Caveat- this is mainly for Oldsmobile 307s, to which I'm more familiar with than say, the 305 Chevy engine. I believe since they use the same sensors, they work the same, but I have done absolutely no further research on them, thus, if you know something different, please correct me where I'm off my rocker. Also, I do not claim to be an expert, but I've piddled around with emission equipment on G-body Oldses enough to feel confident I can troubleshoot the Olds emission stuff enough to keep my butt out of a bind.
NOTE: A MAP sensor is NOT an MDP sensor. Period.
One of the sneaky things that can get your Emission fixes in trouble on your G-body is whether you need a MAP (manifold absolute pressure) sensor or a MDP (manifold differential pressure) sensor for your 307 V8. The MDP sensor is also called a VAC sensor sometimes, but I don't usually call it that. Doesn't matter if you have a Buick or Olds, or whatever, if you have a stock 307 Y or 9 in your car, you need to make sure you get the right sensor for your application. (You Buick turbo 6 guys, you have a special MAP sensor that takes boost into account, and that won't be discussed here, either. P/N 16009886, 12569241.) Fuel injected engines typically use MAP along with MAF sensors, not MDPs.
There's also another sensor to deal with that is similar, called the barometric pressure sensor, aka "Baro", but they're pretty straightforward and I'm not going to deal with those here. I'll only say that they're under the dash behind the kick panel on the passenger side in the interior of the car. They have no hose attached to them like the MAP/MDP. GM p/n 1616834,16006833, 16006835, 12219931. Ironically, over time, GM simply included the MAP and BARO sensors into the same part number since they're effectively the same sensor and act the same way. The only difference is the tube to the intake manifold on the MAP.
As a rule of thumb, 307Y and 9 engines (and cars with 305's) take the MDP (GM p/n 16006834) and the V6 and diesels take the MAP (GM p/n 16006833,16006835, 12219931). The sensor is located on the passenger fender top behind the coolant overflow tank. I don't know any other G-body configuration that's different off the top of my head, but if there's one, I'm not sure what it is. It's very important to realize and use the correct sensor. GM made it super easy to tell by making them all look virtually the same!
So you can see it can be very easy to get the wrong type. And the aftermarket doesn't help with alleviating the confusion on this either. More on that in a moment.
First, in a nutshell, the MAP and MDP work similarly to give input to the ECM as far as manifold pressure or vacuum, but send out different voltage signals for the same situation. Think opposite of each other. This signal helps the engine determine the vacuum signal strength and output a voltage signal to the ECM to help adjust fuel to the engine based on what the air pressure in the intake manifold is doing. There are 3 wires to the sensor regardless. A ground (black), a 5V reference voltage (gray) into the sensor, and the signal wire that feeds the info to the ECM (light green or other color).
In cars that use a MAP, the voltage tracks with engine load. Throttle opens, manifold air pressure goes up (vacuum drops), and the voltage output of the MAP goes up. Normally this is 0-5V, so as engine load goes up, so does the voltage output to the ECM. Typically, you probably won't see exactly 5V at all with these reference voltages. Due to materials, connection quality, length of wire, and other resistance issues, you may see stuff close to 5V or whatever. So don't freak out if you're not seeing the exact 0-5V on either end of the spectrum.
The differential pressure sensor that the Y and 9 engines do things a bit differently. As vacuum in the intake increases, or air pressure goes down, the voltage goes up. So as load increases, the vacuum drops and the voltage drops accordingly. Opposite of what the MAP does. The higher the vacuum or colder engine, the more electronic advance you get on the distributor, and the lower the vacuum or hotter engine, the less advance. So you can see rather instantly if you get the wrong sensor, your day won't be a fun one.
You can check your MDP with a volt meter and vacuum hand pump. First, check for any "34" code. This is If you measure voltage from the signal wire (terminal 20 on back of ECM) to ground (terminal 22 on back of ECM) when key is on but engine off, you should see between 0.50-0.64 Volts. Place 10" of vacuum on the sensor port and you should see voltage rise 1.2 to 2.3 volts above whatever it was with key on/engine off. When engine is running with minimum 15" vacuum, you should see over 2 volts. If everything is good there, the MDP is working the way it should. These should be checked with a digital voltmeter because using a scan tool may have a built-in delay. Here you want to see voltage jump rapidly, otherwise it could cause detonation/lag. If you don't get the readings above, then your connector isn't making good contact, or the MDP is likely toast deeming a replacement. The CSM also states that using the vacuum pump you MAY set a false "41" code, which is Loss of Distributor Reference Signal. So it may be good to go back and check for/clear codes when you're done.
Now, here's the fun part. If you go to find an MDP sensor, you must do it by part number only. You can't go off of just looking at one, although if it's an original it may have an 833, 834, or 835 stamped on it, which will tell you which it is. But new ones? Make sure you know what you're getting. A lot of places call everything a MAP sensor, and that just ain't right. MAP and BARO sensors are interchangeable, but not the MDP sensor. To note, these sensors are pretty durable, but they probably weren't used to living 40 years under the hood. So if you're getting quirky timing or things aren't acting right, it could be your MDP sensor.
GM part numbers for a G-body V8 MDP sensor is 16006834, 16054920, and 16231141. THAT IS IT for GM part numbers. Standard Motor Parts shows as AS 10, and Borg Warner is EC1618. Another I saw was Niehoff FE1618 and NAPA is CRB219011. At least NAPA website calls it a differential pressure sensor.
Borg Warner takes the cake. Their MDP sensor EC1618 is listed BOTH ways in their e-catalog, in the same picture. Check it out. Notice it lists MAP sensor, and right underneath it lists it as an MDP sensor. It IS an MDP sensor. Don't know of the newer ones, but I have an older BWD in the box that has the GM logo on the top of it so that should give you an idea who made them for GM at least for a while. It also has "MAP SENSOR" on the end flap right under the part number. Again, this part is NOT a MAP or BARO sensor. It is an MDP.
NOTE: A MAP sensor is NOT an MDP sensor. Period.
One of the sneaky things that can get your Emission fixes in trouble on your G-body is whether you need a MAP (manifold absolute pressure) sensor or a MDP (manifold differential pressure) sensor for your 307 V8. The MDP sensor is also called a VAC sensor sometimes, but I don't usually call it that. Doesn't matter if you have a Buick or Olds, or whatever, if you have a stock 307 Y or 9 in your car, you need to make sure you get the right sensor for your application. (You Buick turbo 6 guys, you have a special MAP sensor that takes boost into account, and that won't be discussed here, either. P/N 16009886, 12569241.) Fuel injected engines typically use MAP along with MAF sensors, not MDPs.
There's also another sensor to deal with that is similar, called the barometric pressure sensor, aka "Baro", but they're pretty straightforward and I'm not going to deal with those here. I'll only say that they're under the dash behind the kick panel on the passenger side in the interior of the car. They have no hose attached to them like the MAP/MDP. GM p/n 1616834,16006833, 16006835, 12219931. Ironically, over time, GM simply included the MAP and BARO sensors into the same part number since they're effectively the same sensor and act the same way. The only difference is the tube to the intake manifold on the MAP.
As a rule of thumb, 307Y and 9 engines (and cars with 305's) take the MDP (GM p/n 16006834) and the V6 and diesels take the MAP (GM p/n 16006833,16006835, 12219931). The sensor is located on the passenger fender top behind the coolant overflow tank. I don't know any other G-body configuration that's different off the top of my head, but if there's one, I'm not sure what it is. It's very important to realize and use the correct sensor. GM made it super easy to tell by making them all look virtually the same!
So you can see it can be very easy to get the wrong type. And the aftermarket doesn't help with alleviating the confusion on this either. More on that in a moment.First, in a nutshell, the MAP and MDP work similarly to give input to the ECM as far as manifold pressure or vacuum, but send out different voltage signals for the same situation. Think opposite of each other. This signal helps the engine determine the vacuum signal strength and output a voltage signal to the ECM to help adjust fuel to the engine based on what the air pressure in the intake manifold is doing. There are 3 wires to the sensor regardless. A ground (black), a 5V reference voltage (gray) into the sensor, and the signal wire that feeds the info to the ECM (light green or other color).
In cars that use a MAP, the voltage tracks with engine load. Throttle opens, manifold air pressure goes up (vacuum drops), and the voltage output of the MAP goes up. Normally this is 0-5V, so as engine load goes up, so does the voltage output to the ECM. Typically, you probably won't see exactly 5V at all with these reference voltages. Due to materials, connection quality, length of wire, and other resistance issues, you may see stuff close to 5V or whatever. So don't freak out if you're not seeing the exact 0-5V on either end of the spectrum.
The differential pressure sensor that the Y and 9 engines do things a bit differently. As vacuum in the intake increases, or air pressure goes down, the voltage goes up. So as load increases, the vacuum drops and the voltage drops accordingly. Opposite of what the MAP does. The higher the vacuum or colder engine, the more electronic advance you get on the distributor, and the lower the vacuum or hotter engine, the less advance. So you can see rather instantly if you get the wrong sensor, your day won't be a fun one.
You can check your MDP with a volt meter and vacuum hand pump. First, check for any "34" code. This is If you measure voltage from the signal wire (terminal 20 on back of ECM) to ground (terminal 22 on back of ECM) when key is on but engine off, you should see between 0.50-0.64 Volts. Place 10" of vacuum on the sensor port and you should see voltage rise 1.2 to 2.3 volts above whatever it was with key on/engine off. When engine is running with minimum 15" vacuum, you should see over 2 volts. If everything is good there, the MDP is working the way it should. These should be checked with a digital voltmeter because using a scan tool may have a built-in delay. Here you want to see voltage jump rapidly, otherwise it could cause detonation/lag. If you don't get the readings above, then your connector isn't making good contact, or the MDP is likely toast deeming a replacement. The CSM also states that using the vacuum pump you MAY set a false "41" code, which is Loss of Distributor Reference Signal. So it may be good to go back and check for/clear codes when you're done.
Now, here's the fun part. If you go to find an MDP sensor, you must do it by part number only. You can't go off of just looking at one, although if it's an original it may have an 833, 834, or 835 stamped on it, which will tell you which it is. But new ones? Make sure you know what you're getting. A lot of places call everything a MAP sensor, and that just ain't right. MAP and BARO sensors are interchangeable, but not the MDP sensor. To note, these sensors are pretty durable, but they probably weren't used to living 40 years under the hood. So if you're getting quirky timing or things aren't acting right, it could be your MDP sensor.
GM part numbers for a G-body V8 MDP sensor is 16006834, 16054920, and 16231141. THAT IS IT for GM part numbers. Standard Motor Parts shows as AS 10, and Borg Warner is EC1618. Another I saw was Niehoff FE1618 and NAPA is CRB219011. At least NAPA website calls it a differential pressure sensor.
Borg Warner takes the cake. Their MDP sensor EC1618 is listed BOTH ways in their e-catalog, in the same picture. Check it out. Notice it lists MAP sensor, and right underneath it lists it as an MDP sensor. It IS an MDP sensor. Don't know of the newer ones, but I have an older BWD in the box that has the GM logo on the top of it so that should give you an idea who made them for GM at least for a while. It also has "MAP SENSOR" on the end flap right under the part number. Again, this part is NOT a MAP or BARO sensor. It is an MDP.
