Howdy all. I have a problem with the speedometer on a '99 F150 I just bought. It reads about 1/4 of the speed at which the truck is acutally traveling. The odometer is off as well. It does have larger tires and different gears, but nothing which would cause such inaccuracies. The previous owner had both speed sensors replaced, and has had numerous shops look at the truck in an effort to fix the problem, which was there when he bought the truck as well.

Here's the weird stuff. The truck was a 4wd. It has had the transfer case removed as well as the front dif and other 4wd parts. A different 5 speed manual trans was also installed. The original ECU was damaged and it was replaced with a new one that has an appropriate part # as researched by the VIN of the truck. The ECU has been reprogrammed twice in an effort to remedy the speedo issue. A speedometer shop confirmed that there was no input to the speedo cluster. An electrical shop "did something" (exactly what I don't know) to get it to read 1/4 speed ( something about jumping one of the sensor wires). The previous owner tried to use an aftermarket speedo calibration unit, but it would only increase the reading to about 1/3 actual speed. A mechanic friend of mine hooked his scanner up to the truck, and it read the same speed that the speedo displayed when we were driving, which is leading me to believe that the speedo is ok, and the problem lies somewhere between the sensor and the computer.

So my main questions are, can anyone tell me how the speed sensors work in terms of voltage translating to MPH? Is there a difference in the computer translating that voltage into mph on 4wd vs 2wd? Does anyone see any red flags in my description of this problem that might shed any light on the situation?

Any help is greatly appreciated. I can't get the truck through emissions until I get this sorted out. The OBDII emissions reads that most monitors aren't reset so it won't pass. The drive cycle won't reset the monitors when the computer is saying the truck isn't getting over 20pmh. So I'm kind of stuck.

Thanks again.

-Nick

 

nick,
Respectfully, the truck has such a mix of parts and changes that you will never get the speedo to work even close.
First, depending on the specific parts and if the dash is elecronic, the PCM program contains the software 'info' the truck is supposed to have in order to drive the dash micro processor hence drive the speed indicator.
Right there you have a big issue because of the PCM change, the gear changes, the tire diameter changes have no close match.
The shops that tried to help apparently do not understand the complexity of the total system plus all the changes that would even affect a stock struck with just one change.
My opinion is you will have to live with what you have as there is no 'easy way' to make it read even near correct with all the changes made to the truck.
I some one has a good way to make the sppedo even close, I wish you all the best.
Good luck.

 

Any feedback at all? It appears as though the electrical shop cut and spliced two wires going into the computer. I can get a wiring diagram and understand what wires are what, but I don't specifically know the PPM (pulses per mile) that the computer is supposed to read. What little I can find online tells me that it might be expecting a signal of 32000ppm and it may be getting a signal of 8000ppm, thus reading 1/4 speed. Again, does anyone have any specific insight or knowledge about how the speedo, computer, and the sensors work on a 99 F150. Thanks.