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I have been working on moving wires off of my ignition switch. I have a six terminal block installed with the overdrive solenoid, the electric fuel pump, and the reverse lights so far powered from that. I'm working now on getting an HEI made to fit. I will power that also from this terminal block.

I am making the HEI per the instructions given here: https://www.stovebolt.com/ubbthread.../brand-new-hei-for-stovebolt-100-see-how

It is recommended in that thread (and in other places) that I include a relay, and I will do that before I power the HEI. I think I need a 50 amp one with a fuse and use a 10 gage supply wire.

Am I correct with what I think I need?

I have a Standard Motor Parts RY-12 that I will use to make the OD solenoid work with the HEI. I like the metal case look of that.

Is there a recommendation of what I need that looks like the RY-12?

Thanks!

I bought four from eBay to power my headlights and HEI. Mine are 60 amp Bosch style relays that came with the plugs and pigtails. I went with 60 amp because of the 10 gauge wire size. Using a bigger relay than required is OK. HEI distributors draw a high current on startup.

These are 70 amp BOSCH 4 terminal 0986AH0082 relays made for large wire connectors. We use them on school buses to power the 3-fan A/C condensers mounted under the bus. Here are the 70amp Sockets. The 40/60 amp Bosch would also work, but they have the normal 1/4" spade connectors.

No need to fuse the ignition, GM never did, just fuse the feed line to the terminal block at the battery. Use a MAXI fuse with holder from NAPA, which come pre-made with 8 gauge wire.

These guys can hook you up with any sort of electrical part, even the correct color and size to match your GM wires.

Thank you all for your advice. I will update when I get what I want sorted.

Robert the high amperage is from the inrush of current when the engine is started. The coil can draw around 25 amps or so and then the amperage use declines from there so that while running it may be 8 or so and in some cases maybe less. I'm using a 30/40 amp Bosch style relay and it has worked just fine for several years. The main thing you're doing with the relay is protecting the contacts of your ignition switch. Running 20+ amps through that switch can be fatal for the contacts in there. Some of those ignition switches were only designed for 10 amps and you will find some today that are rated for 20 amps. Not common to find one (honestly) rated much higher than that...at least one that will work in the AD trucks. You will see some Chinese switches that claim to be rated for 30 amps. I don't trust that rating any further than I could throw an elephant. Too easy to use a relay and any type of switch to control it. Good luck! Please let us know what you decide.

Thank you, Jon, for your further advice. Poster Phak1 also noted the voltage draw above. Your advice seconds that. This high voltage draw is a new thing that I learned from these postings. My original reason for adding the terminal block was that I was running out of length of screw on the ignition switch (aftermarket bought in 1977) to add more accessories, but I've now learned that all of those added draws can over burden the switch. In the last few years I have added an electric fuel pump, reverse lights, and the wiring for an overdrive. These three things are now powered from the terminal block. The HEI (when I install it) will be powered also from that block, and I will move the fan motor to it.

If I correctly understand what I am learning, people are using 30/40 amp relays just for their HEI units. I was thinking I should use a 50 amp relay to power the terminal block. I could also leave the terminal block as is and add a 30/40 amp relay for the HEI after the terminal block just for the HEI. Does anyone have advice on that?

I bought a 1978 C20 new. The odometer quit working at 385K miles maybe 10 years ago. I figure it now has well over 400K miles on it. The HEI on that has that very high mileage engine starting easily and purring nicely. I look forward to having that same service for my 1958 270.

I believe you could do that, Robert. The modern approach (at least from some makers) is to use a few different relays (usually found in the fuse box) to activate different items. I think the thought behind that is twofold: not to overstep the ability of a relay and by keeping the amperage across it lower not to shorten the life of the contact points in it. The extra draw isn't voltage but current (amperes). For years I've told folks to think of voltage as the size of your arm and amperage as the strength of your arm. My original comparison was "would you rather arm wrestle with Mahalia Jackson or Arnold Schwarzenegger?" Their arms at the time were about the same size.

To your question, yes...30/40 amp relays for the HEI are common and popular. They're cheap and dependable. Also if you have a high amp electric fan, most of those will work with the same relay. The draw from an electric fuel pump, reverse lights and the overdrive solenoid won't be too much, really. With just what you've given us I would say you could come off the terminal block with a 30/40 relay and dedicate it to the HEI. Here's the thing: your HEI at start up might draw 25 amps or so...maybe only 20 but that's still a good bit. If you have a 16 inch electric fan, it might take another 25 amps to run. We're already to 50 amps. Even if your fan is thermostatically controlled, if your engine is shut off at operating temp and the temp increases, the fan and the HEI will operate at the same time. At that point you will be right on the edge with 50 amps. So, to hark back to my second sentence above, I'm generally more comfortable with proper size wiring and separate relays. Hope this helps.

My preference is to use a single relay for each system. In my case, I have four relays, one each for the HEI, horn and two for the headlights (one for each side). I opted for two as I didn’t want the high voltage passing thru the old foot operated dimmer switch. Maybe overkill but it does work and the headlights are noticeably brighter after installing the relays. They are 60 amp aside from the horn which is the original style.

There are two ways to do this.
First way is use one main relay rated for total amperage usage. This will be turned on by the ignitions switch moving all current usage to the relay. The relay will supply power to the power strip. Each application will need its own fused circuit and proper wire size coming off of the power strip. The power strip will need full amperage usage fuse and wire diameter.

The second way is to turn on the power strip right from the ignition switch. Then add relays from the power strip to each application using the proper wire diameter and fused power source. This system requires addition wire from the main battery source to power each relay, but you can tailor each circuit according to its needs. A single point power feed right from battery will be needed to power each relay.

Both system requires minimal current from the ignition switch.


For cooling fans, the relays are usually mounted out near the fan with small wires running them from the switch panel. The power feed is right off the alternator or battery with heavy wire to the relay then a short wire to the fan. Race cars do the same with electric fuel pumps, relays mounted at the pumps so the motor draws power through the shortest, heaviest wires possible. The long run, small trigger wires are less likely to burn much if they get shorted to ground with 3 or 5 amp fuses.

Thank you all for your further advice. I will update when I figure out which way to go.

I think that what I have in mind is similar to what Joe H recommends above. Bill Hanlon sent me his wiring diagram, and I used some of his ideas. Here is my sketch of what I have in mind and a photo of a 50 amp circuit breaker. I have not yet chosen a relay. I will power the circuit breaker then the 50 amp relay from the battery connection to the starter motor.

Am I wrong somewhere with my idea?

Thanks!

I have a set of cube relays that have a blade fuse socket built into each relay. Haven't found a project for them yet, but would cut down on the number of connections.

If the circuit doesn't need a switch, like HEI, don't double relay it, just go straight to it. If you are going to be turning something on and off, then add a second relay ONLY if you are using light duty switches or you want to keep the wires small under the dash. With heavy duty switches, you can wire right off the power strip with a fuse to the switch, then out to the item you are switching on and off. I would use a resettable circuit breaker ( 30 amp )for the cooling fan and try it off the power strip. If it keeps tripping the breaker, then you will need to move the power source and up the amperage. Add a relay if you are switching the fan on and off, skip the relay if its on a thermostatic switch out at the radiator.


This is the type of circuit breakers we use at work on school buses and trucks, https://www.etrailer.com/Accessories-and-Parts/Buyers-Products/337CB50PB.html There's lots of brands of the same type and amperage, the mount easy and will take quite a bit of over amp rush in and not trip. This type can be tripped with a push of the red button, hidden under the dash, this would make a pretty good no start security system!

Thank you all for your further advice.

I did look at the relay sets with fuses that Bill Hanlon mentions, but I don't believe I want to go that way. My power strip has a fuse for each item, and the reverse lights, electric fuel pump, and overdrive relay are not high draw items.

There has been more than one mention of an electric fan. That is a mislead originating from me, sorry. I may power the interior heater fan from my terminal block, only to move one more wire off my ignition switch. My temperature gage never varies beyond 180 degrees, so I do not foresee a need to add a fan to the radiator.

Joe H mentions double relaying. That does seem to be unnecessarily complicated, and I do not know what potential problem may come from that. The overdrive that is powered from my terminal block operates through a relay, so putting a relay before the terminal block would be double relaying that. The 2nd relay I show in my sketch above would be for allowing the HEI to accommodate the kickdown function of the overdrive. That would be a 2nd double relay. I plan to wire that 2nd relay for the HEI as shown by poster ray_mcavoy in 67-72chevytrucks.com. I find that in a search for "HEI overdrive" in that forum.

There is also mention somewhere about fusing or not fusing the HEI. I would be okay with not fusing that, if that is what GM did, but I would want it to be somehow tied to the ignition switch.

I still have some things to learn, and I will before I start soldering.

Thanks!

I thought You were using a engine cooling fan, heater blowers can run on 15 amps. Buses are wired with 15 amp breakers and 16 gauge wire up to 15 feet long. I changed out a ton of 15 amp circuit breaks because they kept overheating with heaters on high speed till we found the manufacturer uses a janky ground terminal hidden in the wall. Once we fixed the grounds, dual blower motors would run on 15 amps all day long.

HEI will run just fine with a little more or less amps then you think. We have always just wired them into the ignition switches with no thought given about amp draw. Since the early 80's I have been installing HEI to standard ignition switches with no problems. I am not sure what HEI and overdrive have to do with each other, so please show the link or explain that one.

Thank you for your further advice. The advice I mentioned in my first post on this recommends a relay. As you point out, I can also put the HEI to the ignition switch. I will figure it out and do something, not soon though, so I have time to think. There will be room on the terminal post of the ignition switch when I have everything else off it if I want to go that way.

One instruction showing the need for making an HEI unit accommodate an electric overdrive is given by poster ray_mcavoy in 67-72chevytrucks.com. I find that in a search for "HEI overdrive" in that forum. I plan to follow his instruction.

Thanks!

I read through the whole overdrive post you linked, I had no idea GM did that, guess they knew what they were doing. I ran a Mallory tachometer for years in my race car that had a built in rev limiter. It had twin dials, one for a shift light, one for the limiter. The hookup was simple, one wire each for power and ground, one for lights, and one to the TACH terminal on the HEI. The limiter killed all cylinders at once using the TACH terminal, not sure what it was doing in the module to kill the signal, but it worked really good. If you just hit the limit, it would be a quick on and off till the rpm came down, but if you stayed on the throttle, it would hit hard.

I asked around about the Mallory rev limiters, the limiter electronically grounded the tach wire while reading the rpm. So for you overdrive, it would be the same hookup as GM did, ground out the negative side of the distributor ( HEI negative side is the tach signal ).

" On a HEI the tach wire terminal is the same as the Neg post on a conventional coil.
On the Mallory Tach the Rev Limiter simply grounds the green signal wire to the black wire.

Some drag bikes use a micro switch to temporary kill ignition while clutchless shifting and in road racing you can get this feature for a dog-box [Flat shifting] where a micro switch is connected to a load cell on the shift lever."

It will be a much easier job using small 18 to 20 gauge wire with no amps involved, you can even use the original GM relay. The other plus is it will be easy to disengage if you ever want to remove it, the main power feed will still be one piece with no relay to worry about.

As always with HEI, throw a spare module in the tool kit. I've carried the same one in four different cars for 40+ years and never needed it, but I know I have it if I do !

This is how they work,

On an overdrive the relay was used to supply power to the OD solenoid. When you floored the throttle pedal a switch under the pedal closed a circuit to a set of grounding points inside the OD solenoid. The other side of the circuit was connected to the coil primary on the distributor side. The kickdown switch also killed power to the OD solenoid. So the solenoid is not energized and the ignition circuit is interupted. With no torque on the sun gear, the spring inside the OD solenoid is able to retract the pawl that was locking the sun gear to the case and you go back to direct drive. Retracting the solenoid also opens that set of points that allowed the ignition to be interrupted. It all happens in a split second and you never notice the ignition being interrupted.

So what you need is the working parts in the overdrive unit and a kick down switch. No external relay is needed. If try do wire it with any other way, the ignition will be shut off every time the throttle is wide open. The spring paw in the overdrive retracts turning the ignition back on, this is the key! You need the ignition back on ASAP once the overdrive disengages.

Thanks! I will look at that. I hope to know exactly what to do before I start.

Borg Warner made a nice relay for Ford and Studebaker. They have a mounting bracket for firewall installation, they have a Buss fuse and they have screw terminals. It is a Borg Warner 1539595. But they have gotten expensive. The cheapest ones I have seen are around $150.00

Thank you, Kenneth. You recommended that relay when I was planning to get my overdrive working. I took your advice, and the overdrive is working fine. I paid $125 for mine.

I'm starting to think that relays do mostly the same thing, and that I need only to find one of a correct amperage capacity and guess at dependability. I need to draw the options for what I want and make sure I understand before I start messing with anything.