86 Mustang trouble codes

tsiemens said:

That sensor is pretty cheap. If the vacuum lines are ok should I just unload the 50 bucks?

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MAP/BARO sensor operation and code 22

Revised 24 Oct 2018 add warning about trying to measure the MAP/BARO sensor output with a common multimeter.

On a Speed Density car, the MAP/BARO sensor is connected to the intake manifold and acts to sense the manifold pressure. Lower vacuum inside the intake manifold when combined with more throttle opening measured by the TPS means more airflow through the engine. As airflow increases, fuel flow through the injectors needs to increase to keep the air/fuel ratio where it needs to be. When manifold vacuum increases, the engine is either decelerating or idling, and it needs to reduce the fuel flow through the injectors.

On a Mass Air car, the MAP/BARO sensor vents to open air and actually senses the barometric pressure due to changes in weather and altitude. Its purpose is to set a baseline for the computer to know the barometric pressure. As barometric pressure decreases, it leans out the fuel flow to compensate for less oxygen in the air. When the barometric pressure rises, it increases to add fuel since there is more oxygen in the air. The fuel requirements decrease as altitude increases, since the atmospheric pressure decreases.

Disconnecting the wiring connector from the MAP or BARO sensor will set code 22..

Misconnecting the BARO sensor to vacuum on a Mass Air car will cause the computer to lean out the fuel mixture.

Code 22 or 126 MAP (vacuum) or BARO signal out of range. The MAP or BARO sensor is pretty much the same sensor for both Mass Air & Speed Density cars. The main difference is where it is connected. Mass Air cars vent it to the atmosphere, while Speed Density cars connect it to the intake manifold vacuum. Its purpose is to help set a baseline for the air/fuel mixture by sensing changes in barometric pressure. The MAP or BAP sensor puts out a 5 volt square wave that changes frequency with variations in atmospheric pressure. The base is 154 HZ at 29.92" of mercury - dry sunny day at sea level, about 68-72 degrees. You need an oscilloscope or frequency meter to measure it. There a very few DVM’s with a price tag under $40 that will measure frequency, but there are some out there.

Map sensor wiring:
black/white - ground
orange/white or +5 volts power
white/red signal out.

Measure the +5 volt supply using the orange/white and black/white wires
Measure the signal using the black/white and white/red wires.

The MAP/BARO sensor is mounted on the firewall behind the upper manifold on 86-93 Mustangs.

The Baro or MAP sensor can only be tested using a real frequency meter. The sensor output is a square wave which cannot be accurately measured with a common multimeter. Run the test key on, engine off.. The noise from the ignition system will likely upset the frequency meter. I used a 10 x oscilloscope probe connected from the frequency meter to the MAP/BAP to reduce the jitter in the meter's readout. And oscilloscope is very useful if you have access to one or know of someone who does. With an oscilloscope, you can see the waveform and amplitude.

If it is defective, your air/fuel ratio will be off and the car’s performance & emissions will suffer

Some basic checks you can make to be sure that the sensor is getting power & ground:
Note that all resistance tests must be done with power off. Measuring resistance with a circuit powered on will give false readings and possibly damage the meter.
Check the resistance between the black/white wire on the MAP/BARO sensor and then the black/white wire on the EGR and the same wire on the TPS. It should be less than 1 ohm. Next check the resistance between the black/white wire and the negative battery cable. It should be less than 1.5 ohm.

The following power on check requires you to turn the ignition switch to the Run position.
Use a DVM to check for 5 volts on the orange/white wire. If it is missing, look for +5 volts at the orange/white wire on the TPS or EGR sensors. Use the black/white wire for the ground for the DVM.






Diagrams courtesy of Tmoss & Stang&2birds

Complete computer, actuator & sensor wiring diagram for 88-91 Mass Air Mustangs


Complete computer, actuator & sensor wiring diagram for 91-93 Mass Air Mustangs


See the following website for some help from Tmoss (diagram designer) & Stang&2Birds (website host) for help on 88-95 wiring http://www.veryuseful.com/mustang/tech/engine/ Everyone should bookmark this site.

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