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Pelican Technical Article:

Testing Relays

Tom Morr

Time:

30 minutes30 mins

Tab:

$5 to $9

Talent:

**

Tools:

Multimeter, flatblade screwdriver (2)

Applicable Models:

Mercedes-Benz R170 (1998-04)

Parts Required:

Replacement relay

Hot Tip:

The relay can be taken out of its housing and visually inspected if a multimeter isn't available

Performance Gain:

Electrical accessories work again

Complementary Modification:

Check and replace fuses

Relays use a low-current load to control a high-current one. The most common example is headlights: the switch sends power to the relay, which has an internal coil that steps up the juice.

Most relays have an internal electromagnetic switching system. When the relay's coil receives power from the switch, the resulting electromagnetic field moves a contact arm to complete the circuit, sending the high-current load to the accessory: headlight, Vario top hydraulic pump, power steering, and so on. Among other benefits, relays allow smaller gauge wire to be run to the switches that control them.

Standard Bosch-type SPST (single pole/single throw) cube-shaped automotive relays normally have either four or five leads/pins on their bases. Four-pin relays are the most common. On these, the low-current pins are normally parallel to each other and the high-current ones are perpendicular: Lead 86 receives low-current power, lead 85 is the ground, lead 30 receives constant power, and lead 87 is the switched high-current output. On five-pin SPDT (single pole/double throw) relays, an extra pin, numbered 87a, is connected to the constant power-in "30" lead when the coil isn't energized.

If an electrical accessory that's fed by a relay isn't working, first check the fuse (or, in the case of lights, the bulbs). The circuit's constant-power wire should have a fuse somewhere between the power source and the relay to protect against overloads and shorts. If the fuse is good, then check the relay. Healthy relays normally make an audible click when their coils are energized, when the internal arm makes contact and transfers the high-output current.

To summarize: no click often means a bad relay; a click means that the relay is probably good but should still be tested in order to eliminate it as a cause of the electrical gremlin.

Standard 4-pin relays use pins 85 and 86 (parallel to each other, indicated by arrows) for low current and pins 30 and 87 for the coil-enhanced high current.
Figure 1

Standard 4-pin relays use pins 85 and 86 (parallel to each other, indicated by arrows) for low current and pins 30 and 87 for the coil-enhanced high current.

Small flatblade screwdrivers can be used to pry the relay out of its plastic housing if you want to visually inspect the internals.
Figure 2

Small flatblade screwdrivers can be used to pry the relay out of its plastic housing if you want to visually inspect the internals.

Inside the relay, the coil converts the incoming low-current voltage to higher current.
Figure 3

Inside the relay, the coil converts the incoming low-current voltage to higher current. When the coil is powered, it electromagnetically moves the contact arm (arrow). This completes the circuit, outputting high-current power to the accessory.

Relays can be checked using a car battery or other 12-volt DC power source (such as a battery charger) and a multimeter.
Figure 4

Relays can be checked using a car battery or other 12-volt DC power source (such as a battery charger) and a multimeter. Here, the 12-volt power source is only connected to one of the low-current pins (red test lead). The multimeter shows infinite continuity, indicating that power isn't being transferred between the high-current pins.

With the green test lead connected to complete the low-current flow and energize the coil, this relay shows continuity between the high-current pins.
Figure 5

With the green test lead connected to complete the low-current flow and energize the coil, this relay shows continuity between the high-current pins. The relay is functioning properly: The contact arm moved when the coil was energized, and the power transferred to the high-current pins. If the high-current pins aren't transferring power, check that the low-current ones have continuity when the 12-volt power source is connected to them.

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el feo Comments: my intruments gages after the engine gets warm they stop working can you help?
September 25, 2016
  Followup from the Pelican Staff: Start by checking the vehicle for fault codes. The cluster may be faulty. - Nick at Pelican Parts  

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