Difference Between A 4 PIN And 5 PIN Relay

It isn’t uncommon to run across needing to use a relay when working on cars. Especially if the car is an old one, a “pre-computer” and high in electronics car. So do you know when to use a relay or why? Do you know the difference between a 4 PIN and 5 PIN relay?

Why Do You Need A Relay

Back when I was in High School, there was this girl I really liked. She was smart, funny and hot. She had an import car that the fuel pump stopped working. I heard about it and wanted to impress her, so I came by prepared to save the day. I found the electric fuel pump worked but wasn’t getting power when you turned the key on. The car had been worked on by many shade tree mechanics in the past. This wasn’t going to be a OEM type repair.

I suggested a relay and was going to go get one when these other guys from school showed up to tell me how wrong I was. All I needed to do was just run a wire straight off the ignition switch to the pump. I tried telling them that would burn up the switch. Turned out the girl wasn’t as smart as I thought she was. She went with the idiot boys suggestion. A week later, the ignition switch would no longer work. So why did that happen?

The fuel pump pulled around 15 amps, probably more when it first come on. This puts a heavy load on an ignition switch. That load creates a lot of heat, not to mention the arching on the contacts. The ignition switch already had a load turning on so many other things, adding that was too much. It was also quite possible the pump had issues and is why it burned out whatever was engaging it before. A relay would have allowed very little amps to be pulled on the switch. A 30-amp relay could have been put in between and handled the load. All the while, protecting the ignition switch.

Reasons To Use A Relay

  • To reduce using long lengths high amperage wiring. This can reduce cost
  • To allow controlling of high current circuits with low current switches and controls
  • Allow multi-functions from one control

It is much easier and cost effective to create a wiring harness out of thin gauge low current wire. It is also much easier to route thin wire to control devices as well. If you had a high wattage audio amplifier in your trunk. It would be far cheaper and less complicated to run one high current wire from the battery and a harness of small wire, rather than run a harness of heavy wire. One heavy battery wire, then low current speaker wire and a trigger wire from the radio. The amp has a built-in relay. When the radio sends a low current of 12 volts to it, it then engages the heavy battery wire to provide a powerful sound.

For running multiple circuits off one control device, use this as an example. You want to run an electric fuel pump and turn on power for the electric windows. This can be accomplished with two relays ran from one switch.

The most common reason to use a relay is so that a low current circuit and activate a higher current device. For example, a radiator fan. Just with a small wire run to a coolant temperature switch. When that switch gets to a specific temperature, it grounds. That ground engages the relay, allowing full fused battery power to run the fan. Second most common is to run multiple devices from one switch.

What Is The Difference Between A 4 PIN And 5 PIN Relay?

I know this sounds like a smart answer, but one relay has 4 prongs and the other has 5. So, what is the real difference, only one detail. They are both SPST, (Single Pole, Single Throw). However the 5 PIN does one thing different and is for the most part completely useless. You can watch a full explanation video that covers this entirely, Click HERE

Both relays have numbered pins on them. The 4 PIN has 85, 86, 87, and 30. The 5 PIN has one extra numbered 87a. Pins 85 and 86 on both, are used to activate the relay. On the 4 PIN, 87 and 30 is switched on when power is provided to 85 and 86. However, on the 5 PIN, without any power to the relay, pins 87a and 30 are connected. When power is added to 85 and 86, 87a and 30 break connection so that 87 and 30 connect.

What is sad, automotive parts stores have less and less people that really know cars anymore. So if you are just wanting to hook up like a cooling fan, fuel pump, fog lamps or a stereo amp. They always try and sell you a 5 pin relay when the 5th pin is completely useless. Be sure to ell them that you want a 4 PIN Relay when hooking up said mentioned items.

Why Do They Make A 5 PIN?

To get a better understanding of the differences are and what a 5 pin is used for, watch my YouTube Video.

If you wanted a light to tell you that your device is off. Many call a 5 PIN relay, the “high-beam – low beam” relay. You could use it for that, however you will need another relay to add power to the pins 87a and 30. In my experience, a 5 pin would better serve a car alarm system. It would work perfect for a signal always on, and if a door is opened, it breaks the signal. That is a real brief explanation but seems more reasonable.

Overall, the 5 PIN doesn’t do much for you in the way of adding accessories or devices to your vehicle. 99% of the time, you will be using a 4 PIN.

If you have any questions or ideas, please leave me a comment or contact me through my main website. https://rodsshop.org/contact/

Published by RodsShop

I have decades of automotive experience. From general mechanics to highly professional auto body and paint. I started a YouTube Channel on How-To fix things on cars. It ranges from motorcycles, general mechanics to auto body and paint. I am a giver by nature and I want to help others by sharing the things I know.

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