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Installing A Zener Diode On The MAP Sensor

Description

Here is my procedure on how to install a zener diode on your MAP sensor, to limit its output and avoid overboost shutdown.  This procedure requires that you have a very basic knowledge of electronics.  Although this mod is easy, I would recommend using Gus's Map Sensor Bleed over this setup.  I did not invent this design, but rather it is a common way to circumvent the ECU`s boost measurements.
 

Procedure

  1. You will need either a 4.3V or 4.7V zener diode.  Use the chart below for part numbers.  The industry part number is 1N749A or 1N750A for the 500mW device.  These are low tolerance parts (designated by the "A", which means +/- 5%), which equals about 0.47V variation between parts.  It would be better to get a higher tolerance part, if possible (1N749C or 1N749D), but these are difficult to find.  If you cannot find a better part, then get a few of them so you can mix and match to find one that works well for you.
  2. Locate your MAP sensor.  On all vehicles before 1987 and all L-bodies (Omni/Horizon and Charger/Turismo), the sensor is located on the logic module.  Later models (except L-body) located the MAP sensor on the front of the right, front shock tower with the baro-read solenoid.  For sensors located on the logic module, you will have to remove the sensor from the module to get at the short harness inside.
  3. The center wire is the signal output, which is either white (WT) or dark green with a red stripe (DG/RD*).  The other two wires are 5V and ground.  You also need to find the ground wire, which is either black (BK) or black with a light blue stripe (BK/LB*).
  4. You will need to connect the zener diode with the anode on the ground wire and the cathode on the signal output wire.  The cathode is the side with the lead near the stripe.  Strip the insulation off of part of the wires and solder the diode into place.  See below:
Overboost Cut-off Points by Year, Engine, and Transmission
Year
Engine
Manual
Auto
1984
Turbo I
4.30V
4.30V
1985
Turbo I
4.54V
4.65V
1986
Turbo I
4.65V
4.88V
1987
Turbo I
4.65V
4.88V
1987
Turbo II
4.65V
-
Zener Diode Part Numbers
Zener
Voltage
Industry
Part Num
Radio
Shack
4.3V
1N749
RSU 11928561
4.7V
1N750
-

Now, your MAP sensor output will be limited to around 4.3V or 4.7V (depending which zener and what tolerance you have).  To make sure that your installation has been successful, you can put a volt meter across the diode with the ignition on and pressurize the MAP sensor (don't exceed about 30 psi).  The voltage will rise until it hits the cut-off (zener) voltage of the diode.  Then, the diode shorts out and limits the voltage.  The highest voltage you see is the zener voltage for this diode.

You can now test your setup by driving it around.  Remember that you will lean out when the pressure generates a voltage past the zener point of the diode, so you need to keep your bleeds low at first and watch your A/F very closely.  If the poor tolerance of the diode allows the voltage to get too high, you may get overboost shutdown anyway.  If this happens, try swapping it with another diode and test it to see if your get a lower voltage than before.  The better diodes shouldn't have this problem.
 
 
 
Return to the Boost Bleeding Beyond Overboost Shutdown page

 

This page is maintained by Russell W. Knize and was last updated 04/27/99. Comments? Questions? Email minimopar@myrealbox.com.

Copyright © 1996-2003 Russ W. Knize