How to test a Barometric Pressure Sensor

[t5535]

How do I test the Barometric Pressure Sensor and what should the values be on a 1994 SHO 3.0 five speed. The car is missing and the scanner says its the Barometric Pressure Sensor. I wanna test it before I spend 80 bucks on another one.

 

[tompumped]

I don't remember how to test them from my teachings, but when I got a code years back for a bap it was indeed bad. I guess you can put a vacuum pump on the port and observe the voltage readings. You might need a scope, or you might have to pressurize it instead of draw vacuum. I would replace it anyway as preventative maintenance once you start getting into higher mileage. I did on my sho I own now. I still have my old working ford one.
It's cheaper on rockauto I think, but I got mine from AZ.

 

[t5535]

I have access to a scope but I don't know how to read it. We have a otc pegisys and I am just learning *** to use it.

 

[Racer X]

You need a scope, however I don't recall the pinout, nor what frequency is considered normal.

 

[t5535]

the scanner says to test the ohm's but I don't know what wire to put with what wire to read it nor do I know what the values should be.

 

[t5535]

Should it have a vacuum line running to it? all it has running to it is the pig tail with 3 wires. there is what looks to be a vacuum hook up with a white plastic thing around it.

 

[93rev2sev]

Unplug it and see if it still misses. No signal is better than an incorrect signal. If it stops missing, replace it.

 

[LJRuddy]

If there are three wires, then you will have

+12v from the battery

signal wire

ground

Turn the key on and one wire should have either 12v or 5.5 (IIRC) while the ground has zero. The third wire will have a varying current based on air pressure. You will want to take the signal wire and the ground to find your reading.

 

[gmorrell]

The three wires are:

+5V (Vref) from EEC (this powers the sensor)

Signal out

Sensor Ground

Ford pressure sensors are frequency out, the waveform will be about a 50% duty cycle with low at ~0V and high at ~5V, the output vary from about 90Hz at low pressure to about 160Hz at high pressure. Without getting out my notebooks, that's all I can remember.

If you unplug the sensor, EEC will default to sensing a high pressure, and if the miss goes away with the BAP disconnected, then it might be a bad BAP sensor, but if it still misses with BAP disconnected, then BAP isn't your problem.

 

[rbruso]

Should it have a vacuum line running to it? all it has running to it is the pig tail with 3 wires. there is what looks to be a vacuum hook up with a white plastic thing around it.

For barometric (atmospheric) pressure, the sensor should not have a vacuum line. Ford has also used the same sensor for manifold pressure. In that case, there would be a vacuum line to the ******.

Ours is barometric, so don't run a vacuum line.

 

[gmorrell]

For barometric (atmospheric) pressure, the sensor should not have a vacuum line. Ford has also used the same sensor for manifold pressure. In that case, there would be a vacuum line to the ******.

Ours is barometric, so don't run a vacuum line.

Ford usually puts a loose fitting plastic cap over the hose ****** when the sensor is used in barometric mode, this keeps crap out of the sensor and also keeps well meaning mechanics from connecting a hose to it.

 

[gmorrell]

So I looked around for some more information on Ford pressure sensors...

Found this site: FordFuelInjection.com

Their spec/function page on the Barometric Pressure Sensor. (BAP)

The sensor frequency increases as vacuum increases.

Unfortunately, that's wrong.

I set up a little garage experiment to power a couple of BAP sensors at 5V and check their wave-shape and output frequency with an O-scope and frequency counter. (Yeah, I have this stuff in the garage, I have even better stuff in my real electronics workshop.)
Sensor at Atmosphere = 137.0 Hz.

Just for grins, I looked at the sensor at 20" Hg vacuum.
Sensor at 20" Hg vacuum = 87.3 Hz.

And at 5psi.
Sensor at 5psi = 167.7 Hz.

Fordfuelinjection.com says, "The sensor frequency increases as vacuum increases." Mind you, just this statement is wrong, the graph of pressure -vs- frequency is correct, and the tabular data is also correct except for the entry at 9000 feet/21.0"Hg/72kPa, this should be 134 Hz, not 138Hz.

So, my data on two BAP sensors says that ...the sensor frequency output increases as pressure increases.

Pressure Frequency
-20" Hg = 87.3 Hz
-15" Hg = 99.9 Hz
-10" Hg = 112.4 Hz
-5" Hg = 125.5 Hz
Atm* = 137.0 Hz

One sensor was a SHO spare, the other I robbed from my SVO. At 5 pressures, they both agreed within 1 Hz.

* Keep in mind that I live at about 6500 feet altitude, and there's a winter storm roaring in here this afternoon, so between those two, my normal barometric pressure is quite a bit lower than sea level pressure of 29.92" Hg. Up here, I'm at about 22" Hg. So for me, -20" Hg on my vacuum gauge is really 22"-20", or 2" Hg absolute, or about 87Hz from a Ford sensor.

Now, let's have a look at FordFuelInjection.com's Manifold Absolute pressure Sensor page.

Same (wrong) statement:

The sensor frequency increases as vacuum increases.

As with the BAP sensor page, the pressure -vs- frequency graph is right, but the tabular data is wrong: The frequency column is inverted, it should read 80Hz at the top for 0" Hg, and 159Hz at the bottom for the 30" Hg entry. Just invert the whole column of numbers so it reads 80, 88, 95, 97 etc., top to bottom.

Now let's go back up a few paragraphs to my 2" Hg absolute that I got by applying -20" Hg to my sensor in a 22" Hg atmospheric ambient, and got 87 Hz. Look at the BAP table now that you've re-ordered the frequency column, 2" Hg would be between the 0 and 3 Hg rows, but closer to 3, and somewhere between 80 and 88 Hz, but closer to 88, right?

Apologies for the long post, but I just hate it when I go out on The Internets and find errors. I sent a message to RJM Injection Tech, hopefully they will correct this.

 

[luigisho]

Dude, you're like a car superhero. Who has this stuff laying around?

 

[SHO1]

Guys that work on/repair alot of electronics.

 

[tompumped]

If you have a high quality scan tool can you test them the way you did? I'm curious because i've been holding out to get a better scan tool.

 

[gmorrell]

Thexton used to make a Ford MAP BP sensor tester, #127, there are a few on eBay right now, 280578105766 and 120665897281 from a quick search.

I haven't seen an OBDI scan tool that will do MAP or BP test, it would require getting the data off the internal EEC bus, and none of the OBDI tools can do that. The TwEECer RT might get in in the datalogging function, don't know, but that's an expensive solution.

Rotunda made one for Ford dealer tech use, tho I think Hickok made it.

They just power up the sensor and convert the frequency output to a voltage that can be read on a volt meter.

If anyone wants a Ford BP or MAP sensor tested, PM me, you can mail it to me, it takes me under 10 minutes to set up and test it.

I learned electronics on vacuum tubes in my Dad's shop at age 10 (>40 years ago), I got scopes, meters, signal generators, function generators, and I still have my Dad's bench full of Heathkit test equipment, most of which is barely post WWII. A lot of my "newer" test equipment is the Tektronix TM-500 series modular stuff from the 1970's and 80's, seen in the above pictures, it's good old fashioned analog stuff that's reliable as bricks. I've also got a newer HP wideband digital scope and a 43 channel logic analyzer, so all my stuff isn't stupid-old.

 

[kevinspann]

Dude, you're like a car superhero. Who has this stuff laying around?

See under his username where is says "never been a noob"?

I believe it.