TEST 3: Verifying The MAP Signal With A Multimeter
When a MAP sensor fails, it usually stays stuck outputting a single voltage number (remember, its normal activity is for this voltage to decrease or increase depending on the amount of vacuum applied to it).
So, to find out if the MAP sensor is stuck, we're gonna' manually apply vacuum to it ourselves (with a vacuum pump or the good ole' lungs) and then verify it's voltage output with a multimeter.
If the MAP sensor is working correctly... the multimeter will show a decreasing voltage the more we apply vacuum to it and upon releasing the vacuum, the MAP sensor voltage will shoot back to the same value we started out with.
If the MAP sensor is fried... we won't see the voltage change no matter how much vacuum we apply to it.
Let's get started:
Remove the MAP sensor from the top of the intake manifold.
Reconnect the MAP sensor to its electrical connector, if you had to unplug it to remove it, since this test requires that the MAP sensor stay connected to its 3-wire electrical connector.
Connect your multimeter's red test lead to the wire that connects pin number 3 of the MAP sensor (see photo 2 of 2 in the image viewer).
You'll need to back-probe the MAP sensor connector or use a wire piercing probe to connect your multimeter to this circuit (to see what a wire piercing probe looks like, click here: Wire Piercing Probe).
Ground the black multimeter test lead.
I suggest grounding the black multimeter test lead directly on the battery negative (-) terminal.
Turn the key on (but don't start the engine) and take a look at what voltage your multimeter is registering.
It should register around 2.9 Volts (± 1 V). If your multimeter is registering something different at this point, don't panic because right now you're just setting up the test.
Attach a vacuum pump to the MAP sensor's vacuum inlet nipple using the appropriate size vacuum hose.
If you don't have a vacuum pump, you can use the ‘good ole' lungs’ and provide the necessary with your mouth.
The voltage, your multimeter is registering, should decrease as you apply vacuum with your vacuum pump (or with your mouth).
The MAP sensor's voltage, as registered on your multimeter, should increase back to the original voltage value you saw in step 4 as you release the vacuum you applied with the vacuum pump (or your mouth).
NOTE: Applying vacuum, via the vacuum pump or your mouth, should make the MAP signal voltage get smaller on your multimeter. Releasing vacuum should make the MAP signal voltage get bigger (back to the original voltage you saw at the beginning of this test).
Let's take a look at what your test results mean:
CASE 1: The MAP sensor's voltage decreased and increased as you applied and released vacuum. This confirms that the MAP sensor is good and operating normally.
This test result completely eliminates the MAP sensor on your Honda as bad. If you're still having the MAP sensor trouble code come back, take a look at: MAP Sensor Is Good But Code Doesn't Go Away!.
CASE 2: The MAP sensor's voltage DID NOT decrease or increase as you applied and released vacuum. This confirms that the MAP sensor is bad and needs to be replaced.
If you need to buy the MAP sensor and would like to save some bucks, take a look at the section: Where To Buy The MAP Sensor.
MAP Sensor Is Good But Code Doesn't Go Away!
If your MAP sensor test results show the MAP sensor is good... yet the PCM keeps setting a MAP sensor trouble code, there's a good chance that there's a mechanical problem with your Honda's engine.
As stated earlier in this tutorial, several things can fool the PCM into thinking that the MAP sensor is bad, when the MAP sensor is only reporting what is actually happening.
So, to further assist you, in this section I'll offer you a few testing tips and/or suggestions:
- Check for vacuum leaks.
- An intake manifold gasket leak or a ruptured vacuum hose (especially a big one) that's leaking vacuum will have a direct impact on the MAP sensor readings.
- Check engine mechanical health.
- This means doing a compression test.
- What you're looking for is uneven wear and tear between the cylinders. This means that the compression each cylinder is producing varies more than 15%.
- This tutorial will help: How To Test Engine Compression (Honda 2.2L, 2.3L).
- Check timing belt synchronization.
- If you've just replaced the timing belt (or had to remove it for any reason), double check your timing marks.
- Make sure that the MAP sensor is getting power and Ground.
- Take a look at the following section: P1129 Basics You Need To Know.
- A short in the wiring between the MAP sensor and the PCM.
- You'll need to check the continuity of the 3 wires between the MAP sensor and the PCM using the appropriate wiring diagram.
- A bad PCM.
- This is very rare, but it does happen.
- A fuel system malfunction (that is also setting other trouble codes).
- If there are any other trouble codes, you'll need to diagnose and repair these first (since they may be indirectly causing the P1129 code).
- You've replaced the throttle body gasket and used silicone sealer on it and this sealer has plugged the MAP sensor's vacuum passage and/or orifice (I've seen this happen a lot!).