Did I get the wrong starter relay?

[Golden Helmet]

Go figure, 2 days after getting my truck back with the hydroboost working, the starter relay on the fender craps out on me. And now it looks like I got the wrong relay, because I just can't catch a break sometimes

Anyways, I ordered a Motorcraft starter relay from O'Reilly's, and it said it was the fender mounted one. I went to put it in today, and the new one looks different from the old. Almost the same, but the new one has an extra small post that I'm not sure what to do with. Can I use what I've got, or am I SOL? Here's some pictures:

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[chris142]

Don't use the extra post

 

[The Warden]

I think the extra post is a ground, in case the mounting surface is plastic, painted, etc. where the solenoid won't ground at the mounting holes. I would check that with a ohm-meter, but note that the brass terminal on the top in the second picture actually has a - molded into the plastic next to it

 

[DaveBen]

The extra post is NOT a ground. Do NOT ground it. Just use the 'S' post to start your truck.

 

[DaveBen]

The extra post is NOT a ground. Do NOT ground it. Just use the 'S' post to start your truck. The post that you said is labelled - is actually 'I'. Note the position of the 'S' post and the 'I' post.

 

[Golden Helmet]

Thank you guys yet again. I just put the relay on, ignored the extra post, and the truck start right up with no sparks. Back in business

 

[The Warden]

The extra post is NOT a ground. Do NOT ground it. Just use the 'S' post to start your truck. The post that you said is labelled - is actually 'I'. Note the position of the 'S' post and the 'I' post.

I stand corrected. What is the purpose for the extra post, then?

 

[towcat]

i'd hazard a guess at momentary 12v while the starter is engaged.

 

[OLDBULL8]

All Ford (Motorcraft) starter relays are internally grounded (ground is connected to the base), the extra post is I for Ignition, can be used to supply power to the spark plug coil on a gas engine.

You can plainly see the I sign on the relay. S is always + Positive (Signal wire) like from a key switch.

 

[chris142]

Some gas engines use that extra terminal to supply 12v to the coil during cranking

 

[typ4]

Some gas engines use that extra terminal to supply 12v to the coil during cranking

Finally someone got it right.

 

[HS108]

In order to save money they make the parts apply to as many uses as they can.

Think about how much it would cost to make/distribute the 2 relays (one with 2 posts/one without) when they can just sell one for both applications

all about the $$$$ Its pretty smart if you think about it

 

[jwalterus]

SW-1951-C
E9TZ-11450-B

those are then numbers that should be on the box for a 3 terminal relay, E8TZ-11450-B is for a gas engine application
in this case, a solenoid is a solenoid as far as I'm concerned

S - start circuit
I - Ignition circuit

and as posted previously, it send full voltage to the coil, it's actually a holdover from the old days with points ignitions
Take my mustang, 1973, had points ignition, when running, there is a resistor circuit for the coil, it only gets about 7-9 volts, what that I terminal does is give the coil a full 12v while cranking so you have a hotter spark when starting, as soon as you let off the key, it goes through the resistor circuit instead, a constant 12 will kill the old coils.
That resistor circuit is why when people put on a new high performance coil, HEI distributor, or an electronic ignition box, you can't use the existing wiring to the coil as is, you are actually supposed to replace the entire wire because it can't handle the load on most older cars, but you can go under the dash and remove the resistor, you'll just be slowly frying the wire.

 

[OLDBULL8]

Some gas engines use that extra terminal to supply 12v to the coil during cranking

Oh crap yes. What was I thinking? I knew that.

Will correct my post so that it's not wrong.

 

[Golden Helmet]

I'm just gonna bump this instead of making a new thread. The new relay worked perfect for the first few days, but now it's acting up. When you turn the key, sometimes the truck cranks like normal and other times you turn the key and nothing happens (other than the gauge cluster turning off, stereo off, etc), no relay click or anything. When I jump the two big posts together the truck fires right up, so it's gotta be either the new fender-mounted relay, my ignition switch, or something to do with the POS alarm system (I don't want to think about that nightmare).

I'm pretty sure that the new relay is bad and it's just another bad luck part, but before I waste more time and money, is it possible for a relay to have an intermittent failure like this? Or is it only Dead / Not Dead with these relays?