Are there 4 MAP sensors on the V12TT?

I see one on each air box and one T'd off the passenger IC (mounted above the TB). That's 3 total.

Is there a 4th underneath the ICM that meters post-TB?

 

You're right about the locations, but I can't remember if they're ALL MAP sensors. The ones on the IC's might be temp sensors. The important one is on the intake manifold itself, downstream of the throttle, under the ignition amp. You can almost certainly find the right answer from the link in this thread:

https://mbworld.org/forums/m275-v12-...75-freaks.html

 

Great link, thanks!

 

Old thread, I know. But for future searchers I feel like I should update this. There are actually at least 5 air pressure sensors, but only 4 can be independently replaced. One in the intake manifold, one referenced to the passenger intercooler outlet (pre-throttle body), one in each airbox, and #5 is internal to the engine computer as an atmospheric reference. There could be more that I haven't stumbled across yet.

 

After dealing with a boost issue 2-1/2 years ago I found the same results you did. The two in the air boxes only measure vacuum and are different than the one in the manifold and the one attached to the intercooler by a hose. The air box sensors are not interchangeable with the other two.

 

Agreed. And if you go to the parts store and order a map sensor, you'll get one of those. If it's $50, BE SUSPECT. It's probably not the right sensor. The two pressure ones are a couple hundred bucks. They will physically interchange and plug right in but you'll have issues.

Also, be careful not to switch the plugs on the passenger side airbox and intercooler outlet map. They will reach and it will run and drive fine at low load, but it will cause very unpleasant results when you give it more than about half throttle.