starter motor relay

Fitting An Auxiliary Starter Motor Relay

Tom wrote to us saying that he is having trouble with an intermittent no-start issue with his 1990 model Toyota truck and from what he has written it sounds like either the contacts in the starter solenoid are worn or there is a current drop issue from the battery to the starter. On occasion the factory starter relay can be heard clicking when the key is turned but nothing is happening at the starter motor.

What we are going to look at today is fitting an auxiliary relay to the starter motor circuit in an attempt to overcome this no-start issue. While I have seen this ‘trick’ solve these types of problems on numerous occasions it is definitely not an answer to all starter problems. For example, if the solenoid contacts are completely worn out or the starter is otherwise past it’s use-by date it is going to do very little but I think it’s worth a go if you have intermittent problems such as what Tom is experiencing.

Basically the use of a relay allows the battery’s full current to reach the solenoid. Check out my dodgy diagrams below for a basic idea of how it works.

starter motor wiring diagram

Basic Starter Diagram Without Relay

starter motor wiring diagram

Basic Starter Diagram With Auxiliary Relay Fitted

NOTE – Some auto parts stores and auto electricians will have a pre-wired relay setup if you don’t want to go through setting one up yourself.

Ok, let’s take a look at what you will need to get this operational.

  • One 4 pin – 30 amp (minimum) Relay
  • 5 insulated Female Blade connectors
  • 1 Male Blade connector
  • 2 ‘Ring’ type connectors (1 large, 1 small)
  • 4 lengths Electrical wire – more on the wire in a moment.

The first thing we need to do is take a look at the relay and identify the four different pins. They should be numbered on the underside of the relay box with 30, 87, 85 and 86. If you’ve fluked it for a five pin relay there will also be 87a, that’s ok, that pin won’t be used.

relay pin numbers,electrical relay pin numbers

We’ll start with pin 86. This is the earth for the switching side of the relay. A simple loop with an insulated female blade connector on one end and a small ring connector on the other is all that is needed here. The wire for the switching side of the relay need not be real heavy, all it does is control the on/off side of the relay so it is not carrying a lot of current. The wire for pin 30 and 87 should be of a reasonable gauge as it carries the current from the main positive battery lead to the solenoid terminal.

starter relay wiring

Earth Wire For Pin 86

Next is the other side of the relay switching circuit, pin 85. This wire needs a insulated female blade connector on one end and a male blade connector on the other. The length of the wire needs to be decided now, it needs to be long enough to go from the starter motor to a location that will provide an earth (such as a small bolt in the engine bay that screws directly into the vehicle’s body) and it should be up nice and high to avoid getting covered in crap.

starter motor relay wiring

Wire For Pin 85 

Next up is pin 30. As stated above this wire should of a heavier gauge than the first two however this is not absolutely necessary, it will work with the same gauge wire, it just doesn’t have the capacity to carry as much current. The wire for pin 30 requires an insulated female blade connector on one end and a large ring connector on the other. It should also be the same length as the wire for pin 85.

starter motor relay wiring

Wire For Pin 30

Last but not least is pin 87. This wire should be the same length as the previous two and have an insulated female blade connector on each end. Yes, I know one of mine is not insulated, we ran out of insulated ones ok!

starter motor relay wiring

Wire For Pin 87

Put it all together and you should have something that looks like this –

starter relay wiring

Now it’s time to fit it. The first thing you must absolutely do is REMOVE THE NEGATIVE TERMINAL FROM THE BATTERY!!! The last thing you want is to fry yourself or the ECU or something just as expensive. Don’t forget to make sure you have the code for your radio before you disconnect the battery if you have a pin number security stereo.

The first thing we need to do is remove any covers from where the battery lead connects to the starter motor and disconnect the solenoid switching wire.

starter motor relay wiring

Then we need to attach the wire from pin 30 to where the main starter terminal. In our case the Camry had a 12mm nut securing the lead.

starter motor relay wiring

Next is the wire from pin 85. The male blade connector goes into the original solenoid switching wire.

starter motor wiring

And then the wire from pin 87 goes into the solenoid switching terminal on the starter.

starter motor relay wiring

Now we can mount the relay in a suitable location, making sure to securely attach the earth wire and route the wires away from areas of high temperatures and keep them away from sharp edges.

starter motor relay,starter relay wiring

Now you should have something that looks like this –

starter motor relay wiring

I strongly suggest that you wrap the wires in electrical tape or some of that flexible convoluted tubing to keep everything neat and tidy. This fitment was done only for the purpose of this article so I’m not going to take it any further but it does make things easier to work on when everything is tidy.

The next step is to refit the battery terminal and give the key a hit. All going well the starter should give you months, even years more of faithful service.

7 thoughts on “Fitting An Auxiliary Starter Motor Relay

  1. hi ive hav mitsubishi canter and wen i turn the key i can hear a click noise but it wont start can i bypass it anyway thanks heaps.

  2. Mate this is the best step by step process going
    IT WORKS …….
    Thanks alot it saved alot of time .

  3. Thanks for a brilliant article my 1987 SJ413 has been giving me grief. new battery , starter motor. water pump and cam belt and the same dreaded problem started then dead and so on . I had already fitted a relay but the wiring was different to yours copying your wiring step by step and Bingo thank you so much I can now fish the beach confident of starting every time .

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