I am working on a system from 1986. It has an old-fashioned 12VDC relay like the one in the picture.

When the circuit powering the relay coil is turned on, the relay does not switch on. When I unplug the relay and then plug it back in, with the circuit still on, it flips on.

While testing I found that the relay coil is only getting about 11.1-11.5VDC of the 12.5 that it should have (likely caused by corroded wires.)

Could this lower voltage be the issue? If so, why does taking it out and putting back in cause it to switch on?

enter image description here

  • \$\begingroup\$ A data sheet of the relay is required. \$\endgroup\$
    – Andy aka
    Feb 21, 2023 at 12:24
  • 1
    \$\begingroup\$ Perhaps the force of plugging it back in temporarily improves conduction through those corroded connectors? \$\endgroup\$
    – rdtsc
    Feb 21, 2023 at 13:39
  • \$\begingroup\$ Bad contacts in socket. \$\endgroup\$
    – user263983
    Feb 21, 2023 at 13:58
  • \$\begingroup\$ how is the voltage turned on? if it comes up slowly it could be the reason. \$\endgroup\$
    – danmcb
    Feb 21, 2023 at 14:19
  • \$\begingroup\$ Is it a latching relay with bipolar drive? Normally 2/3 of rated is sufficient, more is faster. Never assume we know , what, where , how , in a question and include all details or link to specs, also is it repeatable and record all voltage states. \$\endgroup\$
    – Hoagie
    Feb 21, 2023 at 16:44

1 Answer 1


You need to provide a datasheet, and confirm it is a simple SPNO relay, but assuming it is, I would guess either rust on the pivots or contact damage.

When the circuit is initially on, and the relay still open, have you tried giving the relay can a few sharp taps with something metal (a.k.a. "percussive maintenance")? You might well find it flips.

You could also use a screwdriver to unbend the crimps which hold on the lid, and see what is actually happening inside. I had a similar problem with a glowplug relay on an outside diesel generator, and it turned out moisture over the years had cause enough rust to make the hinge temperamental.

  • \$\begingroup\$ I am not very well-versed in the nomenclature, but it is a normally open relay. I will try the percussive maintenance and see if it flips. If I can find another relay I will try it and see if it is a problem with the relay or the power supply. \$\endgroup\$
    – Jayj48
    Feb 23, 2023 at 1:11
  • \$\begingroup\$ Did you ever try this, and how did it go? It would help others if you tell us what the solution was. If it was rust, mark this question as answered. \$\endgroup\$
    – colintd
    Sep 10, 2023 at 7:19

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.