Electrical Gauge Calibration

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Page Created by T June 13th 2013:

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Instrument Gauges. Click to Enlargen. Photo by Sam Roberts.

Electrical Gauge Calibration:

The Advantage Of Using Electrical Gauges:

Electrical Gauges can be easily calibrated by adjusting the resistances that drive them. They can also be mounted in any location and long distances from the senders without the concerns metal piping presents.

Causes Of Under Reading Gauges:

Make sure the full Alternator Voltage is getting to the Dash. Insufficient Voltage means the Gauges don't see enough Current and it will make them under read. Also, the Dash Regulator may have failed.

Electric Temperature Gauge. Photo by Red Seat. Click to Enlargen.

Alternator And Primary Feeds:

If the Alternator Feed or Primary Feed is burned, this can make your Gauges under Read. See the Dim Lights Page for details.

Chassis Ground Strap:

It is always a good investment to upgrade the Chassis Ground Strap to make sure there is an adequate return path for all the Electrical equipment. See the Dim Lights Page for details.

Resistance Substitution Wheel. The 470K (470,000 Ohms) value has been selected. Photo by T . Click to Enlargen.

Causes Of Over Reading Gauges:

There might be excessive Voltage from the Alternator so check +14 Volts at 1500 RPM under load (Headlights on). It may also be advisable to regulate the Voltage going to Electric Oil and Temperature Gauges with an MC78T12 (Jaycar CAT. NO. ZV1634) Regulator. This will ensure the Gauges never see more than +12 Volts and remain Stabilised. Also, the existing Dash Regulator may have failed.

HQ Holden Dash. Photo by Qute. Click to Enlargen.

Dash Regulator:

Some Gauges are powered from the Dash Regulator. If this fails it can make Gauges oer-read, inder-read or stop working altogether.

Voltmeters:

These often have adjustable Potentiometers inside them that need to be calibrated against a known Voltage. Use a quality Digital Voltmeter to measure the Battery Voltage and adjust the Dash Voltmeter to match.

Ammeters:

Ammeters that over Read can be fixed by adding a Length of thick Wire in parallel with the Meter. If the Ammeter uses a Shunt, make sure the Ammeter is connected to the innermost Connections on the Shunt (if there are 4 Connections) otherwise the Ammeter will over Read because it will see more Voltage per Amp than it should. The main Cables should be connected to the outermost Connections.

Ammeter Re-Scaling:

It is possible to re-scale some ammeters to allow them to display higher current by adding a resistor in series with the meter. The right value will cause the ammeter to read 1/2 the real value (20 amps will read as 10 amps). That means a 40 amp full deflection be actually be 80 amps. Mark the ammeter as x 2 or be aware that the rescaling has allowed it to display higher current than before.Use a Resistance Substitution Wheel to work out the correct value resistor to use. This can be done if the meter can be taken apart. The resistor is added into one of the small wires that run between the meter's shunt and the meter. Cut the wire and connect the resistor with one end of the wire to one end of the resistor, the other end of the wire to yhe other end of the resistor. The resistor can be connected either way around. It can also be done if the meter uses a shunt. The resistor is added into one of the wires that runs between the meter and the shunt. The resistor limits the meter current by an exact amount so that twice the current has to flow to cause the meter to dsiplay a reading. 

Electric Type Temperature and Oil Pressure Gauges:

Once the Alternator Feed, Primary Feed and Chassis Ground Strap have been replaced and the Alternator's output verified, the Electric Gauges can be calibrated.

Adding Resistance To The Sender Circuit:

A misreading Gauge can be corrected by adding a Resistor to the Sender Circuit . The Resistor is added in series if the Gauge is over reading or in parallel if the gauge is under reading.

Resistor Substitution Wheel:

One way to calculate the Resistance needed to calibrate the Gauge is to use a Resistance Substitution Wheel  from Jaycar. Once the Resistance is known, you replace it with a single Resistor of the same value. Start with the Wheel on the highest value (1M) and turn it down one click at a time until the Gauge reads correctly. The Wheel has Resistors that are 1/4 Watt so don't leave it connected too long or it might burn out.

Multiple Resistor Method:

Rather than buying a Resistance Substitution Wheel, multiple Resistors can be used to calculate the required Resistance.  Connecting multiple Resistors in parallel reduces the total resistance. Connecting multiple Resistors in series increases the total resistance. See the tables below for Values.

Fixing Under Reading Electric Oil Pressure Or Temperature Gauges:

Electric type Temperature and Oil Pressure Gauges that under Read can be Calibrated by adding a Resistance in parallel with  the Sender. Connect the Resistance between Ground and the Sender Wire. This will increase the Current flowing through the Gauge to the right Level.

Worked Example 1:

Connect a 33 Ohm 1 Watt Resistor between Ground and the Sender Wire if the Temperature Gauge in an HZ under Reads. Such a Resistor exists inside the Gauge and can burn out over time. Fitting the Resistor is a lot easier and cheaper than pulling the Dash apart.

Fixing Over Reading Electric Oil Pressure Or Temperature Gauges:

Electric type Temperature and Oil Pressure Gauges that over Read can be Calibrated by adding a Resistance between the Sender and the Gauge. This will reduce the Current flowing through the Gauge to the right Level.

Worked Example 1:

The Temperature Gauge over Reads. Add Resistance in series to decrease the current flowing through the Gauge. You do this with $3 worth of 1 Watt 100 ohm Resistors. Fit One (100 Ohm) Resistor between the Sender and the Gauge. If the Resistance proves too large (the Meter heavily under reads) then twist 2 x 100 Ohm Resistors together in parallel (50 Ohms) and add the Pair in Series into the Circuit. If still too large, (the Meter heavily under Reads) then twist 3 x 100 Resistors together (33 Ohms) and add them in Series into the Circuit. If still too large, twist 4 together (25 Ohms) and add the 4 Resistors in Series into the Circuit. If still too large (the Meter under Reads, but even less so) twist 5 Resistors together in parallel (20 Ohms) and add them in Series into the Circuit, spot on. Replace the twisted mess with a 22 Ohm Resistor worth 20 Cents, problem solved. The 2 Ohms Difference being imperceptible.

Worked Example 2:

The Temperature Gauge over Reads. Add series Resistors to increase the Resistance. You do this with $3 worth of 1 Watt 100 ohm Resistors. Fit One (100 Ohms) Resistor between the Sender and the Gauge. If the Resistance proves too small (the Meter heavily over Reads) then twist 2 x 100 Ohm Resistors together in Series (200 Ohms) and add them in Series to the Circuit. If still too small, (the Meter heavily over Reads) then twist 3 x 100 Ohm Resistors together (300 Ohms) and add them in Series  to the Circuit. If still too small, twist 4 together (400 Ohms) then twist 4 x 100 Ohm Resistors together (400 Ohms) in Series to the Circuit. If still too small (the Meter under read, but even less so) then twist  5 x 100 Ohm Resistors together (500 Ohms) in Series to the Circuit. spot on. Replace the twisted mess with a 510 Ohm Resistor worth 20 Cents, problem solved. The 10 Ohms Difference is imperceptible.

Worked Example 3:

The Temperature Gauge under Reads. Add Resistors in Parallel to reduce the Resistance. You do this with $3 worth of 1 Watt 100 ohm Resistors. Fit One (100 Ohms) Resistor in Parallel with the Sender to Ground. If the Resistance proves too large (the Meter still under Reads) then twist 2 x 100 Ohm Resistors together in Parallel (50 Ohms) and add them to the Circuit in Parallel with the Sender. If still too large, (the Meter still under Reads) then twist 3 x 100 Ohm Resistors together (33 Ohms) in Series and add them to the Circuit in Parallel with the Sender. If still too large, then twist 4 x 100 Ohm Resistors together (25 Ohms) in Series and place them in Parallel with the Sender. If now too large (the Meter under reads slightly) then twist  5 x 100 Ohm Resistors together in Parallel with the last Resistor which makes (20 Ohms). The Circuit is now spot on. Replace the twisted mess with one 22 Ohm Resistor, worth 20 Cents, in Parallel with Sender. Problem solved. The 2 Ohms Difference is imperceptible.      

  Parallel Resistance  
Resistor Value
 Number of Resistors Total Resistance
 100 Ohms 1  100 Ohms
100 Ohms  2 In Parallel  50 Ohms
100 Ohms 3 In Parallel  33 Ohms
100 Ohms 4 In Parallel  25 Ohms
100 Ohms  5 In Parallel 20 Ohms

   

   Series Resistance
 
 Resistor Value  Number of Resistors
 Total Resistance
 100 Ohms  1  100 Ohms
 100 Ohms  2 In Series  200 Ohms
 100 Ohms  3 In Series  300 Ohms
 100 Ohms  4 In Series  400 Ohms
 100 Ohms  5 In Series  500 Ohms

 

I had to install a 33 ohm resistor in line with the sender to get my electrical temp gauge reading right. It was about 12 degrees too hot End of submission by 1963EJ.

Links:

Dim Lights 

Temp Gauge Repair

Dash Regulator Info

Ammeter

Terms:

Terms 

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