View Full Version : Starter Relay "I" post

01/16/11, 08:02 AM
I am confused as to what the purpose of the "I" post does on the starter relay. What application is this used for?

01/16/11, 08:28 AM
The I post has 2 functions.
First is it bypasses the ballast resistor while cranking. Since battery voltage is dropping during crank (starter putting a load on it) and the demand for a strong spark is needed they bypass the ballast resistor. Full (loaded) battery voltage to the ignition system, so about 11 volts. Not the nicest thing for a set of points, drive down the road without the ballast resistor and the 14 volts off the charging system will fry them very quickly.
The second thing is the ignition switch may not keep the ignition circuit energized during crank. That bypass will keep the ignition fired up shile cranking the engine over. Without it there are times where the starter can be spinning, the engine rotating, but no power to the ignition system to spark the engine to life. The ignition switch is a bit flakey about that.

So the "I" wire will give the best spark at the worst eletrical load condition.

01/16/11, 08:59 AM
Thanks for the explanation. This makes sense, I understand now.

01/16/11, 12:51 PM
It should also be noted that in some Ford solenoids it is also the ground (by completeing the circuit in the ignition) for the "S" terminal. So sometimes if you delete the "I", the solenoid will not activate.

01/16/11, 01:49 PM
The I terminal is different from the S2 (ground); you have to check the relay markings or wiring diagram, or test it, to find out how it's wired internally. (

01/16/11, 06:58 PM
Supermotors is missing a few possbilities in their description. They are the most common though.

01/16/11, 08:31 PM
RRRAAAYYY2! With an answer like that, you gotta expect us schmucks to want more. Please, sir, give us the RRRAAAYYY2 low down on this one:).

01/16/11, 09:56 PM
Those were the only relays I had lying around in my junkpile when I took those pics & wrote the captions to match. It wasn't intended to be a comprehensive list of every type of starter relay - just an overview of the common ones used on Ford trucks.

The point is: an I terminal is never the ground for the trigger coil. If you tried to wire one up that way, it would burn the little contact off the first time it was activated. And if you reversed the wiring so I was the hot side of the coil, the relay wouldn't release until the battery died. The whole idea of an I terminal is to be a discrete connection - separate from all other circuits.

01/17/11, 06:51 AM
There are not a whole lot of possiblities when it comes to the black plastic housing ones. But there are ones that are insulated base. They didnt come on a car, but often can be sold as if they were meant for a car. When the base is insulated, the S terminal has to ground some where. There has also been lots of different aftermarket manufacturers over the years of those mag switches. I only mentioned it as I have ran into it a few times before.

The tin can looking one is not a big deal. You should not be using it for your starter in most cases. Most of them are either continous duty, or continous duty prolonged on duty cycle (note there is a big difference between the two). Which means they do not have a strong enough coil and spring to make and break large loads properly. My old catolgues have 4-5 pages of different possiblities of them. Only one or two that have duty cycles longer than 30-60 seconds.

01/17/11, 11:28 AM
When I was researching the swap from the old-style relay to the new-style in my '83 w/'95 EEC, NAPA's book showed ~40 variations of the black-body momentary relay, including (but not limited to):
current rating 50~250A
grounded base
I terminal
diode suppressed
I haven't checked in years, but I'd expect there to be more now.

When the base is grounded, there is no I; there's a second S terminal to ground the coil, as that pic showed.

The metal continuous-duty relay was one I used for a little while when I had an aux battery. I may reinstall it now that I have a winch, and put the aux.batt. under the rear floor.

01/18/11, 03:41 PM
Are you looking at the ones from power sports? I asked because they will not work in a car, nor do they look like a Ford mag switch. The reason they wont work is because they need to have a make/break contact rating a lot higher than 250amps.

Ford orignially only made about 5-6 of them. Could be NAPA is giving you part numbers from different suppliers as completely different solenoids. But of the three biggest manufacturers in the world, they only offer 5 models I think (counting in my head), to replace original Ford mag switches. Actually one offers none, as they only make the tin can style.

In the industry momentary relays are small "Bosch type" relays. What people call Ford fender wall solenoids are really mag switches. Like a relay on steriods. Mag switches are rated in duty cycle, Intermittent which is under 10seconds, continous duty which is under a minute, and continous duty prolonged on which can be left on indefinitely. There are also latching type, which you engage and they latch closed mechanically, and then when you power them again, they mechanically release.

01/19/11, 10:34 AM
The starter relay is functionally & electrically identical to a Bosch/ISO relay - it's just rated for more current. Many Bosch relays are SPDT, but some are SPST just like the starter relay.

Ford specifies 200A as the maximum nominal current for a typical starter. (

01/19/11, 07:35 PM
No, Ford specifies that the maximum load is 800amps according to your link. You are also posting the specs for a PMGR starter. There are also direct drive starters (4 -4 1/2") available for Fords, in fact they are what was found one every early Bronco. Which happens to be what this site is dedicated to. A general guide for DD starters is about 1amp per cubic inch, for most stock motors. Most start out pulling closer to 400-500amps when they are first engaged. Then it tapers off to lower amperages. PMGR starters are around 300-350 pull in and drop off to 180amps, there a bouts.

As for the relay vs mag switch debate. Build a relay that can handle the amperage of the starter. Install it, and it will fail within probably 50 starts. They are called mag switches, because that is what they are. Relays do not have rotating discs in them and do not employ arc burning, etc. Relays just open and close a contact, and thus have to be low amperage.

The public calls the Ford fender wall mag swtiches, solenoids and the industry just let them get away with it. A solenoid is a mag switch that performs a mechanical action. So in reality, the Ford fender wall mag switch has more in common with a solenoid, than a relay.

01/19/11, 07:50 PM
be careful i have had 2 from kragen that had continuity between the I post and large stud going to the battery at all times, not just cranking, in this case when you turn your key on, the ignition circuit back feeds from the coil to the starter solenoid/relay and tries to crank the engine on that 14gauge wire(dead short more or less). obviously blows a fuse instantly making you have a crank, then start to run and when you release the key to run position it dies. could be a hassle to figure out to the normal person in the garage.

01/20/11, 12:44 AM
No, Ford specifies that the maximum load is 800amps according to your link.That's under maximum load - like an overheated, frozen, or hydrolocked engine. The max normal current is 200A.As for the relay vs mag switch debate.All Ford documentation refers to it as the starter RELAY. Helm, Chilton's, & Haynes refer to it as the starter relay. Only parts suppliers categorize it as a solenoid, so the parts monkeys call it that.Build a relay that can handle the amperage of the starter. Install it, and it will fail within probably 50 starts.I don't need to - I can buy one anywhere, and all of them last thousands of starts.They are called mag switches, because that is what they are.You're the only person I've ever encountered who calls a starter relay a mag switch. I can't even find "mag switch" on WikiPedia, Amazon, Grainger, or McMaster/Carr (though they do have magnetically-operated switches). So IDK where you came up with that term.Relays do not have rotating discs in them and do not employ arc burning, etc. Relays just open and close a contact, and thus have to be low amperage.Anyone can find a relay rated for 30-250 amps, with any kind of construction, including solid state, Mercury, toggle, or disk. I have a few of each in my junkpile.So in reality, the Ford fender wall mag switch has more in common with a solenoid, than a relay.A typical "relay" is a solenoid-operated switch. A typical solenoid is an electromagnetic linear actuator. So MOST relays contain a solenoid, and most relays are electromagnetic devices. I have no idea what a "mag switch" is, but if you want to call relays that, no one can stop you. :)

But break out an early Bronco wiring diagram and read the name of the thing between the ignition switch & starter. If you don't have one handy, here's a link to ALL of them:

Oddly, "Mag Switch" doesn't seem to be on any of the Index pages... ?:? I guess that's because it's a relay. ;D

And take one more look at those diagrams - you'll notice that, in those with modern standardized symbols, the starter relay is remarkably similar to the horn relay: a winding connected to a switch, for both relays. And you can see how it's like a combination of the horn switch plus the carb. solenoid.

01/20/11, 07:34 PM
Well lets start with the amp draw of starters. Here is a link to Denso, the manufacturer of a HD starter for Ford, a starter which Ford used in FedEx Fleet production. Its draw is pretty typical of most Gear Reduction starters. And about 30% higher than a Permanent Magnet Gear Reduction Starter (PMGR) and about 30% than the older Direct Drive starters. Here is the link:

In the brochure is the output specs of the starter at given amperages. 6000 rpm is around 420 engine rpm. At that point its output is 2.4kw or 2400 watts. Watts are watts, so we can convert that to electrical energy. Since the starter is about 55% efficient, it means that it requires 2400 divided by 55% or 4363 watts input to produce the 2400 watts.

You then simpily divide the 4363 watts by the 10volts (voltage drop as it is in use) in the chart. Comes to 436.3 amps under normal conditions. That is from the OE Manufacturers website.

Ford does not make starters, neither does Motorcraft. I believe Visteon is their current supplier of choice.


If you compare the 1.8kw Denso starter's output to the Ford PMGR starter you originally link to, you can easily see why the Denso is a major HD upgrade over the DD or PMGR starter.

01/20/11, 08:03 PM
Here is a link to a simple explanation of the construction of a realy:

Things to note are the lack of a rotating disc, and that it does not perform any mechanical action.

Mag switch is what the industry originally called the automotive magnetically operate switches like we are discussing. They are still commonly called that in HD AC applications.

There are several differences between a Mag Switch and a relay. The first big difference is the mag switch has a disc inside that makes and breaks the contact. When it is released it rotates slightly, so that you do not continously make contact in the exact same place.

Manufacturers also vary the metallurgy of the disc depending on the amount of amperage/voltage the switch is gonig to be used for. The idea is to have the arc created when a high amp connection is made, to burn off the slag from the previous contact.

If you use a disc meant for low amperage connections (typically continous duty) to make/break a heavy load, there is a good chance it weld itself closed, or at the least fail to burn the slag build up way before it should. If you use a high amp disc in a low amperage application it will build up arc dust and cause a resistance issue.

If you used a contact setup like in a Bosch style relay, the contact point would be the same all the time, so the slag would build up quickly. It would not burn off because it is always in the same place. Your resistance would build really quickly, which would cause a voltage drop, and increase in amperage, and it would weld shut very quickly.

A solenoid is a switch that performs a mechanical function. On a GM starter it pulles the shift lever in, which cause the drive to engage the flywheel, and just before contact it makes contact between the two posts, allowing power to flow into the start and cause it to spin. A fuel cutoff solenoid just pushes/pulls a shaft to remotely turn a valve.

Many applications use a relay to operate a solenoid.

Gainger, Chilton, etc can use whatever terminology they wish. But they really are not a part of the automotive industry on the manufacturing side of things. They are companies that cater to the public.

This is like everyone calling Optima batteries GEL cells. Reality is that everyone in the battery industry doesnt, because they are not. Just because the public calls them GEL cells, doesnt make them that. Just because the sales guy at parts store calls them a GEL cell, doesnt make it one either.

Hope this finally clears it up.

01/20/11, 09:01 PM
...a HD starter...Is that what an eB uses?...about 30% than the older Direct Drive starters. Here is the link:Why not just post a link to the specs for the starter that a typical eB would use???Watts are watts...No, they're natt. You're using formulas for resistive DC loads, and as we all know, a motor is an inductive load whose commutator effectively turns DC into AC, so the formulas are totally different, and MUCH more complex.Comes to 436.3 amps under normal conditions. That is from the OE Manufacturers website.No, that number came from your imagination. The manufacturer's numbers are 140-200A under normal conditions.Ford does not make starters, neither does Motorcraft. I believe Visteon is their current supplier of choice....and your point is...? Do you think Visteon is over in a corner building starters, and Ford is trying to come up with vehicles that can use them? Or do you think Ford designs the vehicle, and the starter needed by the vehicle, and then has Visteon (a company that was originally part of Ford) build them?you can easily see why the Denso is a major HD upgrade over the DD or PMGR starter.You're so far off-topic that I don't even know what you're trying to show.

So let's get back on-topic:

The starter relay is a relay, and always has been, and always will be, and every available document pertaining to Broncos says so. And it can handle the current needed by the starter. And the I post is a discrete output typically used for the Ignition system, not related in any way to the S post or the relay coil.

01/20/11, 09:23 PM
So does your gay lover "Frank"s bronco use a relay or a mag switch? Just wondering.

01/20/11, 09:51 PM
Damn, guys! Didn't we solve this last year. Here it is, post #22.

01/20/11, 11:07 PM
Ok, lets have some fun. That link said 140 to 200 amps as normal. Or at least that is the way you are reading it. Then you said a watt is not a watt. The PMGR starter in question is rated at 1.6kw, meaning it has to produce at least 1.6kw of power in order to pass inspection.

So can you explain and show the math of how a DC motor can produce 1.6kw or more, on 140amps of battery power, remembering that the battery will drop below 11.5volts during cranking. (Wont even get into how that is possible with only a 140amp load, but I will guess the battery Techs dont know thier batteries either).

Do you need me to give you the pull and hold amperages for the solenoid? Or do you already know those as well.

01/20/11, 11:25 PM
As for the number I gave you, that is right out of Denso's brouchure in the link. Just in case you didnt know, I will repeat that Denso built that sstarter in the link for Ford. They did it because the PMGR starter you listed was a failure in demanding service applications.

So the link is from the original designer, and the original manufacturer of the starter. It is their actual real world test results. Your link, not so much.

Some irony in all this as well. The link you posted has to do with the faulty solenoid on the starter. It is true that the original starters did have crappy solenoids. They werent really the cause of the problem listed in the tech article. But then again neither was the problem listed in the article overly accurate either.

So here is the irony. The manufacuturer that ended up redesigning the solenoid first did because someone in this thread had a customer that was having issues with them on the starters they we installing in a Ford racing series. About a month after they started the tests, another much larger company also named in this thread, came to them with the problem.

Still more though. Our customer's issue was the header exited right over the solenoid, effectively like putting a torch to it for the entire race. The redesign fixed it, well it went from lasting 1/2 a race, to going 1/2 a season. It did not however totally fix the bigger company's problem. They fixed that when they stopped using the fender wall mag switch (solenoid). The reason why it was an issue was already pointed out in this thread, and actually where the information first was learned from.

01/20/11, 11:34 PM
So now on to how the automotive industry works. Seems I might have some learning to do.
You have implied that Ford designed the starter, and Visteon just manufactured it. Do you think Ford did that as well when they had Denso produce starters for them, using Denso's patented designs. Same when Mitsibushi built the 6.9 and 7.3L starters. Did Mitsubshi just give Ford their part design and let Ford use thme to create their own starter?

Last time we did consulting for a large OE supplier, the design of the part was their cup of tea. Think about big trucks. Cat, Cummins, Ford, all use virtually identical starters that inner change with Delco, Mitisbushi, Denso, etc. Do you think Delco gave Ford access to their patents? Or maybe Ford said we need this, can you do it for us?

01/20/11, 11:43 PM
Now back on to topic.

Your discription fits 999/1000 as far as the I terminal goes. I have only seen 4-5 of them were the coil was wound to the I, in the 30 years I have been in the business. There is one original manufactured by Ford with no S terminal and the I terminal grounds through the starter. Never on an automtive application though. The others were likely bastardized aftermarket units.

The switch cannot be a solenoid as it does not perform mechanical work. I dont care that you call it a solenoid or a relay, as 99% of the public thinks that is right. My issue is with you telling people that it is right, because it is not techinally. I hope you can see the difference.

I hope you will also see the fundamental differences in the design between it and a relay. Very similar, but yet still very different. Easy to paint them both with the same brush. And in reality what the "public" thinks they are, doesnt really matter. I say that as most will never need to know the difference because they wont be designing a vehicle's electrical system.

01/21/11, 08:43 AM
Didn't they replace the MAG switch with Blinker valves on the broncos after 72?? :)

01/21/11, 09:51 AM
So does your gay lover "Frank"s bronco use a relay or a mag switch? Just wondering.FINALLY!!! An INTELLIGENT argument! :-*

01/21/11, 05:54 PM
Boy's I think you need to go outside and Play! ;D

01/21/11, 08:57 PM
So, could I use NAPA Echlin part number ECH STH404 which is a heavy duty "solenoid/mag/relay" in place of the stock starter relay?

Qty Starter Solenoid Switch
Product Line: Echlin Ignition

Part Number: ECH STH404

Price: Price

Features & Benefits NAPA Echlin Products Meet Or Exceed Original Equipment Specifications. All Products Are Top-Of-The-Line Quality, & 100% Tested & Inspected In An Environment That Is IS09000 Compliant.

Warranty Limited 12 Months

Attributes # Switch Terminals : 4

Material Safety Data Sheet There is no MSDS for this item

Application Information: 2000 Ford Truck F250 Super Duty 3/4 Ton - Pickup
Engines : 5.4 L 330 CID V8 SOHC
Comments: Fender Mounted;High Performance
Per Car Qty: 1

Engines : 7.3 L 446 CID V8 Diesel
Comments: Fender Mounted;High Performance
Per Car Qty: 1

01/21/11, 09:23 PM
I think i will put a hermetically sealed vacuum contactor on mine, thank you:)

01/21/11, 10:33 PM
So, could I use NAPA Echlin part number ECH STH404...?It looks right, but do they show the internal wiring? You looked it up as a starter relay, right? If that other small post isn't connected to anything except when the relay is energized, it's an "I" terminal, and will work in an eB.

01/21/11, 10:57 PM
That same part number is on every ford truck from about 80s through 2006, so i would think I is only hot on start. heck, even the 81 Bronco had this and the old style as same part.

01/22/11, 12:39 PM
That's certainly interchangeable with all the older trucks, but it's not identical. My truck's original relay had no I terminal, and didn't need one. And it was the old-style, of course, with the big studs pointing to the sides.