tachometer stopped working so I took it off and attached it to a drill with the wires on and the tach gauge did work. So I assume it is the drive dog. How difficult is it to remove and install with engine in car? Thanks for any help.
Although the dog can fail, it’s not super common, compared to the more common failure of the bushing in the generator housing getting shoved back. It should be standing proud of the housing by about 1/16". If it isn’t then it needs to be pulled back out and pinned in place.
The dog can fairly easily be worked on with the cam cover off. It’s just held into the back end with an internal circlip.
The tech drive has a “c” clip that keeps the shaft from being pushed in. I don’t know if it’s factory that way or if Somebody put it but you cannot push the shaft in. The Dog Drive looks slightly worn but not that bad. The tachometer stopped working even though the tech drive when spun does make the tachometer work so therefore I assume that it’s not engaging properly.I was trying to avoid removing the cam cover but I assume that that is very difficult.
It’s not the shaft that gets pushed back directly but rather the bronze bushing the shaft rotates in. That indirectly moves the shaft with it. The clip only prevents the shaft from moving backward inside the bushing but the bushing is just an interference fit in the housing.
I don’t know why it moves but I suspect it’s because the edges of the drive dog wear more on their outer face so that as it rotates it imparts a backward force on the shaft instead of just a rotational force. It’s super common and happened to me twice before it was pinned in place permanently.
1 Like
I couldn’t pull the shaft out any more.
It’s not the shaft but the bushing. Is yours still sitting forward of the housing as much as this new one? They often get pushed back until the clip is sitting against the housing.
1 Like
Erica, can you show where and how you pinned the bushing?
I don’t have a photo, but I drilled a small hole and ran a tap in and threaded to for a tiny screw. The hole goes into the housing and bushing but not into the bushing’s hole so it just creates a lock between them. The screw was glued in with red loctite. It could be a screw, a roll pin, a nail, whatever so long as the fit is tight. You don’t want it backing out and falling into the head.
Charles, I had the same problem with my tach several years ago. The generator can be removed without removing the cam covers. It’s held on by allen screws which, although a bit fiddly, can be removed with the cam covers in place. As it turned out, all I had to do was reverse the drive dog and reinstall. There had been just a slight bit of wear on the side of the drive dog that engaged the generator and by reversing it, it engaged enough to spin the generator. Probably six or eight years down the road and it’s still working just fine. I’d try that first as it’s a quick and easy fix.
1 Like
Thanks all, Mark, how hard is it to remove the circlip holding the drive dog? I tried but is not easy
Hmmm… It’s been so long since I did that repair but I don’t recall any difficulties once I got the generator removed from the cam cover. I do remember that was a bit of a fiddly job due to access while the engine was in situ.
I’ve ran these for years and never realized a circlip was necessary. Learn something new everyday.
Hello everybody. I thought I should revive this topic as I have a question about the tach generator in my 65 OTS. My generator is suffering from the bushing being driven back so far that I believe that the drive disengaged from the cam. I’m planning to pin the bushing as a solution Erica so aptly described. Unfortunately the generator has another issue. The back cover plate is missing (oops). Is this just a simple flat disc that I can fabricate? Is the thickness important? Does it support the back of the rotor? Any help would be greatly appreciated as this is a pricey item so I am keen to repair it.
Thanks
Norman
Norman,
Hopefully others will chime in as it’s been almost 20 years since I repaired (pinned) my tach gen unit. My cover plate was loose, so I removed it just to see what the interior looked like before replacing it. As I recall, the cover was just a simple cover, a flat alloy disc, fairly thin, maybe .060 thick or so. I added a wee bit of silicone around the perimeter before peening it back in place. Still working today…
Norman,
It’s flat but slightly concave like an expansion plug. I’m not sure if it needs to be or not, but you’ll be able to tell once you pull the bushing forward to the correct location. See if it can turn freely with a flat disk nestled in the recess. An expansion plug might actually be a good blank to start with if you can find a close size.
I want to thank you and Steve for the quick and spot on response. Today I made a back cover out of a piece of 0.90 aluminum, Loctited the bushing as well as pinning it per your method (belts and suspenders, I love that expression). The tach is alive again. This forum is so great. I was able to go straight to the problem, execute a solution and save a $275 part with a minimum of time and effort. You can’t beat that.
On to the projects I’ve been trying to get to. First installing a tonneau cover. Second restoring and fitting my hardtop.
Thanks again
2 Likes
Hello,
My tachometer quit working and the bakelite cover on the electrical prongs is broken. I took out the old generator to replace and/or try to repair. Guess what I saw down in the hole of the back of the cam? This is a picture of the hole using a mirror on the firewall.
Have any of you seen anything like this before?
My question is when I put a drive dog in there does the rubber side face toward the cylinder head or the tach generator? My parts manual shows the part but does not specify the orientation of the part.
There is no circlip. So I guess my second question is what size do I need to get and is it the regular type that I can get at an automotive or hardware store?
So are you saying this was working previously? If so I have some bad news. The drive dog and circlip couldn’t have vanished to another dimension. It had to go somewhere, and it won’t be good. Right underneath the end of the cam is an oil return channel direct to the sump. Having any of those parts floating around in your oil is a recipe for disaster.
Have you ever seen the drive dog that was installed? The originals were rubber slathered over steel, but for a while at least someone was making aftermarket ones out of nylon because the original ones are all gone. I had one of those detonate and about 4 parts of it ended up in the sump. I don’t think an original part is likely to fall apart but the clip could come loose if it wasn’t fully seated and then both parts would go flying.
Step one will be to pull the cam cover and cross your fingers that it’s all somehow clinging to a ledge. Otherwise step 2 is pulling the oil pan off and crossing your fingers that the oil pump didn’t eat it. Sorry to be the bearer of bad news.