Technical Why do I need a Mopar starter relay?

Engine is a 354 Hemi with MSD distributor & coil.
Here's its starter (source: late '60's 440):


Relay in question and wiring diagrams:


What does it accomplish?

Why can't I just eliminate it and connect:

• the red wire between the Battery + and the B pole on the ignition switch,
• and the purple wire between the Starter Solenoid and S pole on the ignition switch (with the shifter's neutral safety switch in the purple wire between them) ?

As always, TIA.

 

This should be fun.

 

In general a relay is used to switch a larger load from a circuit that can't handle the larger load. So I assume the safety switch, egnition switch and there wires are not rated for the load of the starter.

Phil

 

That's pretty much it. Also the relay lets you have a neutral safety switch, if it's an automatic equipped car.

The factory wire from the relay to the starter is 12 gauge, and it's not very long. That tells me it's going to load the ignition switch quite a bit, and maybe have excessive voltage drop, if you leave out the relay.

good luck

 

It's electrical and can't be seen, but what goes on inside that starter relay? Does anyone know, better yet can they explain what gets added or taken away?

 

there's a coil and a set of contacts. The coil gets energized when you turn the ignition switch, the contacts make contact and send power to the starter.

this is a picture of a generic relay....so you can understand what's happening.



here's a photo of a relay guts, so you can see what the coil and the contacts look like.

Sorry I don't have any spare Mopar relays to take apart and show you the insides of.

 

I would think a good 20 -30 amp push button to stater solenoid 10 Ga, drop the Relay in drawing ,

But Im someone Who does not have a Key,
Just Toggo & push button,
Toggo power up ignition,
One side hot On Push button ,Other side to start solenoid,
I can turn engine over Power Off @ Toggo

 

And that leads to how is Gary intending to set up his starter (start) wiring?

 

Why not just mount a Ford starter relay nearby the starter with a 10g wire connecting it to the starter.
The relay's primary signal wiring and switches can be connected with 16g because they're drawing negligible current.

 

Relay good........... Fire bad...........
Yes, you need a relay, but it doesn't have to be a mopar brand.

 

This. I use the Ford relay on my 413. Then pigtail the starter's internal solenoid terminal directly to it's main terminal.

 

I have used the Ford relays on many of my Mopars. The Ford relay was always cheaper, and using it gave me an easy access to a battery hot if I ever needed one. The Mopar relay's spade terminals make accessing the the battery hot more difficult. The Ford relay makes it easy to jump across to crank the starter too.

If you use the Ford relay, you can also eliminate the 12 gauge wire running to the Mopar starter if you want. All you need is a jumper from the battery cable on the starter to to the small post on the starter. Then when ever the Ford relay powers the cable going to the starter, the jumper also activates the Mopar starter solenoid and the starter spins.

You do need to be sure what ever relay you use has a good ground at the mounting bracket. .

 

For whatever reason, multiple manufacturers used both a relay and a solenoid on starters. The solenoid should be able to handle the job as you described. But on the off chance that manufacturer did something stupid, hook the starter to the battery. Take a wire, the same gauge as the one you plan to run from the switch to the solenoid, and connect the solenoid end. Then, hold the other end to the battery and let it spin the engine over for a few seconds. If the wire gets too hot to hold, you'll have your answer. Remember, the longer the wire the more amps it will take and the hotter it will get. You could even get fancy and use a VOM (volt ohm meter) to see what kind of amp draw the wire has. Some vehicles even took it a step further with a fuse to the relay hot wire.

 

Voltage drop of the wires and switches can cause the solenoid to not work, usually when engine is hot. Plus, they maybe not rated for the current draw of the starter solenoid. Yes, the relay could be by-passed with a 30 amp rated switch, and 10 gauge wires. But the neutral switch won't be happy.

Back in the day, we installed ford starter solenoids on Pontiacs that had headers. When hot they would not crank.

 

Safety and longevity. The contacts of the relay under the hood are designed to handle 'lifetime' use and are away from the operator.
Plus as mentioned, the neutral safety switch is incorporated into the trigger circuit by providing ground.

 

The starter relay is a work around on Chevy's with a "hot; no start" problem as well

 

The neutral safety switch goes to ground in neutral and park and you need the Mopar relay for this to work.

 

All good points. I'll rephrase my question.

My '55 Chevy (SBC & Muncie 4-speed) didn't need a separate starter relay so why does my Model A (Hemi & T700r4)?
The answer to that question is the Chevy's starter relay is inside the starter solenoid. Right?

 

Yes, the solenoid functions like a relay on the chevy starter. No the stand alone relay isn't required to start the car. But, as pointed out, the relay serves as the mechanism to make the neutral safety switch function. If you changed the neutral safety switch and ran the power through it to the starter, you wouldn't need the relay. That would require more wire to get to the neutral safety switch and then the starter and the switch and wires would have to be able to handle the amp draw. (refer back to some used the relay to over come the amp draw required when the starter was hot).

 

Not quite. I think the reason Mopar used a separate relay is that they had a higher current draw from the solenoid/relay unit (which functions internally just like the GM design).

Here are the insides, you can see they both have a plunger and contact washer, to act as a high current relay to switch the battery terminal to the windings



The Mopar design is more reliable, as it has another relay (the one you don't want to install) in the circuit, and isn't subject to the heat soak no start issues the GMs experience.

 

An earlier posted the 'chevy hot start problem' as a reason for the Mopar design. So true and Squirrel explained it well.

On a lot of yard and small farm tractors they cheap out, even the green ones, and use the starter solenoid's internal relay. Then offer a kit to install a relay. At about 10 times the price of a generic Bosch type. The problem is the
voltage drop through the safety switches (PTO, seat, neutral) and undersized wiring.
I have my version of the John Deere kit on my 455, and will be putting one on my Hustler mower.

 

Thanks, everyone.
I'll be in the garage tomorrow, attaching a starter relay.

 

I’ve got a full stomach of Taco Bell (gas reference) half a pack of chewing gum (gave up cigarettes) it’s morning (because it’s light) and I’m wearing sunglasses hit it.

 

Use a push button instead. 8^)

 

Ironically, 45+ years ago I used a gear reduction Mopar starter in my race car and it had one of these heavy duty switches mounted within easy reach in the cockpit and operated by hand. It was coupled directly between the main battery cable and the starter mains post, and had a simple metal strap bridging the solenoid terminal to the mains post .... it worked flawlessly.
Unfortunately they don't provide a Neutral start safety capability.

 

If I recall correctly, the neutral safety switch grounds the relay coil. So if you are using the original neutral safety you would need the mopar relay.

Chris

 

you could use a normal "bosch" relay, instead of the Mopar relay....it's not rocket surgery....

 

Or make your own.

 

^^^^^ " RELAY".......FFT.......

 

do you know if they are volt specific , as in 6 or 12v