Alfa Romeo Forums banner

1 - 12 of 12 Posts

·
Registered
Joined
·
549 Posts
Discussion Starter #1 (Edited)
Hi folks,
I have an electric tach in my S2 that has been reading off since I got the car. I've gone without one, but now that I'm checking off all the little boxes of work I've got left I'm looking to restore the tachometer back to stock.

Problem Background:
1985 Jaeger Electric Tachometer. Does not read correctly at anything more than 1K RPM - looks sluggish.
Used Electric Tachs: Going for $40-$90, but may have problems down the line
Professional Rebuilding: Costs around $160 and involves shipping the tach unit to a far away place.

So I decided to open it up and see whats going on...

Disassembly:
-Opening the case is really annyoing.
-I pried open the case using two/three small screw drivers. It took about 10 minutes, because I did not want to break the glass. The glass is just below the edge of the tach, so be careful on this step.
-unscrew the tach-driver from the tach housing.
-the tachometer circuit is taped to the housing.



Tachometer face testing: The needle sweeps well, with no sticking. There doesn't seem to be any issue on this end.


Circuit analysis:





Circuit Parts List:




Conclusions:
I've checked out all the resistance values, and all but one (R7) is right where it should be! I will be replacing the electrolytic capacitor, as I have no way of testing it at home.

I will also be buying either a stand-alone function generator or an Arduino shield attachement to produce a calubration waveform (3000hz for 3000 RPM).


Questions going forward:
What does the three position slide switch actually do? In testing, the switch seemed increase the RPM range -like high/medium/low, but did not change the sweep of the tach.
 

·
Registered
Joined
·
63 Posts
This is a good piece of information. For what it's worth, I would expect the capacitors to be the biggest trouble, as electrolytic capacitors are known to deviate rather wildly from their OEM value as they age - particularly in a mobile environment where the temperature / humidity varies wildly. My tach has the same issue as yours. It reads "normally" up to 2000 RPM or so but won't go much higher than 2500 RPM no matter what. I am pretty sure that rebuilding the tachometer drive unit (the electronics) will probably fix this. I just haven't gotten around to it yet - there's a LOT more to do before I get to that. Another potential trouble spot would be the variable potentiometer, as these also tend to deteriorate badly as they age. You may need to "exercise" this as part of your rebuilding effort, which will (necessarily) un-calibrate the tach. You're going to have to re-calibrate it anyway, though. In fact, I would replace the potentiometer if a suitable replacement is available. As to the purpose of the selector switch...hmmm. Not sure on that one. Perhaps the tach drive circuit was used in other models? It might be a "cylinder count" switch - I've seen such a thing in some Ford Mustang tachometers. They used 4, 6, and 8 cylinder engines in Mustangs during the eighties but all had the same tachometer.
 

·
Registered
Joined
·
549 Posts
Discussion Starter #3 (Edited)
Hi pgman, you make good points. The evaporation of the electrolyte in electrolytic capacitors means that they should always be the primary suspect when old circuits stop behaving as they should. Also, the selector switch just changes the resistance in that part of the circuit - likely for a cylinder count.

Its been way too hot to work in my garage this past week, but before the heat set in I replaced all capacitors (the 2 mylar, the 1 ceramic, and the 1 electrolytic) with new components.

I tried to supply a square wave pulse with a function generator at work at 100 HZ (tried all amplitudes 2V to 12V - 50% duty cycle), but the circuit did not respond. When attached to the coil with the engine running, it works well.

I'm not sure why it won't take a square wave. I can't bring the scope home to test the coil - so I'm not sure what kind of signal or at what frequency I should be supplying.

In any case, I also had to replace the variable potentiometer, as they apparently will fall apart if disturbed. I replaced it with the following 0-300ohm potentiometer from ebay (its a 6mm size, slightly smaller but it fits anyways).



http://www.ebay.com/itm/220732953889?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1439.l2649


I tried calibrating off the engine with another tach/dwell meter, but it looked like it was still reading low at high RPM. I wonder if these tachs are accurate at all RPM when calibrated?

I have a whole bag of transistors which I can use to replace these older one. I know that transistors are unlikely to fail, but I could just go ahead and rebuild the entire circuit with new parts.
 

·
Registered
Joined
·
549 Posts
Discussion Starter #4
I'd like to add that changing the ceramic capacitor seems to have done the trick. I still wasn't able to supply a test waveform, but used a dwell meter/Tach to set the calibration. Also, I've updated the above post with correct capacitance figures for the Mylar capacitors.

My tach is reassembled and back in the Spider!!


Remember not to unplug any tachs from the coil when the car is running (you can get a nasty shock).
 

·
Premium Member
Joined
·
696 Posts
My tach is reassembled and back in the Spider!!

And I assume its working? Ours is not working also, is this a reasonably straightforward operation, I am all about rebuilding, but a little apprehensive with the electronics. Have many other Alfa enthusiast's have much luck with this?
 

·
Premium Member
Joined
·
13,330 Posts
It is almost certainly a monostable circuit. Do you think that the pot is for fine calibration?

My Tach reads about 10% high which points to either the resistor or capacitor in the timing circuit being 10% too high.

For a while I was using the tach output signal from my MSD unit to fire both my tach and the rpm input to my Zeitronix AFR data logger. The tach would indicate normally until the revs hit about 6k then the needle would flop to zero. It would come back to life when the revs dropped below 6k. The tach appears to load the signal more at higher rpm.
 

·
Registered
Joined
·
549 Posts
Discussion Starter #7
My tach is reassembled and back in the Spider!!

And I assume its working? Ours is not working also, is this a reasonably straightforward operation, I am all about rebuilding, but a little apprehensive with the electronics. Have many other Alfa enthusiast's have much luck with this?

Yeah, its still working. Whether you want to go into the electronics is ultimately up to you. All you need for this project is a soldering iron and the patience to wait for the different components to arrive from your ebay source. All my caps were out of spec. All the resistors were still OK. My tuning potentiometer dissolved as soon as I tried to use it. The hardest, or most troublesome part for me, however was the actual opening of the tach. Unwrapping the outer lip, lifting the glass out without breaking it. A bit nerve wracking even though the glass is not difficult to source locally anyways.
 

·
Premium Member
Joined
·
696 Posts
different

Yes, great thread. Was this tach from an S2? The innards from our '85 spider graduate are somewhat different. The movement mechanism is screwed to the pc board, the configuration is similar with 2 "can" type capacitors. How does one measure the capacitance or simply change them out?
 

·
Registered
Joined
·
549 Posts
Discussion Starter #10
Yes, great thread. Was this tach from an S2?
To be honest, I don't know. There is no way for me to tell the provenance of this tach. In any case, the electric tachs began being used from 78' to 86-87, I believe. Thats the transition period from mechanical tach to monopod.


. How does one measure the capacitance or simply change them out?
Totally just change them out. Digikey is a good resource. I'd suggest radioshack, but RIP that store...

Anyways, these caps deteriorate so don't hesitate to change them all. Even the ceramic ones.
 

·
Registered
Joined
·
29 Posts
Hey sorry to revive an old thread. But I hate to have multiple threads on the same topic.

I I noticed there were diodes in your schematic but you don't specify a part number or voltage rating. Did you replace those? I'd like to think after 30 years they expire like anything else. Plus these components are all less than a dollar.

I imagine it's 12v or 5v, but not sure which diode is which.

Thanks
Jeff
 

·
Premium Member
Joined
·
13,330 Posts
I expect that one of them is a Zener diode to eliminate error due to changes in supply voltage. Also, the pulse from the coil will be well over 12 volts and there could well be a zener in the front end of the circuit to protect the transistors.
 
1 - 12 of 12 Posts
Top