Speedometer re-visited

[BobS76]

Posted several weeks ago about a speedo problem in my 79B. Following some forum advice, chased the problem down to the internal gear driving the odo. Returned the speedo, got the replacement, advertised as "new or rebuilt". After installing, noted a much more frequent but minor blip in the needle. Greasing the cable reduced this to almost nothing. HOWEVER, both new speedo read high, about 75 MPH at 3500 RPM. My recollection is that 3500 RPM should be about 60 MPH. I guess that is my first question.

Original and replacements have dial faceplates with identicle numbers. Is it possible that seemingly identicle speedo's would have different internal gear ratios?

The cable and angle drive are both new. I assume that the angle drive is always a 1:1 gear ratio, no alternative possibilities. Correct?

My final question surrounds the internal pinion drive gear. I replaced this about 2 years ago. I believe that my original speedo was working at that time and gave a normal reading (not positive - in recent years, my speedo has probably be down more than up, despite rebuilds, so I drive by the tach). How many teeth should the pinion gear have for 1979, where can I get the correct gear?

Bob

 

[Sarastro]

The speed isn't a good check on the gearing. Check the odometer, since it's directly driven by the cable. The speed indication depends on a kind of magnetic coupling and spring, and those can be way off. I'll bet you'll find that the odometer is pretty close.

It's easy for the speedo to be waaaay off. It works by a magnet spinning in an aluminum cup. That generates so-called eddy currents in the cup, and there is a magnetic repulsion on the cup, causing a torque on it, so it rotates, moving the needle. Then, a spring determines how far it rotates. So, the distance from the magnet to the cup, the strength of the magnet, the stiffness of the spring, all affect the reading. If something gets out of alignment or the magnet somehow moves in or out relative to the cup, it's all over. It might just be that the spring has come loose, or something like that.

Dunno about the internal gears, but, again, they affect the odometer, not the speed. You can check the odo against a GPS.

 

[BobS76]

Steve

Checked the Odo against the GPS and it ran high by about 15% - far more than any intenional manufacturing bias. All gauges have the same set of numbers and the new one was sold as a 79 MGB gauge. So that leaves the pinion gear. How many teeth should it have? Was more than one gear option available (presumably based on alternative transmission options)?

Bob

 

[NC Mike]

Bob,

You wrote that original and replacement units have the same numbers on the face -
Based on the speed errors you've mentioned, it sounds like you are supposed to have a 1,000 TPM speedo, but have a 1240 TPM speedo installed.
I unknowingly had a simliar problem when I first returned my 74.5 to the road, that remained unknown until the error was brought to my attention by the local constabulary.
It's possible that your replacement unit was "rebuilt" using a 1240 body and a 1000 face. If the rebuilder doesn't understand the significance of those numbers (as I must admit I didn't until my error) then he would have no reason to try to keep the correct parts together.

Mike

 

[BobS76]

Mike

Hah! Thank you. I think you have got it. I do have a '1000' amrked dial and knew that had something to do with calibration, but could not remember what. The error I have is close to what I would expect from a 1240 speedo - may recheck my odo. I will have to give Victoria British a call. I may choose to put the new dial and lens on my old speedo.

Bob

 

[NC Mike]

Bob,

After revisiting your post and re-reading it. You're getting an erroneously high speedo reading. Mine was roughly the same difference, except the reading was on the low side, so the opposite seems to apply to you - 1,000 tpm installed where a 1240 should be.
In either case, if your original still functions but needs a new face you can swap the new face onto the old body. My original was vandalized prior to my acquisition of the car. Some destructive little bugger broke the speedo glass and bent the needle. I had a spare in my parts car, or so I thought, and promptly swapped them out. Not long afterwards, those little numbers took on more meaning.
I pulled the bent needle, replaced it and the glass on the original, and reinstalled it.
Trick to removing the stuck needle was to use the smooth jaw hemostat from the first-aid kit to clamp and hold the shaft. A good grip on the shaft reduced needle removal to about a quarter turn twist.
Don't forget to index your shaft before you remove the needle, so that you get the needle put back in relatively the same position. I believe that there's a certain amount of preload on the spring to ensure that the needle returns to the pin at 0 mph, but don't know for certain. Note where the needle is pointing when you affix the clamp, and remember where the clamp is "pointing". Try to clamp the shaft behind the face, "inside" the works, so you don't have to take off the clamp to remove the face.

Good luck.

 

[BobS76]

To all who have offered help - I hate it when I wrap my self around the axel. NC, you are right about the direction of the gear error. Understand that in the last 5 years the speedo has not worked much. I have rebuilt the speedo and replaced different parts at different times - the history is vauge. This time around I chose to replace all parts from the transmission to the dash. Even if the speedo was not at fault, it was getting hard to read. Having re-read the various response, I checked the old speedo and installed it - it had the same error! At one time I had replaced the pinion gear, I thought sure with the correct number of teeth. Maybe not? Any ideas how many teeth a 1979 gear should have - 4 speed, no OD?

Bob

 

[BobS76]

Help. I finally got some decent weather and pulled the pinion gear and counted the teeth - 30. I my understanding of gears serves me well, I need to increase the number of pinion gear teeth in order to reduce number of cable turns per mile, thus reducing the indicated speed for a given vehicle speed. But here is the problem. I cannot find a gear with more teeth. The info that I have gleaned from the internet indicates that 30 teeth was correct for the last several years non-OD transmission. Do I have an older tranny? I feel confident that the old speedo read correctly after I had the tranny worked on years ago. How would I tell? Where do I go from here?

Bob