erratic tachometer (sanity check)

[MountainRoads]

The tachometer on my '71 -E suddenly started jumping around near the end of a nice long drive this past weekend There are multiple threads on several forums concerning this topic, all of which seem to point to voltage regulator, alternator, or grounding issues. Oddly (to me), none of them mention the RPM sensor or RPM transducer. My symptoms are pretty classic - the tach suddenly started bouncing wildly from what seemed about right (3000 RPM), up past 6000 RPM, then down to zero, then up to 6000 RPM again, etc. This continued for the rest of the drive home. Everything else seemed to work fine, including the radio. Unfortunately from a diagnostic standpoint, I started the car up the next morning to check it out, and tachometer behaved normally.

The other piece of information to offer is the alternator warning light goes on dimly at idle speed (around 900 RPM). This has been going on since I bought the car several years ago. It gets progressively brighter as the RPMs go down, and goes out completely at around 1500 RPM and up. I've been meaning to check out both the voltage regulator and alternator (a possible faulty diode ?) due to the warning light, but it hasn't been a top priority since it doesn't seem to affect battery charging and the car otherwise runs fine.

My question for the wiser than me here is: Where should I start first?
1) Obviously, the ground issues are cheapest to fix, but a challenge to be sure you've gotten them all.
2) Batteries are brand-new Sonnenscheins and seem to hold charge well. I use a battery maintainer for extended storage periods.
3) I've been meaning to check out the alternator / voltage regulator combo anyway due to the warning light. This new issue with the flailing tachometer needle gives me increased impetus. Especially if I'm risking an overcharging situation.
4) The voltage regulator is obviously easier to pull than the alternator, but neither is particularly difficult so I'd pull them both to have them tested.
5) I can put a voltmeter on the car to diagnose any low voltages at idle or over-voltages, but it might be difficult to time that with the intermittent tachometer issue. Especially if it doesn't reappear.

Thanks much in advance. Candid advice always appreciated.

- MR

[DaveA]

I've had tach problems when the points were not adjusted properly - when the points were not opening
much, and were near closed on the lobes.

R/Dave

[Chuck Miller]

MR,

As Dave said, check the gap on your points.........
Also take the black wire with the purple stringer off the distributor and clean the spade and connector and reconnect.

Hope this helps

cm

[Bob Harriman]

Points. Mine disappeared with a Petronics ignition and has never returned. It's a 912 though.

[MountainRoads]

Thanks for the suggestion, guys. I'll check out this coming weekend. Wouldn't a points problem show up in engine performance, however? Also, are points problems likely to disappear as suddenly as they appeared? Just askin'.

- MR

[Dmill]

joining the chorus...points

[304065]

Hard to see how it could be a point issue unless the rubbing block has worn SO much that the opening interval is long enough to trigger the CDI, but not to send a signal to the tach.

But before we begin we need four pieces of data.

1) What is production date of the car? Before 10/70? Probably without knowing more.

2) Is there a metal can on the engine electrical console that says "einbau senkrecht klemmen unten" on it?

3) On the back of your tach: what is the date stamped in blue ink, and does the third terminal from the left say "/1" or "C?" This may be harder to get, a flashlight in the inverted yoga position in the trunk may help, or you can GENTLY ease the tach out of the dash to check, be careful that you don't mix up the wires and cause an electrical fire (ask me how I know). "No Screwdrivers unless they are made of butter." -T. Bridgers

4) With a digital voltmeter connected between the + and - battery terminals, what is the voltage (a) with engine not running (b) with engine running at 900 rpm and (c) with engine running at 3000 rpm?

Armed with those data we can give you a precise answer. You were 90 yards down the field with the voltage regulator answer by the way. . .

[DaveA]

Just try slightly filing the points with a points file ( or a fine nail file), and resetting the gap, then reset the distributor timing.
If the car isn't driven much, a film can develop on the contact surface. The car will run, but the tack won't work properly.
R/Dave

[denson]

I have had that and it was related to the voltage regulator.

[MountainRoads]

Thanks again to all the great suggestions/questions. I'll be checking it this weekend and reporting back early next week.

- MR