Because the most of us knows, that a Bad Main Relay is the most common problem. So I thought to write this DIY
What It Does
Our Main Relay is also known as "PGM-FI Main Relay". It controlles all power going to the ECU and fuel pump. This one operates the fuel pump, and supplies power to the fuel injectors. Without this thing in proper working order, your car is going nowhere.
Symtoms Of Bad Main Relay (Outside)
Getting symptoms of a bad main relay means usually, that on very hot days, say 90 °F +, sometimes our cars are difficult to start. Sometimes it seems to be very random. You notice that during the engine turns over with no problem, but will not start. It sounds like it's not firing. After 5-10 min's, it will eventually fire and the engines starts. The other getting reason could be, if the starter cranks but don't start, then in most cases, you may have a bad main relay! I suspect the main relay is temperature sensitive. Sometimes I'm afraid it eventually will leave me stranded, but it's not happened till today.
Another common problem also caused by the main relay is, that our car starts fine, but stalls as soon as you release the key. In this situation, it may take many attempts at starting before the car will continue to run with the key released. Don't laugh, it really happened. Really !
Symtoms Of Bad Main Relay (Inside)
Bad Dry Joins
The first cause could be the well known "Hairline Cracks". A bad main relay is a common fault like "Possible Hairline Cracks" or "Bad Dry Joints". The last item is sometimes possible through any vibrations and changes in temperature cracking the solder around a joint. This can cause intermittent failures, which are especially likely to happen when the car has been parked in the sun on hot days.
Hot Main Relay
Another cause could be a Hot Main Relay. That means, that this relay should not get hot even if the pump is drawing too much current - until the excess current starts to kill the contacts. There are 3 possible reasons for a hot relay:
- Cold Solder Point
- Bad Crimp Joint
- Bad Relay Contacts
Over time these bad joint gets worse due to the heat (heat cool cycles), the resistance goes immediately up, the joint gets hotter, the voltage at the pump goes rapidly down. So then it needs more current to do the same work so the joint gets hotter, it gets worse.
How To Check
Sometimes it helps to see what's going on in this relay. Our Main Relay will click 3 times during the starting process. If problems arise, one of those clicks (usually the third) is missing. If you can hear all 3 clicks, the Relay is fine. You don't need to replace it.
Turn ignition to ON
: Click 1
CEL light goes off
: Click 2
Turn the key to start
: Click 3
When you turn on the ignition switch to "ON" => Position II (don't turn to start) you should hear a click and a hum sound at the fuel filter. That means that the fuel pump gets energized. If you don't hear it, it's probably the main relay.
This one is located behind the lower dash panel on the left side. It's a small box labeled "Mitsuba Relay Main Assy RZ-0105" in white letters on it. It's easy to get access for it. It's held with a 10 mm screw and a small self-locking hook.
All you need is a simple screwdriver and a 10 mm nut and rachet.
How To Remove The Dash Panel
Please see Dash Panel Lower Removement
How To Remove The Main Relay
After you have taken off the panel, you will see now our Main Relay.
Now take a 10 mm nut and rachet and turn it counterclockwise to remove.
Now unclip both connectors and you got it
Inside The Main Relay
A relay is an electromagnetic switch. Anytime you want to switch a device which draws more current than is provided by an output of a switch or component you'll need to use a relay.
When power or ground is applied to a coil, the relay clicks on or off. The result is, that a power wire to a function is either closed (connected) or opened (disconnected). When the line is connected, the function is turned on and works. When the line is disconnected, the function is turned off and will not work. A relay can be used on alarm systems, things like to disable a starter, to flash the parking lights, and so many more of them.
Now let's take a look inside our relay...
Actually our Main Relay contains 2 separate relays. One of them is energized whenever Ignition (IGN) is powered ON. It supplies batterie (BAT) voltage to the ECU, power to all 6 injectors and power to the second relay.
The second relay is energized for exactly 2 sec's, when ignition is switched ON, and when the engine is running. It supplies power to the fuel pump.
How To Repair
Like above described, a typically "Bad Dry Joint" looks like, when it have small cracks around the pin and PCB track. But you can see that when you look very closely. These have a characteristic look...
Try to inspect your circuit board of your main relay. That's the cheapest way. But you need somebody who's a genie or have the well know know how about electronics. This a tricky work, but for somebody who does have the needed knowledge it's really easy to fix. To fix it, you will need to...
1. Remove the solder from all connections
2. Re-soldering your connections
3. Inspect the joints for any imperfections
4. Spray on conformal coating, and wait till it's dry
5. Re-assemble in reversed sequence
To get more infos I recommend you to read the PGMFI Main Relay Repair
tutorial. Excellent taken pics will guide you through the entire knowledge to get your Main Relay repaired. I strongly recommend you to read all pages! It's worth a shot. A must to see this, really