OC

So finally after months of inspecting I was able to figure out that I have a bad ground on my MAP sensor. My question is if this is normal? How could the ground leading to the sensor be bad? Is there an easy fix for this? If anyone could help that would be great. Thank you

Biggiesacks

Inspect the harness and all pigtails. There are many ways it could happen. Ive seen rodents chew up a harness, it could have been in an accident and poorly repaired etc..

OC

Whoa. Back at it again. So when I tap into that sensors ground cable and ground it to the battery. My CEL disapears. If my MAP sensor was bad wouldnt that cause my car to run funky?

barneyb

If you are talking about the black wire, that ground runs all over the place and is the ground for the crank sensor and the cam sensor before finally becoming a chassis ground at the ECU. Obviously the ground is okay at the ECU or the car wouldn't run. As for running a ground to the nearest metal piece, well I don't know, the ECU probably has some protection in it that you would be bypassing.

What is the resistance between the ground at the maf and the chassis?

TheBoz

iTrader: (9)

Not all sensors use the batt ground.


Cam and Crank sensor use a ECU 5v Ground signal, but take in a +12v MFI switched power.


MAP, Throttle, Engine Coolant, Oxygen Sensor, VSS Sensor, Fuel tank pressure sensor, use a battery ground.

barneyb

TheBoz has kindly afforded you a circuit diagram. As you can see, splice S060, located under the dash, provides ground for cam, crank and maf by way of pin C119.. It used to be the makers would ground these devices to the chassis but there is less electrical noise, some filtering I guess, via the ECU.

Edit: What code is being set?

TheBoz

iTrader: (9)

Map Sensor
1 - Yellow -> ECU pin 73 (Sensor data in)
2 - Gray -> ECU pin 81 (ECU +5v)
3 - Black -> Chassis Ground -- big black 14

OC

Thanks TheBoz. I have been looking for something like this. I will look into it and let you guys know. Its weird because the senor wire provides a signal and I get 5v on the Gray wire. Thanks for the heads up on it.

TheBoz

iTrader: (9)

Yes the gray wire is the supply voltage for the sensor, with the ECU powered up you should always see ~+5v. On my fluke meter I see +4.96v.


The yellow wire is a 0-5v output signal from the sensor, and the black is just a ground. So if you have a bad ground you can try to clean all your ground positions. There is one where the battery grounds to the chassis and there are lots of grounds from the engine harness there. Make sure that all of them have been cleaned using a wire brush, and make sure that the bolt and bolt hole are free of surface rust and crap.