2006 F150 speedometer sometimes shows the speed incorrectly, always too low, never too high. To be clear, the odometer records the mileage correctly so it's most likely in the instrument cluster.
I've read a lot about the possibility of it being the stepper motor, but considering this only happens when it's hot in the cab, I'm thinking perhaps a bad solder joint it opening up in the heat and because I've already had solder issues on the instrument cluster board for the brake light. Before I go trying to replace a stepper motor has anyone seen/heard about solder problems affecting the speedometer? I don't want to go around randomly trying to resolder parts and a replacement cluster is not in the budget.

 

this isn’t a direct answer, but might be helpful:

when it is off, how far off is it (major or minor, a set and consistent amount or random and different)?

I would start by having a gps speed handy, then use cruise control while this happens; that way you can determine if the pcm is seeing correct mph, and if problem is limited to physical gauge readout.

If pcm is seeing correct mph and you can’t fix (or paying to fix isn’t worth it), you could either live with it (estimate via gps and/or rely on cruise) OR buy a digital gauge device that would plug into obd port (and would likely allow you to also monitor other things at same time (ex temps/volts/fuel pressure, etc).

Other people will probably be along with other/better ideas soon

 

A couple of more ideas. You could first remove the dash panels to access the instrument cluster. Then while driving, grab the connectors and twist and pull to see if it affects the speedometer. If not you could try tapping the cluster to test for a faulty solder joint..

 

I tried the tap tap tap on the cluster while driving with no change, the connectors seem to be connected well and while driving it doesn't seem to be making any difference if I attempt to move them.
Also, if it's working incorrectly and the cab has cooled down it will start working right after coming to a complete stop, but not until, unless it's still really hot in the cab, then it won't.

So its as if the speedometer needs to shut off before it starts working right but it won't work right if it's overheated.

I guess it's the random soldering avenue for now.

 

Yeah, touching each solder joint on the instrument panel circuit board with a small tip and the temperature hot enough ( around 600° F) to immediately melt the existing solder, is what I'd do next too. Curious to know whether that fixes it, so let us know.