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20. Main Relay Harness Tests

If you know the main relay is good, hopefully because you replaced it after 120,000 miles or 10 years, whichever came first, but it still doesn't operate properly, the main relay harness needs to be inspected. Here's how to do it on many Honda 7-P relays:

1. Disconnect main relay connector. Check for continuity between BLK Terminal 2 and body ground. If there isn't continuity, repair an open in BLK wire between the main relay and the main ECM ground, located on the thermostat housing. If there is continuity, then:

2. Measure voltage between YEL/WHT Terminal 1 and body ground. If there isn't battery voltage, replace the ECM (15 A) fuse in the under-hood fuse box or repair an open in the YEL/WHT wire between the main relay and the ECM fuse. If there is battery voltage, then:

3. Turn the ignition switch to ON and measure voltage between BLK/YEL Terminal 5 and body ground. If there isn't battery voltage, then replace ACG (ALT) (S) (15 A) fuse in the under-dash fuse box or repair an open in the BLK/YEL wire between the main relay and the ACG (ALT) (S) (15 A) fuse. If there is battery voltage, then:

4. Turn the ignition switch to START. For manual transmissions, the clutch pedal must be depressed; for automatic transmissions, the gear selector must be in Neutral or Park. Measure voltage between BLU/WHT Terminal 6 and body ground. If there isn't battery voltage, replace STARTER SIGNAL (7.5 A) fuse in the under-dash fuse box or repair an open in the BLU/WHT wire between the main relay and the STARTER SIGNAL (7.5 A) fuse. If there is battery voltage, turn the ignition switch OFF, then:

5. Disconnect the negative battery cable. Next, disconnect the "A" connector from the ECM and check for continuity between GRN/YEL Terminal 8 and Terminals A7, A8. If there isn't continuity, repair an open in the GRN/YEL wire between ECM (A7, A8) and the main relay.

 

test #1 didn't pass, no continuity on the black wire. Where exactly does it go and ground to?

 

asdf

 

"located on the thermostat housing" 94

 

Hmm, well that's all connected properly. With the ignition on the "on" position, (2nd click) what's supposed to be sending the 12v to kick on my ecu?

 

ttt

 

If PGM-FI Main Relay is not turning on, [no ground] the ECU will not get the 12V+ signal to IPG1&2 from the injector relay, so ECU will not "turn on".

Try this, use a jumper to supply the PGM-FI Main Relay with a ground, [black lead] direct from chassis.

The injector relays coil must have a ground for it to turn on when you turn on the ign. switch.

If injector relay does not turn on, the fuel pump relay will not turn on, the fuel injectors will not have power, the ECU will not have power at IPG1&2, the ECU will not trigger the fuel pump relay, the ECU will not pulse the injectors among other things, in short, engine will not run or even start.

You need to start by getting the PGM-FI Main Relay working. 94

 

Jump the slot #2 [black] wire with a direct ground chassis? I used a volt meter, and when i switch the key on the "on" position. This shows a 12v power.



The BLACK and WHITE wire side-by-side get a 12v on my voltmeter when I click the key to the "on" position. i believe these power the ecu?

So it's Ignition > Main relay > ECU > Fuel pump. correct?

 

Okay, I just jumped it with the ECU unplugged, key on the "on" position. Relay clicks when I jump the slot #2 with a chassis ground. But it blows my ECU fuse under my hood. wtf?

So instead of my black wire being a ground, seems like it's getting power from somewheres....? Where does the slot #2 on the main relay harness go too?

 

Slot #2 Black is a ground, [should be a ground] it is the ground for the fuel injector relays coil, [#85 of relay] it will test as 12V+, [if it is not grounded] when you turn on the ign, because the other side of the fuel injector relays coil, [#86 of relay, probably yellow/green] is supplied 12V+ by the ign, switch, you are "reading" the 12V+ on the other side of the ungrounded load.

If a fuse blows, you have a short on the circuit the fuse is on.

Sorry, I do not have a wiring diagram for a 99 Civic, and I can not see all the wire colors of your PGM-FI Main Relay harness.

I need a better pix so I can see all the wire colors.

You can also unplug the relay and test the leads for power, you should have one that is hot at all times, [white/black] you should have one that is hot when ign. switch is in run or start position, [probably a yellow/green] and the 3rd one that is hot only when ign. key is in start position, [probably a blue/white].

No matter what the colors are, you must have the above 3 powers, test them and let me know if the colors are the same as I have described or if they are a diff. color.

Also let me know which lead is the one with the blown fuse, replace blown fuse, find the 3 power leads, remove the fuse and see which lead loses power, the 12V+ hot at all times, or the 12V+ when ign. switch is on.

Once I know which lead is losing power, [constant or switched] we can test for the short]. 94

 

wow, so i re looked at the plugs into the ECU, and two plugs are shaped identical so they fit into one another's slot. switched em around and everything works!

only one problem, she doesn't crank i hear the ignition clicking, but starter isn't moving. where should i start now?

 

Is the batt. charged?

Try a bypass jump...
https://honda-tech.com/forums/showth...&highlight=fcm 94

 

yesir, brand new battery, the bypass jump works. first thing i tried.

 

Then it is either the ign. switch, the starter relay, the CIS, [A/T Gear Position Switch if it's an automatic] or the wiring between the above. 94

 

whew* got it all figured out. thanks for your imput

 

So what was it? 94

 

two plugs that go into the ECU are identically shaped. i had them in eachother's slots. it's funny cuz it made the Main Relay read all retarded which was difficult to diagnose