Speedometer goes down when I hit the gas?

[jjacumin]

My truck is a 1973 Series III. The Speedometer has worked fine for years, then a few months ago it started acting strange. When I hit the gas and accelerate it goes down, the faster I get the further down it goes. When I'm doing 55mph with my foot on the gas it shows about 20mph. Here is the kicker, when I take my foot off the gas it jumps back up to the accurate speed. Then when I give it gas again it dives back down to a speed about 30mph less than what I'm doing. It will go away and act normal, then come back. Any ideas?

[Wander]

Did you forget to turn off the retro-rockets???

First check the speedo cable connection to the tranny if that is OK.

Sounds like you've got either a generator or alternator that is going bad or a bad relay/ground/short in the wires. My gauge light flickers a bit-but just on the speedo side. I'm planning to take my speedo apart this winter as it is about 20mph too ambitious and the ODO doesn't work.

[JackIIA]

wonder if the end of the cable is partially stripped. it's supposed to be square. as you accelerate it doesn't grab as well in the housing and the cable doesn't keep 'in tempo' with the actual worm gear revolutions. as you slow down, it does catch, and gives an accurate reading.

admittedly, just a stab in the dark. try taking the end of the cable out of the transmission as suggested, and see what both it, and the housing area look like. my 2 cents.

[jac04]

Sounds like you've got either a generator or alternator that is going bad or a bad relay/ground/short in the wires.
The speedo is purely mechanical. An electrical issue should have no impact on it.

As other have stated, check the cable conenctions. Other than that, it could be internal to the speedometer. IIRC, it is a magnetic drive inside the speedo that produces a torque on the needle, causing it to move. When you accelerate, the 2 parts could be moving away from each other, reducing the torque, and causing the needle to drop.

[Wander]

Sorry-I guess I was confusing the issue.

Replacement series III speedos are available if needed.

[Andrew IIA]

Put the truck in a forward gear and see if that helps....... ;)

[bkreutz]

one way to eliminate the worn cable/drive socket is to disconnect the cable at the trans, chuck it up in a drill and see if it does the same thing, if so it would be in the speedo head as suggested, if not, then it's in the drive (maybe the drive gear can't keep up with the output shaft:D)

[Cutter]

mine does this exact thing, I've been using an iphone speedo app but I do intend on resolving it for the odometer at least, this is a good prompt. I have high output gears in the transfer which I assume makes the speedo inaccurate along with larger tires than original.

[jjacumin]

Thanks for the replies, I'll have to go check the cable and see if I can pinpoint whether the problem is in the speedometer, cable, or gears. Also, thanks for the iphone app, I have always been curious as to how fast I was going. All I knew is that it was not fast.

[amcordo]

mine does this exact thing, I've been using an iphone speedo app but I do intend on resolving it for the odometer at least, this is a good prompt. I have high output gears in the transfer which I assume makes the speedo inaccurate along with larger tires than original.

Ha. Me too, only it's an Android app. What would I do without a smartphone?

[mfreeman]

I have found this is usually the result of the gear that runs the speedometer drive slipping. This ring gear fits over the transfer case rear output shaft and engages the little plastic pinion gear that the cable drive fits into. This ring gear is something of a press fit on the output shaft. If the shaft runs loose and has too much slack the drive gear will slip and not turn at the same rate as the output shaft it rides on.

Check it this way; let off on the gas petal, if the needle moves to a more "accurate" reading then that is probably what is happening.

To correct disconnect the rear prop. shaft at the transfer case, remove the cotter pin from the castle nut and tighten nut on output shaft.

Mark

[Nium]

I concur with those that have pointed you towards a bad cable.

The other thing I wanted to say was that most GPS units have a speedometer that you can view, instead of using an iphone or droid app, of course that'd only be helpful if you have a GPS.

Cheers!

[Cutter]

I have found this is usually the result of the gear that runs the speedometer drive slipping. This ring gear fits over the transfer case rear output shaft and engages the little plastic pinion gear that the cable drive fits into. This ring gear is something of a press fit on the output shaft. If the shaft runs loose and has too much slack the drive gear will slip and not turn at the same rate as the output shaft it rides on.

Check it this way; let off on the gas petal, if the needle moves to a more "accurate" reading then that is probably what is happening.

To correct disconnect the rear prop. shaft at the transfer case, remove the cotter pin from the castle nut and tighten nut on output shaft.

Mark

I just happened to do this when replacing the U joints, mine was quite loose, but alas it didn't solve it. Regarding the GPS, that's what I use the iphone for as well when needed, I kind of hate the standalone things quacking at me. If there was an app to track mileage I'd probably just live with the speedo.