Starter relay and huge power consumption?

[Commander Kinsey]

I bought a (new, aftermarket replacement) starter relay for a vintage Vauxhall (GM/Opel) (which was mis-sold as a starter solenoid, but nevermind) with the intention of using it for another purpose, a heavy duty relay. But I find it uses 5 amps at 12 volts to power it! Clearly, dissipating 60W of heat in it won't take long to kill it and it's only intended to operate for a short period. The question is though, why does it use so much power? I've just bought a relay that takes 200A (so similar contacts) that's for continuous use, at half the price and will presumably not use that much power. Why is the first one so inefficient?

[Dean Hoffman]

How do you know it takes five amps to power it? Is there a mistake somewhere in the specs?

[Commander Kinsey]

Because while determining which connection was which, my meter showed 4 ohms which concerned me. So I connected it to a 12V source in series with an ammeter. It uses 5 amps continuously and makes a very loud clunk when switched off and on, as loud as a solenoid would. It's as though someone's taken a solenoid and converted it into a relay! There are no moving parts external to it, and it's held closed with rivets, so it's not like you can join it to something mechanical.

[Anonymous]

IF "no moving parts" then it is not a solenoid. Solenoids need to connect to the starter pinion lever so the pinion gear meshes with the flywheel teeth. And that does take more power than just closing contacts

[Commander Kinsey]

Agreed. I understand the difference, and the seller has mistitled it. I new I was getting a relay and didn't want a solenoid.

But that's not what I'm asking. What I've got is clearly a relay. I'm wondering why it needs 60W to stay engaged.

[Anonymous]

Perhaps what you have was intended for an older car with a "Bendix drive" starter that does not activate a lever. Maybe the windings, etc are the same minus a plunger.

[Dean Hoffman]

I tried Ohm's Law here:

"That does not compute." Remember Star Trek?

[Ralph Mowery]

Many ohm meters are not very accurate at that low of value. The place where the probe is placed will probably not make very good contact and show a few ohms high.

[FromTheRafters]

One should also consider that the 12 volt battery under load wold only deliver 6 or 7 volts and the solenoid would still have to hold the contacts closed or chattering would result.

[Fredxx]

I don't see any mis-selling.

A "starter solenoid" is industry standard name given to a heavy duty automotve relay.

Examples:

The second has a manual override. Both are intended for a inertia/Bendix type of starter motor.

[Dean Hoffman]

That's what I was getting at. The math just didn't work. The power supply would have had to be 20 volts if the resistance and amperage were correct. I used either a Fluke 83 or 87 at work. Even the leads have some resistance. I think my meters used to come close to zero resistance when I checked them though.

[Commander Kinsey]

Good point, a 200A one I found was sold as a split charging relay, which connects the starter battery to the leisure battery while the engine is running. I guess there isn't a huge inductive current going through that as most will come from the closer starter battery.

[Ralph Mowery]

I have 2 of the 87 meters and sometimes it is difficult to make good contact with the probes and what I want to test.

Good thing about them is you can use the button to 'zero out' the leads and other resistnce if present.

With very low resistance it helps to measure the voltage at the device under test. I was using some China clip leads and was getting some strange values . Traced it down to the 2 feet or so of wire between the alligator clips. They must have used some iron or aluminum wire. I replaced the wire with some good copper wire of about 20 gauge and the voltage/current/resistance was ok then. I have noticed that some of the

1/2 watt and smaller resistors seem to have leads that are magnetic.

[Commander Kinsey]

Many things change it slightly. The 12V power source is a 15A 13.8V PSU I use to charge batteries. It's a bit out probably due to a bulging cap I haven't got time to fix right now, so it's 13.9V. So there's another error. And I bet the resistance doesn't stay the same when the relay powers up either. Take a lightbulb and measure its resistance when off, you'll find it calculates to nothing like the wattage you get when it's lit, as the resistance drops significantly when the filament's hot.

I can't believe Dean expected a precise measurement. And also that he needed an online calculator to do basic division using a formula with only 3 variables.

[Commander Kinsey]

Try maths, it's more accurate.

I was doing a quick check on it to determine which contacts to use, not designing a f****ng spacecraft.

[Commander Kinsey]

Yes, it was sold for a Vauxhall Magnum 73-77.

So you think they made a universal one with a plunger for other models of car, then removed it for this one? AFAIK a starter solenoid doesn't have electrical contacts, or can you get combined ones?

[Dean Hoffman]

Two batteries? On what, pray tell? Everything in my little world has only one battery. My work was working on irrigation. Poor farmers couldn't afford batteries, dontcha know.

[Commander Kinsey]

Wrong. A solenoid moves something mechanically, a relay switches something electrical. The correct naming is quite simple.

then I don't expect mechanics to be that bright. It's like sports commentators saying "off the pace" to mean faster, because they think pace is time not speed.

[Dean Hoffman]

What's to say? People don't accuse me of being smart.

[Commander Kinsey]

Motorhome, campervan, RV, whatever you call them over there. Or a car with a whopping great stereo.