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Precision Speed Instruments, Inc


2022 W Clarendon Ave
Phoenix, AZ 85015
 
Phone: 602-973-1055
Fax: 602-242-8577
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Troubleshooting Guide

Resistance Senders for Electrical Gauges

The following Troubleshooting guide is intended to aid in

diagnosing potential problems with Resistance Senders for Electrical Gauges

. There are two fundamental types of Resistance Senders for Electrical Gauges based on their resistance gradient. All

Datcon Match senders

(and all temperature senders) have NEGATIVE resistance gradients. Thus this Troubleshooting Guide is specifically intended to apply to Negative resistance gradient Datcon-Match Senders.
The following troubleshooting procedure is presented in a specific sequence intended to minimize redundant effort. Failure to follow this sequence could result in miss-diagnosis of possible problems.
1.) Without power applied to the gauge, disconnect the wires connected to the Sender (S) and Ground (G) threaded stud terminals of the gauge. Connect an Ohmmeter (such as a Digital Multi-Meter) between the Sender and Ground wires. You should get a reading between 30 and 450 Ohms. Typical Sender resistances for various readings are as follows:
a) Fuel Level - "E" = 240 Ohms, "1/2" = 103 Ohms, and "F" _ 33.5 Ohms_ Actual reading will depend on the amount of fuel in the tank.
b) Oil Pressure - 0 PSI = 240 Ohms, 1/2 scale (such as 40 PSI on an 80 PSI Full Scale Sender) = 103 Ohms, and Full scale = 33.5 Ohms_ Obviously with the engine OFF you should get a reading of about 33 Ohms for 0 PSI.
c) Temperature - There are two "Datcon Match" Temperature Senders_ Typically the "Datcon-High" Sender is 1,000 Ohms at 100F to 87 Ohms at 220F while the "Datcon-Low" Sender is 450 Ohms at 10OF to 46.4 Ohms at 220F. Assuming the engine is cold when the measurements are taken, the Senders should read somewhat higher resistance than their respective 100F resistances. NOTE: If the gauge was powered just prior to performing this test, the current through the sender/gauge circuit will cause some internal power dissipation within the sender_ This power dissipation (heat) will - in turn - cause the sender s resistance to drop somewhat (typically 10 - 20%) , and this factor should be taken into consideration when analyzing the senders resistance readings.
If the Senders resistance is within the expected typical ranges, the sender is OK_ Check connection continuity when re-wiring the Sender and Ground wires to the gauge.
2_) Apply power. The gauge's pointer should MOVE. If OK, go to step 3. If not.....
a) Power may not be reaching the gauge due to a fault in the gauge's wiring circuit_ Measure the VOLTAGE between the Ground terminal (G) and the Ignition terminal (I) of the gauge (Note - Make sure that the test voltmeter is connected to the gauge's threaded stud terminals, not the wires or any crimp terminals that may be present)_ Rated power supply voltage should be present_ If not, troubleshoot the wiring circuit to determine the cause for no power. If power (rated voltage) is present at the threaded stud terminals, go to step 3b_
b) If power (at rated voltage) is present, check the voltage on the Sender (threaded stud) terminal (S) It should measure some voltage between OV and the rated voltage measured on the Ignition terminal _ If the voltage on the "S" stud terminal is 0 Volts or exactly the same as the voltage on the "I" stud terminal, the gauge is internally open-circuited and defective - replace the gauge. If not, go to step 3c.
c) Momentarily short the "S" stud terminal to the "G" Shorting "S" to "G" should cause the pointer to go CW off-scale. If the pointer doesn't move, the gauge is defective - replace the gauge. If the pointer does move as indicated above, the gauge m be OK, Go to step 3.
3_) With power removed, disconnect the wire connected to the Sender (S) threaded stud terminal_ Re-apply power_ The gauge should go CCW off-scale. If it does, go to step 4. If not, momentarily short the "S" stud terminal to the "I" stud terminal.
CAUTION! The Sender wire MUST be disconnected for this test! If it is not disconnected, severe damage to the sender could occur due to the direct connection of system voltage across the sender!
Momentarily shorting the "S" stud terminal to the "I " stud terminal should cause the pointer to go CCW off-scale. If not, and the pointer still doesn't move, the gauge is defective - replace the gauge_ If the pointer moves, go to step 4.
4_) By reaching this step in the Troubleshooting Guide you have determined that the gauge basically functions correctly_ At this point gauge calibration can be checked by connecting a "simulated sender" to the gauge. In step 3 you disconnected the sender wire from the Sender (S) threaded stud terminal.. . . leave it disconnected for this calibration-check test. A "simulated sender" can be one of the following.....
a) A spare "Datcon Match" Fuel Level Sender (NOTE: A Datcon Match F.L.S. can be used to check any Datcon Match Fuel Level, Temperature, or Oil Pressure Gauge).
b) A Variable Resistor (Trimmer, Rheostat, or Potentiometer) having a resistance range from <30 Ohms to >250 Ohms such as a 0-250 Ohm or 0-500 Ohm, 1/2 Watt Potentiometer_
C) A pair of fixed value 5% tolerance 1/2 Watt resistors, one with 33 Ohms and the other 240 Ohms.
d) A Decade Resistor Test Box with a range of <30 to >240 Ohms, 5% or better accuracy, and capable of dissipating 1/2 W.
If "a" and "b" is used as a "simulated sender" it will also be necessary to have an Ohmmeter (such as a Digital Multi Meter or DMM) available to confirm resistance values_ Remove power and connect the "simulated sender" between the "S" and "G" stud terminals_ Re-apply power_ A "simulated sender" resistance of 240 Ohms should cause the gauge to read approximately "E" (Fuel), "0 PSI" (Pressure), or about 1/3 scale on Temperature gauges. Similarly a "simulated sender" resistance of 33 Ohms should cause the gauge to read approximately full scale_ NOTE: This test is not a precision calibration test which must be done under carefully controlled test conditions, with calibrated sender resistances, and per specific ascending and/or descending pointer travel criteria. However the gauge should function generally as noted above. If it does, the gauge is OK_ If not, the gauge should be removed and its calibration checked and verified via a precision calibration procedure.
5_) Other checks:
a) During normal operation the pointer should move smoothly from its unpowered position to its powered position and visa versa. If the pointer jumps rapidly, overshoots, and moves back and forth around its rest position in a dampened oscillation, it is likely that the dampening fluid has leaked out. Replace the gauge.
b) If the pointer hesitates, stops/starts, or shows any other evidence of sticking, the bearings may be worn or contaminates may be present within the movement. Replace the gauge_
c) If pointer moves EXTREMELY slowly during tests 2c and 3 there may be physical damage to the pointer shaft, bobbin, or bearings. Replace the gauge.
d) Remove power and re-attach the sender wire to the Sender threaded stud terminal (S) and check operation with the system sender. If the gauge fails to operate correctly, it is likely that there is a problem in the sender or in the circuitry / wiring to the sender_ Refer to the Troubleshooting Guide for Senders.
Troubleshooting Guide

Electrical "Sender-Type" Gauges

The following Troubleshooting guide is intended to aid in diagnosing potential problems in electrical "Sender-Type" gauges such as Fuel Level, Oil Pressure, and Temperature gauges. There are two fundamental types of electrical "Sender-Type" gauges. These two types are based on whether the gauge is intended to function with a NEGATIVE or POSITIVE resistance gradient sender. "Datcon Match" senders, whether Fuel Level, Oil Pressure, or Temperature, are NEGATIVE resistance gradient gauges. Thus this Troubleshooting Guide is specifically intended to apply to gauges intended for use with Datcon-Match Senders.
The following troubleshooting procedure is presented in a specific sequence intended to minimize redundant effort. Failure to follow this sequence could result in miss-diagnosis of possible problems.
1.) Without power applied the gauge's pointer should rest at the Counter Clockwise (CCW) end of its scale. It may rest slightly above the minimum dial scale graduation, but will more likely rest at some point below minimum. If OK go to step 2. If not.....
a.) Pointer stuck at the full CW position (possibly wedged under the mask). Lightly tap the gauge to dislodge the pointer. If it remains stuck, replace the gauge. (Note that a pointer stuck at the full CW position can only be caused by a fault somewhere else in the gauge circuit, such as incorrect wiring, a short circuit between the sender terminal (S) an the ground terminal (G) of the gauge (or elsewhere in the wiring between the sender and the gauge), or in the sender itself. Thus a sticking pointer should be viewed as a warning to continue the troubleshooting effort to find out WHY the pointer was driven so far off-scale.
b) Pointer resting at some point well above minimum dial scale reading, but not stuck at full CW (off-scale). Replace gauge. This is an abnormal condition that should not occur and could only be caused by a fault within the gauge or damage to the gauge caused by the application of an excessively high input (power) voltage or subjecting the gauge to extreme environmental abuse. Once again the cause of the failure should be investigated.
c) Pointer stuck at the full CCW position (possibly wedged under the mask). Refer to step 1a. However in this case the cause may be a short between the sender terminal (S) and the Ignition (+ Battery) terminal (I) of the gauge or elsewhere in the wiring between the sender and the gauge).
If the resistance reading is very low (less than 20 Ohms) or very high (more than 300 Ohms for Fuel Level and Pressure Senders, more than 900 Ohms or 2,000 Ohms for Datcon Low or High Match Temperature Senders respectively), a short or open circuit in the wiring or sender may be present - go to step 2.
2.) With the sender wiring still disconnected from the gauge, disconnect the other end of the sender wire from the Sender itself. Measure the resistance between the sender wire and ground. Assuming that there are no other connections to this wire, you should get an open circuit (at least several megohms) reading. If not, there is a fault in the wiring circuit - troubleshoot and repair. If OK, go to step 3.
3.) Using a suitable extension wire measure the resistance or "continuity" from the gauge end the sender wire to the sender end of that wire. You should get a very low Ohms (short) reading. If not, there is an open circuit fault in the wiring - troubleshoot and repair. If OK, go to step 4.
4.) Measure the sender's resistance (to ground) at the sender. Refer to the list of typical sender resistances in step 1. If the resistance reading is within the expected typical range, the Sender is OK. If not, the sender may be defective - remove, inspect, and replace if necessary.