The Stovebolt Site Forums The Shop Area The Electrical Bay Continuous Duty Relay

[Context: 1951 truck that was long ago converted to 12v. In need of a re-wire……]

Getting back to planning out my re-wire project. Going to have separate fuse blocks (or perhaps one split fuse block) for constant power vs switched power circuits.

“While I’m at it” would it be a good idea to power up the switched power fuse block with a continuous duty relay or solenoid? Basically, take all the load off the ancient ignition switch, and it just becomes a relay trigger.

Is that overthinking things? Just adding another failure point? Or worth pursuing?

Sounds good to me. Other than ignition, what devices will you have on the switched circuit? What might you add? Are you still using an ammeter?

I have a set of these that I'm waiting to install if I ever decide it is time to move my fuses and relays out from under the dash. A/C, radio and a 78 year old overweight body don't make it easy to work under there. Mine came with pigtails that have the dovetails on the sides to lock multiple relays together.

The same company makes 80 amp capacity (normally open contact that you'd use, 60 amp on the normally closed contact that you'd not be likely to use) waterproof relays (but no fuses) in a similar sized package.

If you convert to an alternator, convert to HEI, add A/C, use high cfm electric fans and things like that, Bosch-style relays are in my opinion a great and in some cases necessary choice. The ramp-up amperage on an HEI distributor will cause failure in an older style ignition switch...even one that was rated for 15 or 20 amps. I can mail you one which failed magnificently...the contact points lasted only a few weeks after the conversion. By converting to relays you'll be standing on the shoulders of GM engineers who began doing this around 40 years ago. Good luck.

1) I already have some relays that I plan to use for high and low beam headlights - both to take some load off the light switch and to hopefully maximize how brightly they glow. And I was going to mount a couple placeholder relays for other future accessories.

2) I don’t really don’t plan to have anything particularly high amperage wired through the ignition switch, but I’d rather not worry about how many total amps are flowing through it an inch from my knee, or have it conk out and leave me stranded.

3) Still using an ammeter and generator and points, but always considering future mods. If I did anything soon, it would be alternator, but I’d probably still use the ammeter and just shunt accordingly.

4) Regarding WHICH TYPE of continuous duty relay to power the switched fuse panel: any particular advantage to the “modern” style Bill mentions vs the old school, fatty “solenoid” (ie Cole Hersee) ones?

The newer work in a smaller footprint, which would be nice. But I’m more concerned about bulletproof, dead-nuts reliability than the size. In principle, I don’t like adding new failure points or complexity to anything. This whole idea is a little against my nature, but it still makes overall sense in my simple brain.

I've been using Bosch 87A style relays for exactly that purpose in my 49 and also in other projects that see a lot of use, without issue. The 49 has been running the switched fuses through that same Bosch relay now since 2021, now it only gets driven in the summer so not necessarily "daily" but several thousand miles (hours I can't say). In an OT LS swapped project I drive regularly, and in my plow truck, I also service all the switched fuses using the same relays. Obviously watch your particular load and use additional relays on specific circuits where it makes sense, so all the load for any given device is not necessarily going through that single switched 87A that is feeding the switched fuses, if you follow me there.

I think a standard Bosch style relay, which are generally available in 40A capacity would be fine to use to power your switched fuse block. Unless you're putting some crazy loads on that fuse block, which it doesn't sound like you're doing. If you need additional relays, the ones that dovetail together are nice. I used 4 of them to run my turn signals and mounted them to the steering column clamp support.

A switched fuse panel section has a power buss bar that is powered by the ignition switch. One side of each fuse is attached to that power bar. The other side of the fuse goes to the component. So each component is then fused. The fuses are sized per the component draw. The headlight switch has a fuse/breaker. I don't see an ignition switch burning up. Haven't had one burn up or even get hot in 70 years.
The universal prewired fuse panel type harness kit I always recommend and use does not have a relay. DIY harness is too much trouble and causes overthinking.

If a big draw non original component is used, use a system of direct feed with an applicable mix of it's own big fuse, fusible link, switch, relay.

I use a lot of the Bosch style cube relays, usually in the 60m amp capacity size- - - -not because that amperage capacity is really necessary, but the wire gauge of the connector plug for the heavier relays is a couple of sizes larger. No point in using a relay, if the wires going to and from it are tiny!
Jerry

The 2/3 amp coil draw on the Cole-Hersee type relays would be much higher that the amps drawn by a cube type relay. Given you are using a generator (for now) the extra current may not be in your budget.

Good points, I didn't preface my response to clarify mine is far from stock.

I did not know about the constant draw.

Any relay will have a constant draw as long as it's pulled in (or energized.) In general, the larger the relay, the higher the draw from the energizing coil.

A 70 year old ignition switch, or even a brand new foreign made one would appreciate only having to move milliamps though it rather than many amps.

I certainly wouldn’t have guessed it was 2-3 amps. I always assumed it was some insignificant fraction of an amp that switched the relay.

2/3 as in 0.666 amps according to their spec sheet.

Hahaha! Unexplained fire under the dash?

Ahhhh. That makes more sense. Still higher than I assumed.

Would something like this work well?

https://www.amazon.com/Bosch-332-002-168-0332002168/dp/B0117FAGVO

That one looks pretty heavy duty. Says it's rated at 75A. Has screw terminals on the load side rather than spades, so yes, I'd say it would work well for a main relay.

I like that it’s not made in China.

Yeah, that's a plus, too.

Minimum switching voltage on that Bosch relay is 10.8 volts. I wonder if it will cutout while cranking. Let us know how it works out for you.

Usually once a relay is pulled in, it will stay closed at a bit lower voltage than minimum switching voltage. I bet it'll work OK.