Former User

The ECU connector must be plugged into the ECU for the voltage test. This^ points to a bad ECU, if the ECU connector is fine.

ej1b18c1

the ecu is all plugged in with key in II. Weird because the ecu worked perfect before with no issues. IDK I'm stumped and would hate to buy another ecu and still have the same problem.

Former User

If the ECU does not supply voltage to the Wht wire terminal, then the ECU is bad.

ej1b18c1

So I took the wht wire out of the plug and swap another wire in there straight to the ecu, tested it and was the same result of 0.04. Did this just to eliminate any kind of shorts etc in the new wire but no luck so after I was done I just put it back how it was. So with this said the ecu must be gone?

94EG8

Pull the cover off the the ECU and look for anything burnt. If you get certain plugs backwards you can burn certain components, but normally the ECU is repairable.

ej1b18c1

Ya I opened it up and don't see anything burnt. Weird because like I said it was fine before I swapped it in the hatch. Really trying to avoid spending useless $$$ on it when I'm selling it anyways.

It seems driveable but don't want to sell it like that.

94EG8

have you tried the ECU in another car just for fun?

ej1b18c1

I can try it in my ej1, but has a gsr it and IDK if that will be a good diagnostic are not.

94EG8

If it doesn't throw a MAP code in that car then there's something else wrong.

ej1b18c1

Okay, I will try it out and see what happens. I always thought when an ecu is bad it has a solid cel on with no error codes.

94EG8

Not always.

ej1b18c1

**update**

I put the p28 in my gsr ej1 and zero CEL's. So it can't be the ecu. When the p28 is in the hatch the cel comes on as soon as I start the car, as if the plug is not even connected to the map sensor. I have no idea at this point, and must be wiring but all show continuity from the plug to the ecu. I've tried other map sensors as well and not sure what's going on so any suggestions will help a ton!

94EG8***** THANKS!!! You saved me $80 for your bright idea.

Former User

The problem has not changed. You still lack voltage on the Wht wire. This takes you right back to the ECU connector. Unplug ECU connector D and check for continuity between D17 (terminal side of connector D) and the white wire terminal in the MAP sensor plug.

94EG8

You can have continuity and still have 2 or more of the 3 MAP sensor wires shorted together, or 1 or more of them shorted to ground. Now you get the fun job of tracing the wiring to see just what exactly is going on.

glad to help

Generally speaking when the ECU was fine before the swap and it mysteriously appears bad I'm very hesitant to replace it even when the flowchart seems to point to it.

ej1b18c1

I guess I have to check all the wiring. I know each wire on the map plug has continuity to ecu so there's gotta be a short somewhere. I could just hard wire all three wires to the ecu and see if that works, even though it will take time but so is tracing all the wires back to the ecu.

note* It's not like the cel comes on during driving, it's right away like it's not plugged in and nothing changes when plug is not connected so it's gotta be a short somewhere. Wire hunting in the morning, thanks for all the help much appreciated

Former User

Don't get distracted. By your own test results, the Wht wire lacks 5 reference volts. You either have a wire open in the Wht wire somewhere between the ECU and MAP sensor or the ECU is bad. It's as simple as that. Stay focused.

td1238

Typically, with most MAP sensors, there is a 5 volt supply wire and a 0 volt supply wire, which, theoretically is at the same potential as chassis ground, but due to real world electrical loads on the chassis, this can vary, which is why it is separated from those electrical loads. The MAP sensor is a rather elaborate electronic potentiometer that varies the output voltage from 0 to 5 volts based on pressure/vacuum imposed upon the stress gauge device within the sensor. If you had 5 volts at both input wires, there would be nothing to reference between. That said, one wire should be 5 volts, and one should be 0 volts, and the 3rd output wire should vary between these two voltages depending on vacuum. Also, all voltages read from the 3rd 'output' wire should be referenced to the 0 volt wire, and not the chassis, as this will isolate readings from voltage drop and changes on the chassis ground when things are turned on like headlights.

Former User

The MAP sensor has three wires, two of which supply 5 reference volts. Please refer to the Honda Civic service manual.

ej1b18c1

after looking everywhere im stumped. been tracing wires and everything looks good. have tested continuity from plug to shock tower from shock tower to ecu and even plug straight to the ecu. i cant find a short anywhere. having a hard time thinking its the ecu when i tested out on my other car and it had no cels what so ever. i will keep looking.

Former User

If the Wht wire has continuity and no short from the ECU to MAP sensor and voltage still does not reach the MAP sensor plug, then the ECU must be bad.

This assumes that you are doing the electrical tests correctly.

ej1b18c1

it has continuity.
using 6v scale while all plugs in key on position II testing it from ecu
wht reads 0.04
grn\wht reads 0.01
red/yel reads 4.89

i even swapped the wire with new one straight to ecu and still reads 0.04.
but why would no codes/cel come up while using the same ecu in my other car if it it was bad?

94EG8

It almost seems like that pin isn't making very good contact at the ECU.

ej1b18c1

i will check that. getting really frustrated because usually wiring on Honda's are not too difficult. be different if the harnesses were all hacked up, but it was all good before swap on both cars.

Former User

You might try this:

Unplug ECU connector D and turn the key from off to ON(II). Measure voltage between D17 (+) and D21 (-) from the ECU, not from ECU connector D. Do you now measure 5V?

When you did the original MAP sensor plug tests, did you measure voltage at the Wht wire terminal using the Grn/Wht wire terminal as ground?

ej1b18c1

no, i will try that.
i was using the body ground or neg side of battery