I have a 78 F150 that originally came with a Kilometer speedometer gauge (truck must have been built in Canada) and I purchased an entire used instrument cluster that had the MPH speedometer gauge.

When I installed it and took it for a drive, once I hit 30-35 mph, the speedometer suddenly made an extremely loud (I mean loud!!) buzzing sound and the speedometer needle instantly wound clear around past 80 mph and bounced around at the bottom of the speedometer gauge (down by the odometer). When I slowed down to less than 30-35 mph, it quit making the loud buzzing noise and the needle went back to functioning normal.

The noise almost sounded electrical (kind of), but not sure. The gauge is operated by a cable, so I didn't think it was an electrical problem or a problem with the printed circuit board, but who knows....I'm not sure.

I took the cluster out of the truck and took the speedometer gauge out of the cluster and visually inspected it for any obvious broken or worn components, but everything looked brand new; was in really good shape with no sign of wear whatsoever. Since I'm at a loss, I decided to ask you guys for help....

Questions....

1) Has anyone ever had this happen to them?
2) If so, what was the problem and what did you do to fix it?
3) Could this be a problem with the printed circuit board?
4) Is this an electrical problem or mechanical?

Thanks in advance for your advice....

 

I too have the same question. When it started getting cold here in the morning my truck started acting the same way. Hissing noise and the needle on the speedo going crazy. I have thought that maybe my cable has some tension in it and is now ruined? I have also thought it could have a broken a gear tooth? Or that it just needs some lube on the cable. I haven't inspected anything as it has been cold and wet here.

Anyone care to let us in on what to do to fix it?

 

As a side note (maybe this will help you diagnose the problem)....My original instrument cluster with the kilometer speedometer did not have this issue. It worked perfect. That's why I'm wondering if it's a gauge issue or a printed circuit board electrical issue. I guess I can swap the mph speedometer into the original instrument cluster (with the original circuit board) and maybe that would answer some questions, but I was hoping someone on here could answer my questions before I went that route....

 

I don't believe it to be a circuit board issue. I believe the only thing related to the circuit board with the speedo is the bright light indicator. It is either a gauge issue or speedo cable issue. Mechanical issue not electrical issue. I'm just not sure exactly what the issue is.

 

Sounds like you didn't snap the speedo cable all the way into the gauge. My white plastic retaining clip is broken so the speedo cable works loose and does the same.

 

I'm thinking and hoping you're right. Maybe I just need to put it back in and also check the tranny end of the cable and make sure it's seated and give it another shot. Thanks for the advice everyone...

 

I just spent the weekend rebuilding my instrument cluster in a 78. I agree it is probably the cable or even the gear in the trans. It shouldn't be electrical.

 

Could it be possible the gears that drive the speedo cable need to be changed for the different cluster. Just throwing out idea's.

 

I don't think that's the problem. Doesn't make sense vs how it's acting and when it occurs (30-35 mph and above). If this was the problem, I would think it would do this from the start if it was a gear problem....who knows.....

 

Anybody know where you can buy one of those white plastic clips for the cable?

 

I saw a picture of a speedometer cable on Mustangs Unlimited (Mustangs Unlimited - Mustang Parts Superstore - FREE shipping!) and it had that plastic clip on it. Not sure where they get the cable from, but maybe you can call them and ask them if they have it or can get it. You could probably do the same with Auto Krafters, LMC Truck, CJ Pony Parts, or any of the aftermarket companies. Might even check Ford also....

 

The speedometers are a magnetic cup design, no electronics, either cable acting up or a small chunk of debris getting between the cup and the needle drive.

 

Would blowing the debris out with an air compressor work (between the cup and needle drive)?

 

I would guess also you do/did not have the white clip on all the way and the cable is slipping in and out of the collar? Swap back in the KPH and that will tell you if its a gauge cluster or the cable issue. If you did not remove it from the trans then should not have any issues down there.

 

I'd be careful blowing it out it's pretty delicate, I dont' think I'd use the air hose. My 65 does that when it's below freezing as well, I never checked it out but I figured it had moisture in it and it was freezing.