Bap Sensor Testing:

[FoMoCo17]

1990 Mustang - Supercharged

Been troubleshooting an on-going rough running condition open loop ... closed loop is good now after finding a bad (semi-new oxygen sensor) that was bad. Tested everything under the hood and all looks good except possibly BAP sensor. I am familiar with the testing procedure. I tried measuring the frequency with my meter on the BAP signal wire at the computer and measuring against ground/sig return wire but am getting zero. However, my meter measures only in Khz. Based on this, I thought I should get a reading like 0.157 for 157 hz but am just getting zero. Has anyone been successful reading in Khz??? But, possibly such a low hz reading just reads zero on such a meter. Any expereince would be most helpful. I am not getting a Code 22 but after measuring a Code 22 did show up and I erased it .. haven't run or checked since if it came back ... but it has never showed it head when checking codes. I have heard that they can go bad without throwing a code so I want to check it the correct way but do not know about this meter thing. Don't have a cheapie meter handy which probably only has hz as opposed to khz.

I checked my other meter and it reads in Khz also. When I switch to voltage, I do get 2.5 volts on the BAP signal wire at the 60 pin connector. But I know this is not an accurate way of knowing if it works. Has all other required voltages like 5 volts supply etc.

Thanks for your help!

 

[jrichker]

You have the correct technique, but you need some better equipment.

Soe things to check that you may have already done...

MAP/BARO sensor operation and code 22

Revised 19-Jul-2011 to add functional descriptions for MAP and BARO operation.

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 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.

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

Baro or MAP test using a real frequency meter - 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. An 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.

 

[FoMoCo17]

Yeah, I pretty much know the testing procedure from reading it on here before. I have a very good meter probably too good since it reads high frequencies. Just wondering if a meter that reads in the khz region up to 20,000 hz would pick up the so-called 159 hz put out at idle. I'm sure others have meters that read in this range ... just wondering if it will register at these lower hz levels as 0.159 khz or not. Anyone really know and have done it??? Or, since it is so low a frequency, will it just read zero ... what I'm getting when I hook it up.

 

[tca7291]

If it is a quality meter, it should be able to give you 0.XXX khz reading. Are you using Fluke, Snap-on, Mac, or what?

 

[KraZkyle]

You have the correct technique, but you need some better equipment.

Soe things to check that you may have already done...

MAP/BARO sensor operation and code 22

Revised 19-Jul-2011 to add functional descriptions for MAP and BARO operation.

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 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.

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

Baro or MAP test using a real frequency meter - 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. An 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.

When testing the resistance im not getting a reading going from the black/white on the bap to black/white on the egr & tps im getting a reading going from black/white to orange and its 1.5ohms is something wrong or am doing it wrong?

 

[jrichker]

When testing the resistance im not getting a reading going from the black/white on the bap to black/white on the egr & tps im getting a reading going from black/white to orange and its 1.5 ohms is something wrong or am doing it wrong?

And the answer is...Something wrong...
Re-read the test path - I never said to check resistance between the black/white wire and the orange wire. that pair of wires is for a voltage check for 5 volts VREF. The VREF is 4.7-5.2 volts with the ignition in the Run position.

Set the meter for low ohms and touch the probe tips together. You should see .3-.5 ohms.
Then check the resistance on black/white wire on the MAP/Baro sensor and the black/white wire on the EGR and TPS. You should see the same reading that you got by touching the probe tips together or maybe a little more. but still less than 1 ohm. more that that and you have a broken wire in the 10 pin connectors or some dirty contacts.

See the graphic for the 10 pin connector circuit layout.

The black/white wire connects to the Signal Return for sensors inside the 10 pin connectors

Next check the resistance between the black/white wire on the MAP/Baro sensor and the negative battery cable. It should be less than 1.5 ohms.