Tachometer Question

I wanted to know how you guys manage to put a tachometer in a 3 cylinder car? I wanted to get a tach but all the of them i see say for 4+ cylinders, so I dont want to buy one and have it not work or not be able to be hooked up. Will the tachometers work with 3 cylinders and the companies just dont mention 3 cylinders (i'm assuming so b/c the metro is pretty much the only 3 cylinder car in wide use in the US) or will they just plain not work?.
Let me know if it's possible to put a tach in. thanks!

Look for the Sunpro brand. They have settings for 1, 2, 3, 4, 6 and 8 cylinders. Make sure and research the model numbers. I think they willl all have the 3cylinder setting but I'm not 100% sure. :)

thanks! i'll have to check them out now.

Quick follow-up question...where's a good place to mount it? I am looking to get a smaller tach (2.5" -3") instead of the giant ones...just looking for an opinion on which spot looks best.

If your Geo Metro is a 92 to 94 year model, you can pull the instrument cluster out of a Suzuki Swift GT (same years) or a Geo Metro Convertibles (92 & 93) or later year Geo Trackers & Suzuki Sidekicks (96-98). Those instrument clusters are all interchangeable and had tachometers.

If you have a 95 and up Metro, I believe you can still try the above instrument clusters, and it might work.

For instance, something like this...

http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&item=7965107799

The problem is that unless the cluster came from a 3cyl car the tach will not read properly. They are a bit rare. It is cheaper to go Sunpro. :2cents:

Okay, I reading your claim, but not understanding.

Since the tach is reading RPM's (revolutions per minute) what difference does it make how many cylinders there are making revolutions? How would the tachometer know, the tach is mechanical and counts how many turns per minute of the crank, not how many cylinders are cranking it.

Are the turns of the tach cable geared and different between vehicles with different engines?

When guys do a metro to swift gt swap, I haven't yet heard anyone say that it will throw off the tach...my cent added to your two cents, but dunno if it makes sense.


The problem is that unless the cluster came from a 3cyl car the tach will not read properly. They are a bit rare. It is cheaper to go Sunpro. :2cents:

With a mechanical tach, you would be absolutely right! :biggrin:

However there are no mechanical tachs in any auto I can think of. The last I remember were used long before modern electrical systems and were driven off the distributor. At that time they were superior because of the unreliability of the day's electronics.

The Swift/Metro/Firefly, etc cars all came with electronic tachometers and are driven by pulses from the distributor. The four cylinder tach would, in theory, be 1/4 off it's reading because for every revolution of the motor it would be missing 1 of every 4 of those pulses. :p

Now, someone good with printed circuits might be able to make one work on the other, but IMHO that would be a lot of trouble. :uhoh:

I bought the cheap EQUUS tach from walmart, took it apart, scanned the face plate into photoshop, did the math, and figured out what the engine would ACTUALLY be turning and what the tach would THINK it was turning when set to 4 cylinder. I then mapped out the reference points on a new layer in the image, and printed out the new tach with the newly calibrated markings on it on photo paper. This was cut out and installed in the new tach. I've checked the tach readings vs what the OBD-II system thinks the engine is turning and it's off by, at most 2 or 3%.

I used to have the high res images for the tach online, but I lost them in the grand hard drive crash of '02. Sorry, All.

wow! sounds um....involved. It would have been cool if those images were still available, since I suck at both math and photoshop.
I found out that AutoMeter will calibrate any tach that they sell for about $25. I think that just that service plus shipping would turn out to be just about as much as a SunPro tach from autozone. I think I may just go with the sunpro tach, as I'm finding stock geo parts (i.e. gauge clusters) can tend to be expensive.
Thanks all for your responses so far, hope to hear anything else you guys have to throw out there!

~Rob~

I looked at the Sunpro tachs at Autozone and none say 3cyl on the box. Some on Ebay are advertised as for 3cyl and others.

Did not compare model numbers yet. If memory serves the instruction manual for all Sunpro tachs are the same...but? :uhoh:

http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&item=7966391492

3 cylinder! Vert's had tach's, but this one would be outa a 92 or 93, not a 91 as stated.

My '94 Geo Metro 1.0L 5spd has a tach. Are you talking about a tach on an AT? I'm not catching what the issue is here.

Only the vert and xfi[?] came with a tach. Mose Metros did not come with one be they auto or standard.

I'm converting a Pontiac Firefly Convertible (Geo Metro in USA) from ICE to all electric..

Since the 3 cyl, 1.0l ICE was a Convertible , I have a tacho as well as a speedo in the cluster, and while the speedo is mechanical and should still work, (same wheel diam and gearbox) I haven't any idea what signal the tacho received to drive it.

Tominator2000 wrote:The Swift/Metro/Firefly, etc cars all came with electronic tachometers and are driven by pulses from the distributor.

This confuses me slightly since a plug in particular only sparks every other revolution, (the ignition rather than the exhaust cycle) so getting 3 spark pulses from the distributor actually implies that the drive shaft has rotated twice, ie 1 revolution is only resulting in 1.5 pulses..



In my ignorance I'd been assuming that it just got one 12v 'pulse' per engine revolution from the old engine management unit, (rather than from the distributor) and have seen a magnetic collar that will bolt 'round the new electric motor tail shaft and with a matching receiver will give me exactly that, one 12v pulse per rev of the electric motor. Before I lay out the cash on the magnet & sensor can anyone confirm that one 12v pulse per rev is what the tacho expects? Or, if it isn't that simple can you please tell me how many pulses per rev and the voltage the tach would have been driven by.



Thanks,

Colin McC
Whistler
Canada

hi colinmcc,

please keep us updated on the electric conversion ... i presume you mean a full battery and motor system for the firefly. i'm still considering the possibility as my '91 ages though i'll probably keep an ice metro for the transition.

Have a look at these threads concerning Tachometers in a '93 three cylinder Metro (photos).

"Is there a SMALL, CHEAP Tachometer for a 3 cyl Geo?" 7-22-2005
http://www.automotiveforums.com/vbulletin/showthread.php?t=434486

"Can a 4,6,8 cylinder Tachometer be set for a 3 cylinder?" 11-28-2006
http://www.automotiveforums.com/vbulletin/showthread.php?t=647626

"Now I have a Tachometer in The Phoenix...!" 6-22-2007
http://www.automotiveforums.com/vbulletin/showthread.php?t=717109

May be of some help....

DoctorBill