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Follow the bouncing tach?

Discussion in '"Stock" Alpine' started by Alpine 1789, Jul 7, 2022.

  1. Alpine 1789

    Alpine 1789 SAOCA President Diamond Level Sponsor

    I had a nice drive to the golf course in my SV yesterday; about an hour each way. The drive was a mix of country roads (some with too much traffic as it is tourist season here) and a scenic highway. The car ran great but on the way home while on the highway portion (early in my return trip), I noticed that my tach was bouncing around within a 1000-1500 RPM range. I was driving at a steady 60 MPH when I first noticed red it. Out of curiosity, I took it out of overdrive and the tach settled down to normal. I turned it on and off several times to be sure it wasn't just a coincidence, and it was consistent. It happened in both 3rd and 4th gear OD, although it seemed to be worse in 4th.

    It is a Tom H rebuilt tach and has always functioned flawlessly. What I found especially strange was that it seemed to disappear by the latter half of the drive, although it was difficult to be sure, given that I didn't have any more highway legs and was mostly in 3rd. I did manage to get up to 50 or so and drive in 4th OD and it was steady. I can take it out again for another drive but wanted to get input first on possible causes. I have never heard of this happening before. Any ideas?

  2. hartmandm

    hartmandm Moderator Diamond Level Sponsor

    Was the 1000 to 1,500 bouncing range "straddling" the correct RPM reading - i.e. going above and below, or was it only above what should have been the correct RPM reading?
  3. Alpine 1789

    Alpine 1789 SAOCA President Diamond Level Sponsor

    Good question and I am not certain, but think it was straddling. If anything, I’d say it was mostly below the correct range, but will watch more carefully on my next drive.
  4. belmateo

    belmateo Gold Level Sponsor

    What ignition system do you have in your car?
  5. Mike O'D

    Mike O'D Donation Time

    It wouldn't be running 1500 at 60 mph even with OD.
  6. Alpine 1789

    Alpine 1789 SAOCA President Diamond Level Sponsor

    I have a Pertronix system that has been there for about 15 years. Nothing has been changed recently.

    No, it runs around 3000, but the Tach was bouncing between 1000 and 4000 or so, but I have to check to be sure.

    It will be a day or two before I can get back out with it, but will report back then. In the meantime, my primary objective in posting this now was to be sure there was no danger in another highway drive. I can't imagine there is, but wanted to be safe.
  7. belmateo

    belmateo Gold Level Sponsor

    Two things come to mind, first is the ground wire connection should be checked when you get there. Even if it looks good, I would take it apart and clean it before remounting just to be sure. If that doesn't fix your issue it will be harder to find a high resistance open somewhere (probably the signal wire), but could be any of the wires.
  8. Alpine 1789

    Alpine 1789 SAOCA President Diamond Level Sponsor

    Thanks. I will double check things before I take it out, hopefully tomorrow.
  9. Tom H

    Tom H Platinum Level Sponsor

    Very strange! Does it seem to be due to the OD being on, vs or off, or was it more due to engine revs being higher (due to OD off ) that made it work better. Maybe try different revs while in neutral to see if it messes up at lower revs or higher. Or maybe put it in neutral and rev to say, 3000 and then with the shift lever in the neutral slot move it to the 3/4 side of neutral and turn OD ON and see if it acts up. That could help identify if it's a problem at certain rev level or simply due to the electrical issues from OD electrics.

  10. Alpine 1789

    Alpine 1789 SAOCA President Diamond Level Sponsor

    Thanks, Tom. Strange, indeed. Especially the way it settled down halfway through the drive. It was definitely the OD vs the revs. I will do little more investigating today and report back.
  11. Tom H

    Tom H Platinum Level Sponsor

    I keep trying to think of what might cause this. My best thought so far is that it’s due to a slightly binding needle pivot. I have seen similar behavior before, where the needle bearing is ever so slightly binding at just a couple positions of the needle. IIRC when increasing frequency (revs) during testing/ adjusting, the needle would stick and not increase until freq was upped a couple more steps where the increased current thru the meter overcame the stickiness and the needle then jumped up and actually overshot the correct value and the needle would oscillate a couple times before settling at the correct point. These needle movements are very sensitive and it takes very little to bind slightly. This could also explain why the problem only occurred on your return trip where a slight temperature increase may have slight metal expansion that caused the binding. If this IS the problem it’s pretty easily solved. There is an adjustable bearing just under the needle hub that needs about a quarter turn to add a tiny bit of slack in the end-play of the shaft.

    As I noted earlier, I think it best to test it with the car not moving, tranny in neutral, but shift lever in the 3/4 slot to be able to turn the OD electrics On and Off. My guess is that you will be unable to duplicate the problem until the temperature situation is identical to your return drive.

    If you are not able to duplicate the OD interaction but the problem recurs send me the tach and I’ll adjust the needle bearing.

    If you are able to duplicate the OD interaction I’ll consult with a couple other tach experts and try to find a possible cause.

  12. Tom H

    Tom H Platinum Level Sponsor

    Note that this sticky needle behavior is easily observed on a test bench with a precision test signal. But while driving you would not notice the actual, say, 200 RPM shift in revs while the tach does not move, but you do notice the resulting needle oscillation once a further increase in revs (or a bump in the road) unsticks the needle and causes the osculation.
  13. Alpine 1789

    Alpine 1789 SAOCA President Diamond Level Sponsor

    Thanks, Tom. I will respond to your suggestions in a minute, but want to first post the results of yesterday's drive. I wrote the following after the drive but held off submitting it because I wanted to embed the videos but was having trouble. It sounds like fixing the embedding issue is going to take a while (https://forum.sunbeamalpine.org/index.php?threads/embedding-youtube-videos.31100/#post-227253) so I will post this with just the links.


    I just returned from a short drive; about 15 minutes each way. I first checked to be sure the clip on the white wire wasn't loose and that the ground connections were good. (I grounded each gauge individually when I rewired the car about 10 years ago.) One thing that was readily apparent on the drive was that this is not, as I originally thought, an OD issue. I took a bunch of videos during the drive. They are a little rough and shaky, but demonstrate what I am seeing.

    Cold idle:

    On a local road, shortly after I left home. I am driving around 30-35 and trying to keep it steady. I am pretty sure I am in 3rd gear here.

    This is a few minutes later and shows me accelerating and in and out of OD. You can just see the OD light to the upper left of the tach. My maximum speed here is probably around 45-50. The temperature gauge was in the normal range by this point.

    Idling at a stop light about 15 minutes in, so after the car is warmed-up

    And, my return trip. The same road as the second video.

    So, it is not related to the OD as I originally thought and only happens when everything is cold. I am less concerned about it knowing that the OD isn't a factor, as I don't relish having to deal with a problem with that. What do you think given this new information?
    Last edited: Jul 11, 2022
  14. Tom H

    Tom H Platinum Level Sponsor

    Ok, so it is unrelated to OD. Good! I have seen some tachs over the years where the needle tended to overshoot a bit and oscillate slightly and briefly before settling after a big step in input frequency but not anything like I see in your videos. I also recall a physics experiment where we studied “galvanometer damping” to prevent the kind of action seen in your videos. I will consult with my tach experts in CA and New Zealand to see what they know. Seems 99% likely it’s a failed capacitor in the tach. But there are only 2 caps besides the main timing cap. I’ll conduct some experiments later this week at home also. That oscillation at around 3000 rpm with a smooth sound is definitely not due to ignition miss. The problem is almost surely in the tach itself.
  15. Alpine 1789

    Alpine 1789 SAOCA President Diamond Level Sponsor

    Thanks, Tom. I consider that very good news, as I can live with the cold bouncing for quite a while. I dug out my old dwell/tach and will go for a ride later today, if at all possible, just to have a 'second opinion' on the RPMs. But, a quick question before I do: Could this be related to the Pertronix? And, if so, would the external tach be able to pick it up, as that is attached to the negative terminal on the coil.
  16. Tom H

    Tom H Platinum Level Sponsor

    I cannot imagine any way it could be related to the pertronix. Clearly the engine runs fine, so the coil is firing we’ll every time so the pulse of coil current that triggers each pulse in the tach is getting sensed properly. I note in your videos that the needle oscillation is less likely at higher RPM, which is why it seemed OD related at first.
    If your old dwell/tach shows the same oscillation problem I am baffled!
  17. Alpine 1789

    Alpine 1789 SAOCA President Diamond Level Sponsor

    I just returned from a short drive with my ancient Craftsman Engine Analyzer attached to the coil and can confirm that the RPMs were very steady in the range where the tach is bouncing. (I can also confirm that the tach is very accurate when it is steady, so nice job on the calibration. :)) And, I kept it at a steady 4000 and 4500 RPMs for a period of time and it was steady, so the problem is only below that, as Tom noted.

    The forecast is very nice for the next week and I will drive it as much as possible. I'll report back if I notice anything else significant, but will appreciate any thoughts that occur in the meantime.
  18. Alpine 1789

    Alpine 1789 SAOCA President Diamond Level Sponsor

    Just FWIW, I edited yesterday's post with the videos and added one showing what cold idle currently looks like.
  19. belmateo

    belmateo Gold Level Sponsor

    Interesting how heat affects the movement, Tom's explanation of the needle bearing makes some sense and so do electrical components. Following this to the resolution.
  20. Tom H

    Tom H Platinum Level Sponsor

    The sticky needle bearing sounded like a reasonable answer until I saw the video of it bouncing back and forth even at very steady revs. Still might be the issue, but less likely. Might take a while to solve. We'll try a couple things with the tach in the car. If not resolved there I'll ask Jim to send it to me and see if I can reproduce the issue on the bench and solve it.


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