How to test a relay?
 

How do I test a relay...?

I think the red relay for the fuel pump is acting up, and I want to test it. Do I clip the positive lead to either 30 or 87a, and the negative to the other, and then test the resistance? If so, what should the resistance value be?

I do appreciate your knowledge.

dan crockett

Dan,

The relay coil should measure 70 - 80 Ohms between terminals 85 and 86. The resistance across operated contacts should be less than 0.5 Ohms.

The best test is a functional one ... swap with the horn relay, and see if your red relay works in that circuit!

Be sure to check the relay socket for corrosion ...

Warren...

I did swap it out for the horn relay...which is in there right now. Problem is, the red relay works for the horn, and so I replace it in it's proper spot, and the car starts, and then a few days later, the car won't start after a short or long run (seems to make no difference) and there's no gas to the injectors, and I swap and it works, and around it goes.

I appreciate the specs, however.

Thanks,

Dan Crockett

I work for one of the world's largest relay manufacturers, and we get a lot of customers who ask this question.

Your method of energizing the coil and then checking the resistance is sound, in theory, but I highly recommend putting a small load (a lightbulb for instance) across the contacts and measuring the current instead (light will come on).

The reason being, there is a minimum amount of energy required to push the electrons across the (usually) somewhat corroded and oxidized contacts of a relay. The voltage supplied by your multimeter will be insufficient for this in some cases. I’ve seen brand new relays show infinite resistance on a multimeter when the contacts are closed.

Good luck!

Pull the relay from its socket and use a dentist's pick or a knife blade to force the slit sections of the cylindrical socket contacts apart to ensure a better contact. Have a look at the contacts in the socket too and clean if necessary. Add a light coating of dielectric grease on the relay's pins and reinstall. If problems persist, you can cut the top of the relay off at its base and check the internal contacts as well. They're fairly simple devices.

ianc

Thanks ianc...I'll try all that.

Dan Crockett