MAP sensor/electrical help please
MAP sensor/electrical help please MAP sensor reference voltage is 5v until it is connected to the MAP sensor, then the voltage drops to 2.1volts. Why? This happens on the old MAP sensor & the brand new sensor. Disconnect from sensor & 5v is back. Then tried a 9 volt reference which dropped to around 1.6v when connected to each MAP sensor. Tried a separate external ground w/ the same results. Any suggestions?
There's a pullup resistor inside the PCM. Open
circuit externally produces a full-scale MAP
voltage (and can be detected as a fault, as
MAP normal range is 0.5 - 4.5 or so).
2.1V might be a sane atmospheric voltage for
a 2-bar MAP, or a fairly normal idle MAP at
40-50kPa or so for a 1-bar.
Don't know about the 9V other than that it's
well outside the MAP sensor expected supply
range and maybe the internal electronics are
bent by it.
circuit externally produces a full-scale MAP
voltage (and can be detected as a fault, as
MAP normal range is 0.5 - 4.5 or so).
2.1V might be a sane atmospheric voltage for
a 2-bar MAP, or a fairly normal idle MAP at
40-50kPa or so for a 1-bar.
Don't know about the 9V other than that it's
well outside the MAP sensor expected supply
range and maybe the internal electronics are
bent by it.
TECH Senior Member
Joined: Apr 2002
Posts: 6,080
Likes: 17
From: So.Cal.
You mean the 5V map reference wire and not the signal input wire, right...?
That should stay at 5V.
If the voltage is pulled down by both sensors, then the PCM may be bad...
(...I said "may"...)
That should stay at 5V.
If the voltage is pulled down by both sensors, then the PCM may be bad...
(...I said "may"...)
Thank-you both for the reply. Here's the problem. After getting the new block up & running, I began having a MAP low input code. Long story short, the MAP, EGR, & A/C pressure sensors all share a circuit within the PCM. The EGR is turned off in the PCM & the EGR reference voltage wire shorted, never causing a CEL because the EGR alarms are turned off. This in turn fried the internal circuit for the MAP, EGR, & A/C pressure to an extent (bad PCM as suggested by joecar). This caused the initial MAP code. Recently, we changed the 5v reference to the MAP. Turns out that the reference is there until the ground wire is terminated (found this afternoon). The 5v reference remains as does the PCM feedback when the the MAP sensor ground wire is not connected. This would coinside w/ jimmyblue's comment about the open circuit providing the full scale MAP voltage. So, I'm gonna try to run her this way 'cuz even the circuit in my back up PCM got fried the one time that it was hooked up to while the EGR problem existed. If it throws a code, hopefully it can be turned off. Otherwise, I guess a new PCM is in the future. Or maybe an external 5v reference can be made w/ some resistors feeding from the battery? As long as the feedback circuit remains ok.
Originally Posted by WAHUSKER
Not sure about where you got 9v, but I doubt it did your MAP any good. Over voltage is not a good thing to do to electronics.....
It was applied as a test for 3 seconds to see what would happen. The MAP is fine. The 8.89v reference came from an un-used pin on the red PCM connector. The PCM is a much bigger problem. Thanks for the post though. I do appreciate all replies.
Just trying to help. I've never looked at the schematic for a MAP sensor either. I make assumptions it has over voltage protection on the inputs, which may have saved you. But they might also have been weakened or blown by your test.
From all you say is going on, something sounds amiss. If I had some extra time I'd go check what mine reads. I think that is what you need to hear.....what is normal.
From all you say is going on, something sounds amiss. If I had some extra time I'd go check what mine reads. I think that is what you need to hear.....what is normal.
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Originally Posted by WAHUSKER
Just trying to help. I've never looked at the schematic for a MAP sensor either. I make assumptions it has over voltage protection on the inputs, which may have saved you. But they might also have been weakened or blown by your test.
From all you say is going on, something sounds amiss. If I had some extra time I'd go check what mine reads. I think that is what you need to hear.....what is normal.
From all you say is going on, something sounds amiss. If I had some extra time I'd go check what mine reads. I think that is what you need to hear.....what is normal.
Keep in mind that the 9v MAP test was done yesterday, long after the other issiues had damaged the PCM. I also have more than one MAP sensor that can be used should there be a problem. There is no 5v reference from the original pin location for the MAP. That circuit inside the PCM is gone. An alternated 5v reference is being used.
I think that the issue has been found & it is as described by joecar & jimmyblue. We know for a fact that the PCM curcuit used to control the EGR, MAP, & A/C pressure is damaged. This most likely effects all of the 5v reference curcuits within the PCM because all of them result in the dropped MAP voltage when the MAP sensor ground wire is connected & not all of them work (for example the MAP sensore feed). Likely, within the PCM there is a short to ground. Even when the TPS 5v volt reference was used as a reference MAP source, it too dropped & effected the TPS as well.
The PCM is damaged. I have not yet tried to run w/ the alternate 5v reference from pin 71 on the blue conector without grounding the MAP sensor.
This would be the pull up circuit mentioned by jimmyblue & is the one method used where the 5v reference to the MAP & the signal from the MAP work properly (5v ref & feed back connected w/ no ground). I believe that connecting the MAP this way will work. Who knows for how long. When this method eventually fails, the PCM will be replaced. Good news is that when it is replaced, the issue that damaged the first two PCM's will have been corrected.
A normal MAP sensor w/ MAP sensor in place & wired has a 5v reference, feedback somewhere in the 4 volt range depending on elevation, & a ground.
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