Map Sensor issue

[Joseph Brown]

So other than my idle being bad I have another issue dealing with my Map Sensor so I just created another forum to deal with that instead of tagging it on the other post.

My issue with the map sensor is that it isn’t varying voltage depending on the vacuum applied. I had the 5v power and the 12v ground on the middle wire but the control wire that is supposed to vary the voltage isn’t changing at all, staying at around 5v. I know for sure that my map sensor is good (tested on another honda) and have continuity through each wire to the ECU so what else could be the problem?

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[none]

How did you test for continuity? Are you a voltage drop kind of guy or did you hit the circuit with an ohm meter? If you tested for resistance, I’d still be looking into that signal wire. Ohm testing can fool you with false positives because it doesn’t really tell you what the condition of the live circuit is. Can you run a back probed jumper wire between the MAP signal and the ECU and recheck your signal voltage? Did you check to see if a terminal backed out of the connector on the ECU end of the harness? I have no idea what model or year you’re working on so I have to ask all these questions. Is the MAP sensor remotely mounted and did you check the vacuum supply line to make sure the MAP is actually getting a vacuum signal to measure? A vacuum hose could be plugged, disconnected, or connected on the wrong side of the throttle body.

Edit** am I reading this right? You’ve got 12 volts on a ground wire? Ground voltage in a healthy circuit should be in the vicinity of zero volts. The more zero the better. I might be interested in back probing that ground wire at the MAP and then start disturbing the rest of the harness to look for a change in voltage.

[Joseph Brown]

91 honda CRX HF
Used ohm meter to test resistance (around 8 ohms, which is what I got just touching the two leads together, also used the beeping continuative setting. Terminal does not seem to be backed out of the ECU end, it’s tight with little play with gentle pulling. I replaced the connector on the map sensor end to see if it the connection just wasn’t good and no luck. I used my mouth to apply vacuum while checking with the meter (key on engine off tested, worked on the other Honda so I know it’s a viable way). I can move the map sensor to get to it easily so testing is easier than other cars. 🙂

[Its me]

If you have truly tested the MAP on another vehicle and it is giving you the proper return signal (on the other vehicle), then I would look at the wires to the ECU… You say you have continuity, and that is a good start, but you have to check the circuit while it is loaded…. backprobe at the ECU…. A short to ground on that circuit may give you the full 5v when testing at the MAP sensor… Check what the voltage is at the ECU…Quite possibly the ECU provides the ground and it is grounding prior, but still has connection to the ECU (giving you continuity)…

A schematic could help…

-Karl

[Joseph Brown]

Used this page to test it.
http://troubleshootmyvehicle.com/honda/1.5L-1.6L/how-to-test-the-map-sensor-2
The 12v ground is plugging the second wire (ground wire) with one connector and touching the battery with the other.

I have an ecu pinout so I can trace where the map sensor wires are going to in the ECU but after that I get lost in the sea of wires.

ECU does provide the Ground. So you’re telling me to test the voltage by piercing the control wire near the ECU to see if I’m getting the same reading? Or test the ground?

[Its me]

OK…

I’ll try to clear some of this up…

From the pic on the link…

Wire #1 is a constant 5V from the ECU
Wire #2 is the ground
Wire #3 is the control wire (which has variable voltage)… This pretty much means that I was incorrect in my assumption that the ECU provides the ground.

What I mean by backprobe, is to get T-pins (make it easier) and use a schematic to find which wire is wire #3 going into the ECU, put in a T-pin and check the voltage (T-pin not required, but makes it easier)

So, to explain what you have done so far, if you need it…

You found you have 5v coming from wire #1 when the key is turned… GOOD!
You got a ground by testing the ground wire (wire #2) to 12V and found it was GOOD!
The signal wire (wire #3) is not changing… (that is where I think we are going).

You say you are getting nothing but 5v from the control wire, am I understanding this correctly?

Also, You say you have got continuity between the control wire and the ECU? Is this a correct assumption… A bad/grounded wire #3 will not send the proper signal to the ECU…

Just trying to clear things up…

-Karl

[Joseph Brown]

I’ve tried searching a online diagram to help out but no luck. I have the one from the Haynes manual. All three wires do indeed go to the ECU So I believe the ECU does supply the ground.

Everything else looks correct.

Wire 3 should have the same voltage if it’s not bad/grounded near the ECU correct?

[Its me]

Wire #3 should have the variable voltage (between 0-5v)… This is the wire you should test while pulling vacuum on the vacuum line of the MAP… This is the one that should fluctuate… If you did not check this, then do check it… BTW, Wire #1 should always be 5v…

-Karl

[Joseph Brown]

I’m sorry if I’m confusing but if I’m getting the 5v power on wire 1, wire 2 is grounded correctly, and the map sensor works, I really should see the change in wire 3 whether I’m near the connector or at the ECU.

My view on this: Wire one supplies 5v that passes though the grounded map sensor which regulates the voltage that goes out through wire 3 depending on vacuum. NO other voltage is going through wire 3 other than what the map sensor sends. So why is 5v going through and not varying, Just really confused. Haha

[Joseph Brown]

And yes wire 3 was the one I was reading while applying vacuum, Like I said I did the same test on another Honda.

EDIT: I’ve ran it a bit since doing these voltage test too. I’m going to recheck these readings and post them tomorrow, I’ll also probe near the ecu on wire 3 to see if I get the same result. Thanks for helping so far, I think I’ve done nothing but confuse you too though.

[Its me]

[quote=”crxmpg” post=113821]I’m sorry if I’m confusing but if I’m getting the 5v power on wire 1, wire 2 is grounded correctly, and the map sensor works, I really should see the change in wire 3 whether I’m near the connector or at the ECU.

My view on this: Wire one supplies 5v that passes though the grounded map sensor which regulates the voltage that goes out through wire 3 depending on vacuum. NO other voltage is going through wire 3 other than what the map sensor sends. So why is 5v going through and not varying, Just really confused. Haha[/quote]

OK… You understand perfectly in paragraph 1.

I am not completely understanding paragraph 2. Wire 3 is what the MAP is is sending based upon what it detects… It is a 0 to 5v line, yes, but it is what the ECU uses for data…. it should be somewhere around 2.5v at idle I believe, but will change when vacuum is introduced.

The 5 volts from the ECU through wire #1 is a CONSTANT…. It is not a wire that has voltage that fluctuates… It is just pushing out raw 5v voltage for the MAP sensor to use…

Hope this helps…

Wire #3 should be fluctuating… If it is not, you have an issue with the MAP sensor… Possibly a shorted/broken wire to the ECU.

Try this… using jumper wires, hook up wires #1 and #2 from the wiring harness to the MAP… but do not hook up #3…. Read where wire #3 should be at the MAP sensor with your DVOM… If you see the voltage fluctuating, then the MAP is fine…

Then, hook the harness back up to the MAP sensor and check #3… If it is 5v (or something whacky) then you have an issue with the wire going to the ECU or the ECU…

-Karl

[Joseph Brown]

That’s what I thought, map sensor is good, no reason it shouldn’t be working right but it isn’t.

Good map sensor (varies voltage on the other Honda doing the same test I posted) good 5v power for the map sensor to do what it’s supposed to, map sensor isn’t doing what it’s supposed to, Good map sensor, infinite loop of craziness. Unless I’m just stupid but I think I’ve done everything right in testing…

paragraph 2 was what I thought a better reiteration or 1. Guess it wasn’t worded too well…

I’ll try with jumpers tomorrow.

[Its me]

I added this to my last post… Wanted you to read this to truly test the MAP sensor…

TRY THIS… using jumper wires, hook up wires #1 and #2 from the wiring harness to the MAP… but do not hook up #3…. Read where wire #3 should be at the MAP sensor with your DVOM… If you see the voltage fluctuating, then the MAP is fine…

Then, hook the harness back up to the MAP sensor and check #3… If it is 5v (or something whacky) then you have an issue with the wire going to the ECU or the ECU…

-Karl

[Joseph Brown]

Map did not fluctuate like it was supposed to. Voltage was 2.7v, I’ll get another and test it the same way and post the results when I can.

Also without the Map sensor being plugged in I’m getting 5v on the control wire (wire 3). I don’t think I should be getting any reading on the control wire at all without it plugged in right? Could that be why it’s not working?

[Its me]

Ok you got 2.7v and applied vacuum and it did not change?

The 5v on the control wire (signal wire) with the MAP disconnected is cause for concern. This wire needs to be checked if it is picking up voltage somewhere… ECU may be an issue…

So apply vacuum to the MAP while doing the test as i suggested above and observe for fluctuation…

Next isolate the control wire back to the ECU and ensure it is not broken or shorted before the ECU…

Karl

[Joseph Brown]

I did exactly as you said and the 2.7v is what I got when applying the vacuum (no fluctuation). I will trace that wire back to the ECU to see if it’s shorted.

If it’s not shorted should I go ahead and try another ECU? I have been concerned about that but everything else seems to be working correctly so I haven’t tried to order a good one.

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