electrical relays

Gregster613 · Jan 6, 2022

James, Thanks for the definitive explanation. I don't think I'll go that far; my luck I'd sink the ship!

Dale, Once again you come through with an easy to follow diagram to simplify a question. Thanks once more. Once I figure out how to bookmark anything I'll do so for future references.

LD1, You are correct, a Flasher Unit is cheap and will be much simpler to just replace than spend $ on parts and pieces to construct something that I may never again need to use. Thanks for the insight and thought.

LittleBill21, Thank you for assisting me in showing people that a relay tester is indeed in the marketplace and sold. I probably just had the wrong component plugged into it and that's why it was sparking. So I butchered up a piece of test equipment needlessly.

Oh well, I can always get another. I bought it just for the purpose of testing the glow plug relay if I ever needed to do so. I haven't needed it for that yet and hopefully won't. Although I now see that it's maybe not even a relay but rather a glow plug "Timer". It appears that proper wording is critical with these components. In my defense and lifetime though, I have heard the flasher called a relay before.

My thanks to all who've chimed in to help this electrical dufus get his vocabulary straight. I am quite embarrassed to having admit I'm a dufus in this respect thus I'd really rather put this issue to bed now that I'm better educated on wording. Don't worry, I'll probably have another dumb question for you all to chuckle over whilst educating me again in the future. I do now know something about electrical that I didn't before. Greg

LittleBill21 · Jan 6, 2022

3 pin are generally rare. there is usually a post that is meant to be mounted to ground, this removes the 1 pin needed for coil. there would be generally be hot side coil and then the load, and the battery terminal. sending 12v to the hot side coil, generally activates the coil

Rebeldad1 · Jan 6, 2022

cheapest ... quick and easy...way to check a relay is to swap it for another that's the same.

most vehicles or equipment that have relays have more than 1.

Oz_Kioti · Jan 6, 2022

One instance I had where a multimeter caused me a wrong diagnosis:

The airconditioner on the (previous) tractor stopped putting out cold air. OK then, check that the electrically operated clutch on the compressor was getting 12v. Yep, multimeter registered 12v when ac knob rotated to the ON position. (Engine not running at this time)

So - problem must lie elsewhere, I reasoned.

Turns out that the 12v it was getting was UNDER NO LOAD, but when the engine was running and you asked the clutch to engage, the 12v source could not supply enough current, due to a dodgy connection somewhere upstream.

This was confirmed when I got a "tester pen", which utilised a filament tube (light) which drew some real current when applied to the test point. it didn't light up, even though the multimeter indicated 12v.

So I tracked back through the wiring, found the dodgy connection, and everything was cool again!

WranglerX · Jan 6, 2022

Now there are relays..... This was very popular in early telco crossbar switches . Until most everything went solid state.... Spent many a hour with close association with these. In comparison the average relay found in car/truck/tractor applications are very simple....

LD1 · Jan 6, 2022

LittleBill21 said:
3 pin are generally rare. there is usually a post that is meant to be mounted to ground, this removes the 1 pin needed for coil. there would be generally be hot side coil and then the load, and the battery terminal. sending 12v to the hot side coil, generally activates the coil

A three pin can be a possibility, though I haven't personally seen one

But modern stuff switches the ground.

Meaning that the 12v supply feeding the coil and the 12v supply feeding the high current load "can" be one and the same.

Single 12v supply, capable of handling the current of both the load and coil.

Then the output to the load, and the coil ground side going to the switch.

Not very many "hot" wires on cars anymore with switching the grounds.

LittleBill21 · Jan 6, 2022

Oz_Kioti said:
One instance I had where a multimeter caused me a wrong diagnosis:

The airconditioner on the (previous) tractor stopped putting out cold air. OK then, check that the electrically operated clutch on the compressor was getting 12v. Yep, multimeter registered 12v when ac knob rotated to the ON position. (Engine not running at this time)

So - problem must lie elsewhere, I reasoned.

Turns out that the 12v it was getting was UNDER NO LOAD, but when the engine was running and you asked the clutch to engage, the 12v source could not supply enough current, due to a dodgy connection somewhere upstream.

This was confirmed when I got a "tester pen", which utilised a filament tube (light) which drew some real current when applied to the test point. it didn't light up, even though the multimeter indicated 12v.

So I tracked back through the wiring, found the dodgy connection, and everything was cool again!

in this case a voltage drop test would have confirmed it, with a volt meter.

k0ua · Jan 6, 2022

DL Meisen said:
Now there are relays..... This was very popular in early telco crossbar switches . Until most everything went solid state.... Spent many a hour with close association with these. In comparison the average relay found in car/truck/tractor applications are very simple....

Spent some time cleaning those nasty things in the early part of my career. Before everything was solid state. Didn't like it much. We only had two of those PBX's left. Both were NEC (Nippon Electric Corp.) One was the small NA120 and the other was the NA409

quarencia · Jan 7, 2022

I don’t understand the comments about 3 pin relays being rare. They’re quite common. They’re all over the place. Just Google it. Flasher relays, horn relays, etc. Used all over the place in automotive, industrial, electronics, appliances, etc. In the general case, a three pin relay is just a Single Pole Single Throw (SPST) relay in which the load and coil share a common pin (See picture below). I’ve seen & used both NO and NC varieties. The advantages of a three pin relay are lower relay cost and you only have one common/ground connection out of the relay. Three pin relays are used less often than 4 and 5 pin relays so you don’t see them much in aftermarket because manufactures churn out 4 and 5 pin at such low cost. But for an OE buying large quantities, when a 3 pin can be used, it lowers manufacturing cost.

As for a relay tester, I suppose you could have one for the standard automotive relay form factors (Standard ISO, mini ISO & Micro ISO terminal footprints), but there’s an almost infinite variety of relay footprints out there and I don’t think a universal relay tester would be practical. Even in the ISO relay footprints, there are thousands of non-standard internal wiring configurations that would be cost prohibitive to build a tester to test them all.