Common Relays (With Pics)

[SSTJ]

[URL]https://www.the12volt.com/relays/spdt-and-spst-automotive-relays.asp[/URL]

Thanks. Found that yesterday. It's linked in the original post, and I use some of its pictures in the table.

 

[SSTJ]

We call those a Hella type headlight relay. They aren't, but that is where we first noticed them with the dual 87 terminals so you don't have to splice two wires to one output to run a pair of head lights, foglights, or similar.

Thanks. So if I'm getting this right, there are two types of SPST relays: some that have a single 87 terminal, and some that have two. I think that's what @NskLJ was trying to get through my head.

If so, is the pic I used in the SPDT row of my table correct? Or is that one of these two-pin SPSTs that we call a headlight relay?

 

[SSTJ]

Are you trying to wire up the fogs like stock? Folks have already covered the relay basics nicely so I won’t touch on that, however the fog setup Jeep used is a bit different. Since fogs are disabled when high beam turns on, Jeep uses a SPST NC relay (30 and 87 are always connected) and the high beam circuit is the trigger for the 86 pin (coil). This works out to where high beam triggers the relay which disables the fog circuit power that normally flows through that relay.

Now, I’ll have to check the FSMs to make sure that’s how all the different years work. I believe I saw this function in a 2006 FSM, so it might be different in other years, especially prior to 2001 when they used the MFS for everything.

I wasn’t sure if you’re maybe trying to understand the stock fog light circuit and how the relay interacts with it, hence my explanation.

Edit: earlier years were different. 1997 used 2 relays. Fog relay 2 is where parking lamp power normally flows through, and they used a normally closed relay there. High beam feed triggers that relay to go open when high beams are on.

When relay 2 is closed, that parking lamp power flows through relay 2 to the fog lamp switch. When fog switch is on, that parking lamp power from relay 2 flows through and goes to relay 1, where it triggers relay 1 as fog light power. Relay 1 is normally open, so fog light power triggers the relay to close, allowing battery power to go to the fog lights.

So on a 97 you have 2 SPST relays, one NC and one NO. I imagine they are both 4 pin from factory but keep could have used 5 pins, I’m not really sure without going to the pinouts.

Wow, thanks. Nope, I'm not trying to re-create factory wiring setup. I'm just running my own wires from a switch (with relay) to the fogs. But this is really helpful, and I'll add your comments about how 97s use two relays to the quirks of 97s thread. (Do I understand correctly that's the only year you've found that to be true?)

 

[machoheadgames]

Wow, thanks. Nope, I'm not trying to re-create factory wiring setup. I'm just running my own wires from a switch (with relay) to the fogs. But this is really helpful, and I'll add your comments about how 97s use two relays to the quirks of 97s thread. (Do I understand correctly that's the only year you've found that to be true?)

I’ll check later but it’s probably 97-00 that do it that way because that’s how long they used the separate fog switch for. They ran it off the MFS in later years. I’ll check FSMs by year and figure it out and let you know.

 

[SSTJ]

I’ll check later but it’s probably 97-00 that do it that way because that’s how long they used the separate fog switch for. They ran it off the MFS in later years. I’ll check FSMs by year and figure it out and let you know.

Ah, right, that makes sense.

 

[NskLJ]

Got it, thanks. But I don't understand why the metal one would be called a SPST if it connects pin 30 to two pins (87 and 87b you say), whereas the SPST in my table only energizes one pin.

I think you are overthinking this. The one in you table is spst they just don’t need the extra pin so they did not add it in manufacturing. The headlight relay has one pin 30 that will connect to 2 pins 87 when energized. I have seen this labeled spst or dpst. I would not consider it a true double pole relay as both 87 terminals connect to on pin 30 so it does not have 2 true independent circuits in it.

 

[NskLJ]

Since fogs are disabled when high beam turns on, Jeep uses a SPST NC relay (30 and 87 are always connected) and the high beam circuit is the trigger for the 86 pin (coil). This works out to where high beam triggers the relay which disables the fog circuit power that normally flows through that relay.

2006 uses a standard Bosch spdt relay and attaches the fog lamps to pin 87a which is closed to pin 30 (from fog lamp switch) with the relay at rest. The high beams trigger the coil pin 86 to energize the relay which connects pin 30 to pin 87 which has nothing connected to it thereby turning off the fog lamp when high beams are turned on.

 

[NskLJ]

I’ll check later but it’s probably 97-00 that do it that way because that’s how long they used the separate fog switch for. They ran it off the MFS in later years. I’ll check FSMs by year and figure it out and let you know.

I looked at the 98 wiring the fog lamps use 2 relays, relay #1 connects battery power to the fog lamps when the switch is on and low beams are on. Relay #2 interrupts the power to the fog lamp switch when the high beams are turned on turning off the fog lamps.

 

[freedom_in_4low]

Wow, thanks. Nope, I'm not trying to re-create factory wiring setup. I'm just running my own wires from a switch (with relay) to the fogs. But this is really helpful, and I'll add your comments about how 97s use two relays to the quirks of 97s thread. (Do I understand correctly that's the only year you've found that to be true?)

The 97 is a good example for using relays to perform logical operations. You can set up relay arrangements to implement logic gates just like a programmable controller or transistors in an integrated circuit.

 

[freedom_in_4low]

The 97 is a good example for using relays to perform logical operations. You can set up relay arrangements to implement logic gates just like a programmable controller or transistors in an integrated circuit.

View attachment 487442

bonus if you consider the first and fourth lines together in a real circuit, it actually demonstrates an XOR (exclusive OR) gate, which means IF EITHER R1 OR R2 but NOT R1 AND R2.

 

[Jerry Bransford]

bonus if you consider the first and fourth lines together in a real circuit, it actually demonstrates an XOR (exclusive OR) gate, which means IF EITHER R1 OR R2 but NOT R1 AND R2.

I went through several courses in Boolean equations used in logic circuits and used them extensively way back in my tech days. I assure you they're way above this discussion level lol.

 

[NskLJ]

I went through several courses in Boolean equations used in logic circuits and used them extensively way back in my tech days. I assure you they're way above this discussion level lol.

Yes, we are just turning on a set of fog lamps.

 

[machoheadgames]

I looked at the 98 wiring the fog lamps use 2 relays, relay #1 connects battery power to the fog lamps when the switch is on and low beams are on. Relay #2 interrupts the power to the fog lamp switch when the high beams are turned on turning off the fog lamps.

Yeah, I am comparing both side by side and they are a bit different (97 vs 98). Same overall execution using 2 relays, but a bit differently how they routed the power between the two. Still ultimately 2 relays, with parking lamp power triggering one relay and high beam disabling the whole mess via the other relay.

 

[SSTJ]

I think you are overthinking this. The one in you table is spst they just don’t need the extra pin so they did not add it in manufacturing. The headlight relay has one pin 30 that will connect to 2 pins 87 when energized. I have seen this labeled spst or dpst. I would not consider it a true double pole relay as both 87 terminals connect to on pin 30 so it does not have 2 true independent circuits in it.

Got it. Updated the table.

 

[SSTJ]

I'm wiring some fogs, and realized that I still don't know the basics of relays. If others will help fill this in, I'll make a table that may be useful to others who come along. For example, I found what I think is a SPDT in my garage today, but cannot remember why I bought it, or when it would be helpful.

———

From @freedom_in_4low: A relay has a coil, and contacts. The switch supplies voltage across the coil (pins 85 and 86), and then the contacts change state. See also here.

Name​	Pics​	Diagram​	Notes​	Example​
SinglePole, SingleThrow (SPST)	View attachment 487244 View attachment 487245	View attachment 487273	Most commonly used for an aftermarket accessory. Wiring aux lights, for instance. As the name suggests, this offers only one "on" position for a single switch. A typical SPST relay will have 2 pins for the contacts, which will be normally open (NO) which means the contacts will be open when the coil is not energized, and the contacts will close when coil is energized. These are pins 30 (common) and 87 (NO).	Summit Racing
SinglePole, SingleThrow (SPST) "Headlight"	View attachment 487349 View attachment 487348		This is still a SPST, but instead of providing power to one pin (the 87), it provides power to two pins (both 87s) simultaneously. Can be helpful when trying to put two accessories on one switch, such as a pair of headlights or fog lights, which is why @mrblaine gave them this nickname. The alternative for wiring a pair of fog lights is to use a SPST above, and split the wire connected to 87 into two, one for each light.
SinglePole, DoubleThrow (SPDT)	View attachment 487247 View attachment 487305	View attachment 487306	For a single switch that accommodates two "on" modes, and no "off" mode. A double throw has 3 pins for the contact: common (C), normally open, and normally closed (NC, pin 87a). When the coil is not energized, you will have continuity between C/30 and NC/87a, but not C and NO/87. When energized, you'll get continuity between C and NO but not C and NC. You would use this in an either-or situation, as in either one or the other will be powered at all times. You don't have to use the second 'throw' for it to function, so this can also be used as a SPST if you just ignore the center pin.	Amazon

@Chris how about moving this to the resources section?

 

[SSTJ]

I think you are overthinking this. The one in you table is spst they just don’t need the extra pin so they did not add it in manufacturing. The headlight relay has one pin 30 that will connect to 2 pins 87 when energized. I have seen this labeled spst or dpst. I would not consider it a true double pole relay as both 87 terminals connect to on pin 30 so it does not have 2 true independent circuits in it.

Ok, so they do have a name. "Dual Make" SPSTs. I feel better now.

@mrblaine

 

[Chris]

@Chris how about moving this to the resources section?

Will do!

 

[NskLJ]

Ok, so they do have a name. "Dual Make" SPSTs. I feel better now.

@mrblaine

I have never heard that term before but I see it in the 12v link. Your best bet is to refer to the diagram on the side of the relay. When I did google it the correct relay came up with two 87 pins and not the 87a pin.
I have always referred to them as the “metal body Bosch relay” and @mrblaine refers to them as “hella headlamp relay” which would lead me to the correct relay also.

 

[SSTJ]

Those are spdt like the larger non metal body Bosch relays. They just made them smaller as the need for relays is greater in newer vehicles. Real estate in fuse boxes is a premium

Thanks. Updated the table.

You think I'm missing anything else that would be helpful to someone who's new to relays or looking for a recommended product?

 

[mrblaine]

I have never heard that term before but I see it in the 12v link. Your best bet is to refer to the diagram on the side of the relay. When I did google it the correct relay came up with two 87 pins and not the 87a pin.
I have always referred to them as the “metal body Bosch relay” and @mrblaine refers to them as “hella headlamp relay” which would lead me to the correct relay also.

Only because I saw them in a few Hella light harnesses and thought that was a neat way to hook up two lights off of one relay. Now we just look for dual 87 pins.