The Stovebolt Site Forums The Shop Area The Electrical Bay Basic relay diagnosis and test.

I have a early 60's three prong relay I am suspect of and wonder if someone can tell me a method whereby I can bench test it. I follow directs well but do not know much about the dark side. A continuity tester across any combination of the 3 terminals does nothing. One of the three terminals is common to the metal housing of the relay as it shows continuity. Appreciate the help. I was thinking I could put power to this and perhaps hear it and watch it on a meter?

rasman,

To my knowledge, most automotive relays would have four 'prongs'.

I am wondering if your three terminal relay is actually a horn relay. The power would be applied to either of the terminals that had no continuity with each other or the case. When the terminal with continuity to the case was momentarily connected to battery ground, the relay would close.

There are three terminals.

Terminal 1 and 2 are the normally open contacts.

Terminal 3 is the ground for the coil (pull-in).

Power is supplied to the coil from a internal jumper from terminal 1 or 2. You can't see it as it's inside. Power must be present on the terminal that is internally jumpered...the other becomes energized when the relay closes.

If you apply power to one of the terminals NOT common with the case and touch the terminal that is common with the case to battery ground the relay should click. If it doesn't, try applying power to the other terminal NOT common with the case.

If the whole things blows up, blame your neighbor. Good luck.

Stuart

If it's from the 60's it is probably a horn or maybe a headlight relay. That's about the extent of relays for those years.

Stuart
Okay, I will apply 12V to one of the terminals not common with the case and place the relay case on Battery ground. Is the third terminal the continuing line that runs out to the motor that needs current to run? Thanks Ralph

Just a question and comment...you state one terminal is common with the case. Do you mean zero ohms in common with the case? Is there some resistance from this terminal to the case? If so, then these two connections are the 'coil' for the relay and where you would apply batt. power for testing. You can then use your ohm meter to check for N/O or N/C condition on the other terminals. Hint: Know your test equipment! If using an analog ohm meter like a Simpson 260 you must know how to 'zero' the meter to get good ohm readings. Stuart could also be correct, but then that's why they invented fuses. Good luck.

When I put my little cheapo meter on OHM and then zero the gauge while putting both probes together. I then touch the combination of the prongs on the relay. The needle pegs all the way from left to right at 0.

The to confuse me even more, I take an identical used relay from the parts box and this one tests different, with the meter pegging over to right (0 again ) between two of the prongs, but not any prong and the case.

Then I used my little continuity probe thingy with the little handle that lights up when touching the same two prongs that peg the OHM meter ,,, NADA, does not light. The other relay with the prong and case, it does light up. You guys that understand this electron stuff are what make the world go round. My simple wrench turning self likes to systematically sort things out but this stuff is black magic. Ralph

Ok, this is getting confusing to me. You do have a like relay from your parts box right? Why don't you just replace the 'iffy' relay with the spare one. Test your circuit and if it does as it should...problem solved. After many years of blowing fuses I know substituting like components is an acceptable troubleshooting procedure when two like components such as relays don't 'test' the same.

Ralph,

You're working your meter correctly. I'm getting a tad lost myself. Because this relay has only three terminals I am assuming it is a horn relay.

There should be continuity between two of the terminals, none to the case, and none to the third terminal until the relay closes.

This would be for a typical NO (normally open) horn relay. A three 'prong' relay isn't your typical relay. I tried to find a picture of the insides of one as it would make it perfectly clear.

The terminals can be thought of as, power in, power out and ground for coil. When the coil gets grounded, the relay closes and power flows out the 'power out' terminal.

Pushing the horn button is what supplies ground to the coil. I don't know of any other device on your rig that could be powered by a three terminal relay.



Stuart

Beltfed, you are right.....of course both of these do not work so I was trying to sort out why they are different. I guess they both are broke different.

I decided to test the circuit with the relay with advice I got from here and lo and behold jumpers allowed for power and function. Apparently I was keeping two junk relays. Which proves my wife right in this one isolated rare instance. BOX OF JUNK! Thanks you guys.

What sort of circuit are you trying to operate?
Jerry

I raided a relay from a '70's Cadillac A/C for my electric fan. I pried the cover off to see which terminals are what.

This was an added on blower/heater with a three terminal relay from a mid 60's GM.

The small square relays made by Bosch and several other manufacturers are very handy to use, and they have the added advantage of being available at any parts store, not obsolete and scarce like the 40-year-old ones from junkyards. There are two basic types, 4-pin relays that just have two normally-open contacts and two coil terminals, and the 5-pin types that have both normally-open and normally-closed positions for the load contacts. Many of them have wiring diagrams molded right into the relay housing.

An added advantage to the little Bosch relay is the fact that connections can be made with regular female blade-type connectors, and the plastic case doesn't have to be grounded.
Jerry

Relay holders can be bought from Dell City wire as well as the relays. They attach with 1 screw or if you want to use quite a few relays, late model International trucks use banks of about 10 relays. They are in truck salvage yards and are usually crushed with the cab.