4 to 3 Tach conversion

Ludditeman
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Joined: May 17th, 2011, 2:22 am

June 22nd, 2011, 8:18 pm #1

Rather than do things the easy way, I purchased a tach from a 4 cylinder car to convert to work in my 3 cylinder car. It was cheap, and it was in my hands, what else could I do? The problem that now arises is that there are two resistors that need to be swapped out to achieve a combined 20.5K ohm resistance to get the tach to read properly on a 3 cylinder car. The two resistors are supposed to originally read 14.06K on a 4 cylinder car.
On the tach I have, I am reading 6.25K ohms. The two resistors to be swapped have color bands of red/black/orange = 20K, and white/brown/red = 9.1K. Using the formula R1 X R2 divided by R1 + R2 I get 6.25K ohms total resistance.
I can easily solder in new resistors to achieve 20.5K ohms resistance in place of the original 6.25, but now I am in doubt as to the accuracy of the original postings at:

http://www.teamswift.net/viewtopic.php? ... & start=75

OK, here's the real deal: If you start out with a tach showing 14.06K ohm resistance, you need to go to 20.5K ohms to make a 4 cyl tach work on a 3 cyl car. If you start out with 6.25K ohms resistance like I did, you need to end up with 11.5K ohms resistance to make a 4 cyl tach work on a 3 cyl car. One of the original participants in the thread above didn't mention what resistance he started out with, but his tach was visually identical to mine and he went to 11.5K ohms to make it work. That's what threw me off in the first place, I had no idea where he started from, so it made me doubt my numbers. Read on...
Last edited by Ludditeman on July 6th, 2011, 1:59 am, edited 1 time in total.
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mcmancuso
Elite Member
Joined: February 14th, 2010, 6:19 pm

June 22nd, 2011, 8:49 pm #2

did you try measuring the resistance with an ohm meter to check your maths? Also, what year did the tach come out of? There is a difference in the 4 cylinder tach depending on whether its 92-97 or 98+
Last edited by mcmancuso on June 22nd, 2011, 8:50 pm, edited 1 time in total.

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CityConnection
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Joined: September 8th, 2009, 10:59 pm

June 22nd, 2011, 9:26 pm #3

I really hope you didn't do this to a 99+ 4cyl tach. Those are quite rare and you could have sold it in minutes. 3cyl tachs are a dime a dozen.
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Ludditeman
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Joined: May 17th, 2011, 2:22 am

June 23rd, 2011, 1:49 am #4

The tach is out of a '96. I got the specs from the link listed, and was surprised to find a different reading. I took resistance readings and did the math based on the codes on the resistors, and they both came out to 6.25K ohms. What I am really looking for is some inkling of the correct resistance I need to make this particular tach ('96) work on my '99 3 cyl car. If it is hopeless, the tach will be for sale.
CC, they are a dime a dozen you say? Perhaps you could help me find one then. Nearly every salvage yard I called said they just crush Metros after a short time because nobody buys parts from them. BTW, I live and work in sparsely populated areas of the country, so Pick-n-Pulls are not common.
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CityConnection
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Joined: September 8th, 2009, 10:59 pm

June 23rd, 2011, 2:20 am #5

ebay, car-part.com, the classifieds section on this forum, etc.

99+ 4cyl tach is rare, a 96, not so much.
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Ludditeman
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Joined: May 17th, 2011, 2:22 am

June 23rd, 2011, 3:55 pm #6

CC, I checked all those sources, and after the third time a salvage yard said, "Yeah, we had one with a tach, but we crushed it", I gave up and bought the 4 cyl tach for $40. After all, the 4 cyl tach supply will last longer. Other places weren't sure if they had a 3 or 4 cyl tach, so I would still be rolling the dice.
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JellyBeanDriver
Elite Member
Joined: May 6th, 2009, 4:44 am

July 3rd, 2011, 8:31 pm #7

Recalibrated his tach. His calc in an email to me of what the new resistance value should be was spot on.

BTW, if physically the 4cyl and 3 cyl tachs are the same, why worry about which one is rarer when one can be turned into the other rather easily.


Here's a pic of my 'calibration' setup, resistor substitution box allows me to dial in what value is needed and then a fixed resistor is installed.


One end of 9.1K resistor was lifted and a 27K resistor tacked across the existing 20K resistor.
519,500 miles, original bottom end - donated to a charity 12-2017
- head replaced with rebuilt at 456,000 miles while I was in there replacing a failed head gasket
2003 Jetta TDI - 345,000 miles 50MPG, much more power.
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CityConnection
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Joined: September 8th, 2009, 10:59 pm

July 3rd, 2011, 10:15 pm #8

If you can make it work with a 98+ 4cyl I'll be impressed. And I'll buy one.
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Sabes
Fresh Fish
Joined: March 19th, 2012, 8:44 pm

March 20th, 2012, 1:29 am #9

So to add on to this and keep all the info in the same thread.

I have a cluster out of a 92 gti. Pulled the tach apart, desoldered the resistors and checked their resistances, the smaller resistor was 9.1 and the larger resistor was 17.8 which when you do the math equals a total resistance of 6.02 ohms. This is bit off from what Ludditeman found on his. This cluster is going in to a 93 hatchback 3 cylinder.
The question I have is do I still aim for the 11.5 ohms of total resistance in the parallel circuit?
If the answer to the previous question is yes what would be the ideal resistance resistors to use? The original resistors kind of got beat up from being dropped on the concrete floor at work several times. Not sure if this affects them or not.
What if I just used a big 1/2 watt 11.5k ohms resistor instead of two?
Last edited by Sabes on March 20th, 2012, 6:23 pm, edited 2 times in total.
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kel
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Joined: December 24th, 2011, 8:50 pm

March 20th, 2012, 2:44 am #10

Hope I can get mine converted if sum one can do it for me.
KEL
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