.  This will also cure the problem of a speedometer reading while the vehicle is not moving. 

To check if the erratic speedometer problem is caused by ignition noise (requiring shielding) or by the transmission, drive the vehicle until the speedometer becomes erratic, then shift into neutral and turn off the engine.  Turn the key back on so that the speedometer has power going to it.  If the speedometer is steady without the engine running, then the problem probably will be solved by shielding the speedometer.  If the speedometer remains erratic without the engine running, the problem is in the transmission and might be solved by using the SN74 to buffer the signal.

Sometimes the SN16 plug will be loose when connected to the sender.  This will also cause an erratic reading.  This can be fixed by spreading the wire retaining clip on the plug to create a tighter connection between the wire harness plug and the sender.

Ultimate speedometers are sometimes erratic if using an ECM speed signal.  This can be fixed with a SN79 filter.  This filter will only work with ECM signals to a speedometer using ultimate technology.

Bad plug wires can cause excessive EMI interference and cause the speedometer to be erratic.  Check the spark plug wires and insure they are EMI suppressant wires.

Insure the speedometer has a dedicated power and ground. 

Speedometer Wiring

• What speedometer signal are you using?  (SN96, SN16, VSS or ECM)
• Is your speedometer an 8-pulse, 16-pulse or ultimate?
• Are you using a dedicated ground?  (not touching any other ground wires)
• Are you using a dedicated +12V?  (accessory of ignition switch or separate fuse on fuse panel)

If using a VSS or ECM signal with an 8-pulse speedometer, a SN74 is needed. (Except Ford transmissions, which do not require a SN74)  The SN74 setup is dip switch 3 ON and 1, 2, 4 OFF for a VSS signal.  The SN74 setup for an ECM speed signal is dip switch 1, 2 and 3 ON. 

Ultimate technology speedometers using the SN16 pulse generator require the use of the red wire from the speedometer harness.  This needs to connect to the red wire (filtered power) from the SN16.  Connecting +12V from anywhere else to the red wire of the SN16 might cause the speedometer to read erratically.

Connecting a VSS to an ultimate technology speedometer requires one of the VSS wires to connect to the purple wire from the speedometer and the other to connect at the same spot on ground as the black wire from the speedometer harness.  You should NOT connect the other VSS wire to a ground on the transmission or anywhere other than where the speedometer is grounded as this will weaken the signal. 

It doesn’t matter which VSS wire goes to the purple speedometer wire (of an ultimate technology speedo) and which goes to the ground as long as the signal is not also connected to an ECM.  If the VSS signal is also going to an ECM, where the VSS wires are connected does matter.  (Always use the ECM speed signal if available)  It will not hurt to connect them the wrong way; it will just cause the speedometer not to work and the transmission not to shift.  If this happens, just reverse the order in which the VSS wires are connected.  (This is also valid if connecting the VSS to a SN74)


Speedometer Pointer on Wrong Side of Stop

• What speedometer signal is being used?  (ECM, VSS, pulse generator)
• What were the circumstances right before the pointer got stuck on the wrong side of the stop?
• Are you experiencing any erratic speedometer movement, especially when the vehicle is stopped?

Speedometer lost power while receiving a speed signal.  (i.e. vehicle was moving or was getting false signal caused by interference when power was removed from speedometer)
In order to get the pointer back to zero, a signal of 100MPH must be applied to the speedometer.  There are a few methods to accomplish this without actually driving the vehicle at 100MPH.

 Method 1 [SN16 signal]: 
• Remove pulse generator from transmission.
•  Apply power to the speedometer (turn key on). 
• Attach a high speed drill or Dremel tool to shaft of pulse generator.
• Slowly spin shaft (direction does not matter) and increase speed until you notice the speedometer pointer move counter-clockwise.
• Stop spinning the shaft of the pulse generator and wait for the speedometer pointer to return to zero.
• Re-connect the pulse generator to the transmission.

Method 2 [Computer (ECM) signal]:
• Switch all speedometer dip switches to the closed position (pushed in on the number side)
• If available, connect ECM tach signal to SN74 speedometer signal interface INPUT.  (temporarily remove speed signal from input)
• Start engine and rev motor until speedometer pointer moves counter-clockwise.
• With engine still running, disconnect the ECM tach signal from the SN74 and wait for the speedometer pointer to return to zero.
• Turn off power to the speedometer and set speedometer dip switches back to the previous setting.  (also reconnect speed signal to SN74)
• If an ECM tach signal is not available, drive vehicle until the speedometer pointer moves counter-clockwise (may require you to be driving 60+ MPH)
• Stop vehicle (do not turn key off) and wait for the speedometer pointer to return to zero.
• Now turn off power to speedometer and set dip switches back to the previous setting.

Method 3 [Built-in Transmission Speed Sensor (VSS)]:
• Switch all speedometer dip switches to the closed position (pushed in on the number side)
• Drive vehicle until the speedometer pointer moves counter-clockwise (may require you to be driving 60+ MPH)
• Stop the vehicle and wait for the speedometer pointer to return to zero.
• Turn off power to the speedometer and set dip switches back to the previous setting. 


Tachometer wiring

• What type of ignition system do you have?  (HEI, standard points, MSD, ECM Signal, HEMI)
• Can you find the signal? 
• Diesel or gas engine?

Most HEMI engines don’t provide a tach signal.  A tachometer pickup from MSD or Autometer is usually required. 

Ram Jet engine computers have analog and digital tachometer signals.  Our tachometers require the DIGITAL signal.  [J1-14]

Tach filters are not required on Classic Instruments tachometers.  They should be removed if any problems are experienced with the tach as they are usually the weak link and go bad.

Diesel applications require a signal from the alternator (if available on the alternator) or from a mag. sensor on the flywheel.  If using a flywheel signal, a Dakota Digital DSL-1 is needed to convert the signal.  If using an alternator signal, it can be connected to a 3-3/8” tachometer only if it is a 12 Pole signal.  If using a 4-5/8” tachometer, the alternator signal may be used if it is a 10, 12 or 20 pole.  Otherwise, the Dakota Digital DSL-1 would be needed for the alternator signal as well.