MAP voltage....

ok, the skinny
98 xj
4.0L
5spd

the problem
about every three months, this thing starts to act up (like a vaccume leak). dieing when you stop at a stoplight, finniky starting sometimes, a lil smoke on hard acceleration, idle dips and dives while stoping. the "check engine" light comes on after about 2 weeks of this. i can unplug the map sensor and plug it back in, clear the codes and it runs like a charm for another 3 months. MAP voltage (cant remember the exact code) is what it reads every single time. also, it runs awesome w/ the check engine light on, which makes me think that i'm having a map failure here... any body around va got a map sensor laying around they'd send a guy to try this, i cant see shelling out $100 clams for one and it not be a problem...

MAP input voltage is supposed to be +5VDC, which the variable resistance in the MAP then divides to form the signal (variable resistance causing variable voltage.)

I haven't had a half chance to figure out what the output voltage is supposed to be at a given vacuum level - it's on my list of things to do...

Gimme a bit to finish eating, and I'll see if I can come up with any more for you - but if you unplug the MAP connector, one pin should be +5VDC WRT chassis earth.

I have been working with my MAP recently to build and adjuster so I could richen up the mix. I installed a 3 position switch with one position being 0v instead of the +5v mentioned by 5-90 above. I did this to act as an anti-theft and the engine won't run with 0v, it'll start but only runs for a few seconds. If yours is cutting out it definitely sounds like your voltage must be dropping below (probably pretty far below) the factory 5v (greater than 5 is going to dump more fuel in, less than 5 won't put much fuel in). The purple w/ white strip wire on your MAP sensor should be the supply wire with the 5v. I'm sorry I'm not much more help, but if you could check that voltage, especially while it is acting up it might be able to clue you in. It could be a bad MAP or possibly a bad connection to the source.

I had almost EXACTLY the same problem and it was not the sensor but a bad wire. Or at least it seems that way. It's been 300 miles since I spliced in new purple wire and the problem hasn't returned yet ...

Try jiggling the wires/harness with the engine running and see if you can get it to stumble.

Also, I was daunted by the connector but with a little bit of care you can remove the connectors from the housing, separate the old crimp connection and solder the new one in.

what voltages should be on each wire?

lilredwagn said:

I had almost EXACTLY the same problem and it was not the sensor but a bad wire. Or at least it seems that way. It's been 300 miles since I spliced in new purple wire and the problem hasn't returned yet ...

Try jiggling the wires/harness with the engine running and see if you can get it to stumble.

Also, I was daunted by the connector but with a little bit of care you can remove the connectors from the housing, separate the old crimp connection and solder the new one in.

Click to expand...

how much of the old wire did you have to replace?

i had a cutomer who had a very simmilar problem on his 95 the MAP was reading low voltage and i replaced the MAP and it was still doing it. i found that the vacume line that runs to it was rubbing the intake had worn a hole in it. the truck was not OBD2 so i got no lean codes (PO171) but it was giving the MAP the wrong reading.