FMS and Redline problems
This test will determine if our MAP harness or the truck is to blame. If your truck sets a 237 code and or is low on power, ONLY when our MAP harness is plugged in between the sensor and the factory harness, most likely the circuit board in the harness has failed. The good news is we seem to have fixed this problem from occurring in our newer harnesses!
Turn ignition key in the "On" position, with the engine NOT running.
Remove the phone plug on the back of the box and check the voltage on the two center contacts. It is typically tricky to get the probe of a volt meter to make contact with the contact part of the plug, so my suggestion is to use a single edge razor blade to reach the metal contact with the red lead of the volt meter connected to the blade. The black lead of the meter should be connected to any engine ground. If it is a 1998-2000 truck, the voltage should be close to .5 volt, and if it is a 2001 or 2002 it should be 1.0 volt, on BOTH contacts. If both voltages are virtually the same, but lower than specs, our harness is NOT to blame.
Then try the "enema" trick at the bottom of this page to make the voltages right, before buying a new sensor. If the voltage is more than .2 volt DIFFERENT from one terminal to the other, our harness is bad.
The next step to cure this problem is to call Blue Chip Diesel to buy a better replacement! If you use the "MAP HARNESS MOD" that we previously suggested because we couldn't get parts to make replacement harnesses, your engine will set codes and reduce performance when voltages getting to the ECM are out of range.
Enema Trick Instructions:
With our MAP harness removed from the engine harness and sensor, remove the sensor from the engine and plug it DIRECTLY into the factory harness.
Attach the red lead of a voltmeter, set to a low voltage range, to the SIGNAL wire, the one that doesn't have zero voltage or 5 volts in it, in the factory harness, and the black lead of the meter hooked up to an engine ground, with the key in the "on" position.
Push a small flat bladed screwdriver in the hole of the sensor and push gently against the diaphragm and you will see the voltage rise, and when you let go, it will drop.
Keep increasing your pressure very gradually each time you push on the screwdriver, and when the "at rest" voltage is at or slightly above the spec for the year engine you are working on, STOP. You have now bent the diaphragm and re-calibrated the sensor. If you pushed too hard and end up with a higher voltage than spec, the engine will run fine, AS LONG AS YOU USE OUR HARNESS, but the boost gauge on our product will show a higher than real reading, and alter our fuel apply rates in Street Mode, but do no other harm! If you don't use our harness but do the "MOD", the ECM will see too high a voltage reading, set a code, and cut back fuel enhancement because it thinks there is an over-boost situation. Worst case scenario is if you screw up you have to buy a MAP sensor, after all! This WILL not fix any problem if the SUPPLY voltage to the sensor is HIGHER than 5 volts. If this is the case you are MOST LIKELY looking at an ECM issue. Check the supply voltages to other sensors and if any of them also have high supply voltage then you know the ECM MOST LIKELY has to be fixed. The bad news is all the sensors are NOT powered by the same circuit as the MAP sensor!