Relay - Contact Resistance Test Procedure
Bulletin: P92-007Section: Product
Dated: 4/2/92
Applicable to: USA
CANADA
SUBJECT:
RELAY TEST PROCEDURE - CONTACT RESISTANCE CHECK
The following information, specifications and test procedure should be used to determine relay contact condition if required during electrical troubleshooting.
BACKGROUND
The traditional method of using an ohm-meter to check relay contact or point resistance does not take into account the current which normally flows through the contacts when the relay is energized. This can lead to problems with a circuit even though the contacts checked OK with an ohm-meter.
The only effective test involves having the actual circuit current flowing through the contacts then measuring the voltage drop through the relay.
SPECIFICATION
The maximum allowable voltage drop across relay contacts in automotive application is 0.025 Volts per Ampere at 12-14 Volts.
Example 1:
A circuit which draws 10 amps should have a maximum voltage drop of 0.25 volts since 10 amps x 0.025 volts = 0.25 volts.
Example 2:
A circuit which draws 18 amps should have a maximum voltage drop of 0.45 volts since 18 amps x 0.025 volts = 0.45 volts.
The actual voltage drop will normally be substantially less than the maximum with a relay in good condition.
- Switch on and off the circuit to be tested six to ten times to remove any contact oxidation then leave on.
- Using a digital multi-meter measure the voltage at relay terminal 30 (load "in" terminal) and then at terminal 87 (load "out" terminal). Keep your black meter lead grounded to the same point during the test.
NOTE:
Some circuits are designed with 87 as "in" and 30 as "out." This has no bearing on the test.
- Subtract the smaller from the larger value - this is the actual voltage drop across the relay.
- Measure the operating current by series connecting a multi- meter in the circuit.
NOTE:
Make sure your meter is capable of handling the expected load.
- Compare your results with the specification.
Example:
Voltage at Terminal 30 = 12.85V
Voltage at Terminal 87 = 12.70V
Voltage difference (drop) = 0.15V
Measured current in circuit = 5.8A
From the specifications, maximum voltage drop for this circuit should be 5.8A x 0.025V = 0.145 volts.
Diagnosis: Replace the relay and re-test since the actual voltage drop of 0.15V exceeds the maximum spec of 0.145V.
Relays used in a ground switching application such as the cruise control neutral lockout relay are tested in the same way. Just remember to switch the polarity of your multi-meter leads.