elec. fuel pump relay wiring diagram

BillGrissom said

If you get it backwards, and there is a diode, the coil won't actuate w/ 12 V since the current will flow thru the diode. If you do it very briefly, you may not damage the diode. You can hear the relay click. To tell ahead of time if there is a diode, measure resistance w/ a multimeter, and you should measure low one way (<100 ohm?) and high the other way (>1000 ohm?). But, should be a symbol on the relay if it has a diode. When a diode fails, it usually fails shorted.

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I do not agree with diode test with Ohm meter. Typical Ohm reading applied voltage, is less than a diode forward junction, so the diode appears invisible. The diode setting on a multi-meter is able to supply up to about 3 V open circuit, but the coil in parallel will block the reading, because of the current limit in force circuit. As suggested look at diagram on relay.
The suggestion to quickly test by applying voltage a short time is also unrealistic. The forward voltage maximum is only 1V, applying 12V will result in diode failure, before the relay clicks.
A current limited 12V supply of about 250mA, would enable safe testing. The correct current reading would be the lesser for correct polarity. Current varies, but most are between 75 and 150 mA. Incorrect polarity will hit the supply limit of 250mA, yet protect diode short term.