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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!

Diagnostic Instructions: 

  1. Turn ignition key in the "On" position, with the engine NOT running.

  2. 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.

  3. 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:

  1. 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.

  2. 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.

  3. 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.

  4. 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!

1. Box gets REALLY hot and doesn't fuel consistently
2. FMS box enhances fuel in "Drag Race" mode but not in "Street Mode"
7. Redline doesn't enhance fuel, but display works
10. Display does not light up
3. CODE 237 errors, map signal voltage errors, MAP Harness
4. Display shows "Pump Connection all the time after start-up
5. Pyrometer reads low
6. Pyrometer reading fluctuates rapidly
8. Screen displays "F2 ERROR" (FMS ONLY)
9. Display shows jibberish after start-up not what it normally shows
11. Red light flickers under load

Box gets REALLY hot and doesn't fuel consistently

This usually only happens on really old versions of our FMS boxes and can be diagnosed by shaking the box in your hand and if you hear a rattle, then it probably has melted the power resistors off of the circuit board. This can be repaired and upgraded to the newer style for about $65, plus return UPS shipping. Please call for unique address for where to send it for repair.

FMS box enhances fuel in "Drag Race" mode but not in "Street Mode"

This is usually a MAP harness issue, but if after following the above diagnosis for the MAP harness the problem still exists, send to BCDP for an approximate $125 repair, plus return UPS shipping. Please call for unique address for where to send it for repair.

CODE 237 repeatedly being set OR map signal voltage errors OR truck runs better with MAP harness un-plugged from factory harness.

This is usually a MAP harness issue, and not the box. To diagnose this issue, visit our diagnostics page and repair low MAP voltage issues

Display shows "Pump Connection" all the time after start-up

Check to see if there is 12 volts in the big red wire with the ignition key on. If there is battery voltage there, then you have a failed processor chip on the circuit board. Most likely this failure is due to reversing the big red and black wires during installation, or a static electricity caused voltage spike. Return box to BCDP for an approximate $125 repair, plus return UPS shipping. Please call for unique address for where to send it for repair.

Pyrometer reads low

This is a result of us using an unusual type K thermocouple. It is not low, but rather it is slow, so don't worry about it, or use any other type K thermocouple and tolerate the fluctuations.

Pyrometer reading fluctuates rapidly

You are not using our supplied special type K thermocouple. As there is so much radio interference under the hood of these trucks, it is necessary to use a special style thermocouple to eliminate this problem. Read the product installation directions if you want to try and use your present thermocouple.

Redline doesn't enhance fuel, but display works

Check and possibly replace the 7 amp slow blow fuse in back panel with an ohmeter, as they usually burn out at the end where you can't see the break!

Screen displays "F2 ERROR" (FMS ONLY)

Replace fuse in black fuse holder in back panel of box, 5 amp fuse.

Display shows jibberish after start-up, not what it normally shows.

Box has lost 12 volt power or ground while the engine is running, either 12 Volts on the small red wire or ground on the big black wire, or your small red wire is not on fuse number 9.


Check tightness of the brass tap on the top leg of fuse number 9 and other crimp connectors on small red wire and fuse at fuse-box on left side of dashboard. Redo Skotchlok connection on big black wire from box, at black wire with tan tracer at injection pump plug.

Display does not light up

Verify 12 volts on small red wire, and ground on big black wire at injection pump black with a tan tracer. If this doesn't fix it box probably was either hooked up backwards, or got a high voltage spike, causing it to blow the soldered-on 1 amp fuse on the circuit board, or lose its programming, or both. Send to BCDP for approximately a $65 repair, plus return UPS shipping. Please call for unique address for where to send it for repair.

Red light flickers under load

This is always a bad connection on either the big red or the big black wire. If you have a "Pump Cover" loosen lock nut and tighten the allen screw a half to a full turn, and if not using a cover, redo the big red wire connection at the pump, and if that doesn't fix the problem, then redo the connection on the black wire at the pump.

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