2002 f150 screw fx4 4x4 - When I put my truck in 4 high, the 4x4 light illuminates. When I next put it in 4 low, things sound right and it seems to be in 4 low, but the low range light does not illuminate. This has worked in the past. Of course I am in Neutral with the brake on while changing modes. Any idea why the light does not come on?

 

Burnt out?

 

Is the bulb burnt out?

 

The bulb lights when I turn on the ignition with the engine off.

 

Are you actually moving at half speed when you think you are in low range?

Adrianspeeder

 

Everything feels right. The speedometer shows about twice the speed when in 4 low. It definitely seems to be in 4 low and not 4 high when in the 4 low position. The "4x4" light stays lit and the "low range" light stays dark when put in the 4 low position.

 

********* made a mistake - it is a 2002 not a 2000. Sorry.********

 

This is still fine. Same info applys to all ESOF from 1988 till current.

I'm not sure what you mean. The speedo should show your exact speed and you top out at 40 in OD.

Adrianspeeder

 

Actually, it'll do 65 at the rev limiter in OD and low range.

As for the lack of illumination, it'll take some testing with a multimeter and test light. How comfortable are you chasing wires?

Also, to do it right, you really need to see the inputs and outputs on the GEM, but that's a little tough to do without a good scan tool.

-Joe

 

That's seems to be different that how my speedometer functions. When the truck is moving at 20mph, the speedometer will read around 40. Mine will not show the actual mph while in 4x4 low range.

 

I am very comfortable tracing wires and using meters but I don't know where to start. Is there a sensor that gets tripped when everything locks in on "low range"?

 

The speedo picks up the speed reading from teh rear diff. The PCM and ABS module don't care which range the transfer case is in, it'll read the speed regardless. If it reads double, there's something wrong.

I'll need to skim the schematics for the colors, so hopefully I'll get time later today to take a look.

-Joe

 

Ok, I tested the speedometer in 4 low. It definately shows twice the speed the truck is moving. Are you sure the sensor is in the read dif for a 2002 F150?

 

I mis-spoke (got my F-150 confused with my Explorer)

The speedometer gets the signal from the PCM. The PCM gets the signal from the Output Shaft Speed Sensor on the transfer case. 15mph is 15mph whether the transfer case is in low range or not. How, exactly, did you confirm the speed? GPS would be the best way.

The high range light is controlled by the GEM. The Low Range light appears to be controlled by a microprocessor unit in the instrument cluster itself. The IC is linked to the PCM, so it's likely either the PCM isn't getting the proper signal from the transfer case, or the PCM isn't outputting the proper signal to the IC, or the IC isn't getting the signal from the PCM. Without a diagnostic unit attached to the truck that can read the GEM and PCM PIDs (inputs and outputs) it's going to be a little tough to troubleshoot the light problem.

-Joe

 

Are GEM readers expensive? Is this something only a dealer can do?