Wondering if any of you folks have ventured into "home repair" of the /5 motometer tach/speedometers? Mine stopped working some years ago at 60k +-. I've seen the tech article here giving an overview of what appears to be a /6 or later unit. I have viewed a web site that sells the replacement gears with their somewhat generic instructions.
What I hope to get here is personal experiences in executing said repair on a /5 motometer.
Are there instances when replacing gears is not necessary?
Should the whole gear set be replaced at the same time?
Is it difficult?
Are there any "gotchas" that need to be looked for and anticipated?
Thanks!
Steve
I have worked on 3 or 4 sets of speedometers for the /6 and later side-by-side gauge sets.... but one of my other hobbies (when I'm not out in my Roll Royce cruising for Grey Poupon ?) is working on watches and clocks.
► Forgive me while I point out the obvious... Both of your instruments are cable driven, so the place to start is by giving both cable a thorough going over. And by that I mean pulling the inner cable completely out of the outer sheath and inspecting it. These cables break all the time, but when they do it's not always the result you might think. If you simply inspect the end that goes into the instrument, then you may indeed see rotation while riding. However, under the load of the instrument, the broken cable will cease to turn. The instrument applies a load that is missing from "open air" rotation testing. So check the drive cables.
► Secondly, never ever lubricate a speedo or tach with anything but silicone watch "oil". Any petroleum based lubricant product will simply require a complete disassembly and cleaning in about 1 year... so don't do it.
► Thirdly, I will warn you that working on a speedo internals is very much like a watch in that such a huge array of small screwdrivers, picks, oilers, non-magnetic tweezers, magnifying glasses, etc is required that it's hardly ever worth buying all that for a single instrument. Send it to one of the repair shops listed in the Airmail.
Hope this helps.
Owning an old Airhead is easy.
Keeping an old Airhead running great is the true test.
Thanks for the info, Richard. Don't assume you're pointing out the obvious, ever, in my case. ?
I've disconnected the tach / speedo cables from the instrument, and the speedo remains at the 80mph position. I'm inclined to speculate that lessens the likelihood of a failed cable significantly, at least for the speedo portion.
Good warning re lubrication; exactly the sort of "gothchas" I am looking for.
OK then, in most analog speedos and tachs, the rotating cable drives a magnet. The magnet spins inside a co-axially located steel cup. The cup and magnet are in close proximity. So the spinning magnet imparts a tiny "drag force" into the close-fitting magnetic cup. At the other end of the cup's shaft is the dial and indicator needle. To oppose this tiny force, a flat spring made in the depths of the Black Forest from frog hair and fairly dust resists the movement of the needle shaft, and makes it return to zero when the spinning stops. It is these miniscule opposing forces of magnetism and return spring that allow a mechanical speedo (or tach) to have that graceful sweep of the indicator needle. As you might well imagine, any friction in the indicator shaft completely disrupts this extremely delicate balance.
So although the cable is able to drive the primary spinning motion of part of the instrument, the secondary needle shaft has become fouled. This is how the cable can continue to drive the instrument without any indication from the needle. There is simply not a direct mechanical connection between the cable and the needle.
Owning an old Airhead is easy.
Keeping an old Airhead running great is the true test.
Great explanation. Is it the same type of arrangement for both the speedo an tach functions?
@sebascodegan If the drive cable and remainder of the Speedometer are good, then I will assume the Odometer still works OK.
Owning an old Airhead is easy.
Keeping an old Airhead running great is the true test.
i’m still waiting for my gearbox to come back from a rebuild (end of Aug!), but perhaps I can give it a go with a drill chuck to confirm.
I have refurbished speedos on a few (1976-95) Airheads over the decades, most recently my 95 R100RT; not difficult, but painstaking. For the 95 RT, I used the guy's article on gunsmoke site (sry, I dunno his name) for reference, which was very helpful, especially since it's been 15+ years since the last one. I have also refurbished several speedos on old Harleys and 1950s-60s Studebakers. When the needle hairspring is accessible, I have also calibrated them. (Not accessible on my BMW though.) My experience is the BMW's speedo is 99 percent same as Harley & Studebaker, but I have no experience with /5.
For oil, I use something akin to 3-In-1. For grease, I use distributor cam lube, made to lube the wear block on points type ignitions. I am more used to working with bronze and other metal components, but the BMW is mostly nylon. Will see if there is any compatibility problems with the lube I used and the nylon, but I do not recall any such problems before. For sure, it's better than it was when I disassembled it, as the factory grease had all been pushed aside, and it was basically running dry. The 95 RT's needle was still steady, but it was beginning to squeak at 30-40 MPH, which is why I refurbished it.
I cannot figure out how to insert a link here, but the site I mentioned above was on gun smoke.com
Thanks Joe. I appreciate the info. I'll poke around on the gunsmoke.com site some more. There are a lot of rabbit holes to get lost in there. Good stuff.
If I ever get into instrument I'll take [and post] some pics. I suspect I'll have questions.
Steve
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