JAlvarez

I'm having a main relay problem. The fuel pump isn't getting any power. When I jump the 5 and 7 terminals on the main relay plug, I can hear the pump whirring, so that is fine.
I checked the fuses 24 and 31, there is no break in the little bridge inside the fuse, and both notches on top read 12V to ground.
I tried resoldering the main relay, and there were two bare looking posts, but fixing that didn't help.
I tested the main relay by using some clips, one to positive terminal, one to ground, and it tests fine. During the test I realized that for the 5th terminal to connect to the 7 terminal and give it power, there must be voltage to the 3 terminal and ground from the 8. I tested terminal 3 on the main relay, and there was no voltage.
Looking at a wiring diagram, it seems that the power to the 3 terminal comes from terminals A23 and A24 in the ECU. I tested posts A23 and A24 in the ECU and it read 4.5V, should it be higher? I also checked those wires to main relay terminal 3, and it read 3.75ohm, is that too high?

Oh, I drive a 1994 EX with auto transmission, no mods.

Thanks,
Juan

Former User

Terminal 5 delivers voltage from the ignition switch via fuse 24. If you jump terminal 1 (from battery via ECU fuse 31) to terminal 7 (to fuel pump) in the main relay plug, does the fuel pump run?

JAlvarez

Yes.

Former User

Is the ground wire attached to the thermostat housing clean and tight? If so, is there continuity to body ground at terminal 2 (Blk wire) of the main relay plug?

JAlvarez

Ground is clean and tight.

No, it fluctuates between 40 and 70 ohms.

JAlvarez

I'm sorry, i wasn't testing it on a suitable ground, there is continuity.

Former User

Explain how you did this^ test. For example, was the main relay connector unplugged? Was the key in ON(II)?

Also, have you checked for continuity of the Grn/Yel wire running from main relay terminal 8 to ECU terminals A7/A8?

JAlvarez

Key was ON(II) and main relay was unplugged, I put one end of the multimeter into terminal 3 and the other on the ground up by the driver's side headlight.

Main relay terminal 8 and ECU terminals A7 and A8 do have continuity.

Former User

The way you did this^ test, you should have measured 0V at terminal 3, as you did.

Try these voltage tests. Leave the main relay connector plugged into the main relay but unplug ECU connectors A and B. Now turn the key to ON(II) and individually measure voltage to body ground at ECU terminals A25, B1, A7, and A8. What voltages do you measure?

By the way, you can use the metal frame under the dash as ground.

That's fine.

JAlvarez

I'm reading 12V at A25, B1, A7 and A8.

Former User

With ECU connectors A and B still unplugged, if you ground ECU connector terminal A7 or A8 and then turn the key to ON(II), does the fuel pump run?

If the fuel pump runs, leave ECU connectors A and B unplugged and then test for continuity to body ground individually at ECU connector terminals A26 and B2.

JAlvarez

Individually nothing happens, ground them both and it goes.

Former User

Very good. Now test the ECU connector A26 and B2 terminals as I mentioned above. If they have continuity to body ground, the final test is to determine whether the MAP sensor gets the expected 5 reference volts from the ECU. I'll tell you how to do that voltage test later.

Former User

IF A26 and B2 have continuity to body ground, then:

1) Reconnect the main relay and ECU connectors
2) Unplug the MAP sensor connector
3) Turn the key to ON(II)
4) Measure voltage to body ground (thermostat housing) at the Yel/Red wire terminal of the MAP sensor connector
5) Measure voltage to body ground (thermostat housing) at the Wht wire terminal of the MAP sensor connector

What voltages did you measure?

JAlvarez

A26 and B2 are reading 17ohms, is that too much?

Both wires at the MAP sensor are reading 5V.

Former User

Your test results are pointing strongly to a bad ECU. Do you know anyone who has a known working ECU that is the same as yours?

JAlvarez

I do know another guy with a `93 Civic. His ECU number is P28A52, mine is a P28A51, will they be compatible?

Also, what typically causes an ECU go bad? Can it go bad on its own?

Former User

Your buddy's ECU should be fine for testing purposes.

Sometimes ECUs are blown by swapping sensor plugs, but in your case i think it probably just went bad. The ECU is basically a computer and once in a while a circuit is damaged.

94EG8

It's basically the same ECU, it's still an automatic P28.

If nothing changed and the ECU just randomly died there's a good chance the single brown capacitor finally dried up and blew. You can buy these for next to nothing and solder them on pretty easily.

Former User

Do you know of a DIY for this job? That would be nice to add to the FAQs sticky.

94EG8

No, but I could probably put something together. I'll have to order some caps first though. I've got a stack of ECUs here I could do.

Former User

That would be great (as long as it doesn't take you away from something more important).

JAlvarez

New ECU worked, fuel pump started right up.

Thanks man!

Former User

Sweet