Anybody out there using a zener diode at the MAP sensor to limit the boost signal to the PCM in order to avoid defueling.

Just curious if there is a downside to this method..?

Mike

 

Popular opinion says it works. There are a few guys who caution against it for reasons I can't remember, but it had to do with damage to the MAP sensor. I have one some place... I wish I could find the bugger and install it.
I had a thread on this a while back, I'll try to dig it up.

 

Why not just use an air regulator installed in the map line?

 

The zener diode is easier to install and looks basically factory when done- no need to mount a regulator, run new lines, etc. Just an electronic way of accomplishing the same thing.

link 1
link 2
link 3

 

That was some intresting reading Chase. Thanks for the links!

 

I think the closest zener is going to be 4.7 volt.

Not sure what the output impeadence/drive capability of the MAP sensor is but there may be an issue with the zener clamp current, depending on what diode you use. I believe the EEC-IV inputs are BJTs with input impeadence in the 10's of kohm range so it will only ask the sensor for 1ma or so. most zeners need way more current than that to hold there voltage rating (4.7volts or whatever)

I went with the ITP OBA because it is proven, simple and I didn't want to hack up the factory wiring.

Now, if you really wanted to over engineer a voltage clamp with nano second delay, use a mircrocontroller with a A/D & D/A converters and write some software

 

No need to hack wiring, just stuff the wires of the diode into the plug.
You lost me after EEC-IV

 

Chase, Thanks for the links. Not sure I'm seeing a definitive answer. Maybe effects the map, maybe not... Maybe need a to add a resistor, maybe not... Both the diode and a mechanical solutions have some "non-ideal" characteristics.

Being a mechanical guy who doesn't tust anything electrical, I guess I'm leaning toward the mechanical solution.

Thanks again.

 

electronic "do-dads". a number of these were replaced due to their effect on low speed, low boost drivability. don't mess with the electronics, the PCM and MAP Sensor maintain 100% normal connectivity with no electronic interference withthe OBA..the OBA lets the map see live ###''s at that moment.. the resistor is slow to re act..

 

Ron- the OBA keeps the MAP from seeing boost over 23 psi (or wherever you have it set). The diode does the same thing, only by limiting voltage instead of psi. I can see where the diode could effect the sensor in other ways (I'm no electronics expert) or may be slower to react but all it does is limit the voltage the MAP sends to the PCM. Ifthe diode has no other effects on the MAP sensor I think it's a way better option than an OBA- less clutter, less cost. I really wish I could find the one I had...

 

all electronics operate at the speed of light.

it is the non-ideal charateristics of a diode that give the precepion of a 'delay'. the plot by ernest eugene in one of the links above shows it well

adding a resistor in series to the diode circuit to protect overloading the MAP will only make it preform less ideal, or not at all. a typical zener needs to be reverse biased and fed 20-100ma to get it to drop (or hold) it's rated voltage. the MAP isn't going to support that, it would need a voltage follower current amplifer.