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High Reading Tach Repair

 
Old 08-18-2010, 10:59 PM
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Pop Evans
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St. Jude Donor '10
Default High Reading Tach Repair

Did this last night since there were several folks seemed interested.

REPAIRING THE "HIGH READING" TACHOMETER IN THE 90 CORVETTE FIRST ....... DISCONNECT THE BATTERY..... Do it NOW

1. Remover the instrument cluster. To do this it is strongly recommended that you first drop the steering column. It is possible to remove the cluster without dropping the column but you will do damage to something before you finish, trust me.
2. Remove the carpeted panel under the dash, this is held in place by about 5 - 7 phillips head screws.
3. Disconnect the courtesy light and the screw holding the ALDL connector.
4. Remove the metal support panel. It is held in place by 4 screws (10 mm). There are 2 on the left side of the dash and 2 from the bottom on the right side almost hidden by the carpet on the console.
5. There is a yellow wire and connector close to the column, stay away from it. It is the SIR connect and goes to the air bag on the steering wheel.
6. Unscrew the tilt lever from the steering column. Insert your key in the ignition and turn it to the 6-12 O'clock position. This will allow the column to clear the dash when it is dropped.
7. Remove the 2 15mm bolts the hold the column in place and gently allow it to lower. I used a jack stand with a cloth on top to provide a rest. otherwise just be gentle.
8. Remove the bezel screws and remove the bezel. There are 4 screws that hold the cluster in place, now you can remove them.
9. . Reach behind and unplug the wire connector, 1 push clip on each of the long side ends, you'll have to wiggle it out but it will come out. Now the cluster is free.
10. Place the cluster on the bench and remove the plastic cover from the back. (If it has been worked on the cover may be missing). The back should look like the picture below.




11. You can see the red arrows pointing to the 2 solder points that you will be working with. On the other side of the board there is a resistor "pac" that has changed value. The "fix" for this is to find the correct value and solder a resistor of that value across these two (2) points.
12. Go to your favorite electronic parts supplier (Radio Shack) and purchase a 1 meg ohm potentiometer. This will cost around $3.00. Solder 2 wires to it, one to the center leg and one to one of the outer legs.





These wires should be twisted together and about 3 feet long. Solder the other end of the 2 wires to the points on the board insuring that they will not short out to anything. Now comes the fun. Re-install the cluster, 2 screws will be enough. Bolt the steering column back up and remove your key.
13. Re connect the battery and go for a drive. You can adjust the potentiometer to make the tachometer read just about what ever you want. In mine at 65 MPH 2000 RPM was about right. After adjusting for the correct reading be very careful not to disturb the setting. (If you have access to a portable tachometer, use it instead of driving)
14. Do as necessary to remove the cluster again and remove the wires from the printer circuit board.
15. Using a digital ohm meter measure the resistance between the wires. Write this down. You will need a resistor of this value to solder back on the cluster. If it is not close to a standard value you will have to use several resistors in series to make up the value.
16. Once you have the "new" resistor solder it to the board. Place some electrical tape under it and over it to insure that it will not short out to anything.




17. You now should be able to reassemble the tachometer, steering column, etc back to normal and enjoy a correct reading tachometer.
Again, just be careful about not shorting out any wiring and make sure nothing is pinched. If you don't understand this process, ask somebody that is comfortable with electronics and soldering to assist you.
You will not need the potentiometer anymore unless you do this process again.

Thanks to the following folks who provided input.
Stu
Johnny Evans (no relation)
John Gardner

Regards Pop Evans
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Old 08-18-2010, 11:17 PM
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ccrazor
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Great fix. Only thing different I would do is use shrink tubing instead of the tape. Should be able to get at your electronics store..
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Old 08-25-2010, 02:56 PM
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Default Thank you to Pop Evans!!!

Great Job!!!
Thank you so much!
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Old 08-25-2010, 08:09 PM
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Pop Evans
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Sky Air,
Glad you found it useful. There were a couple of folks that provided information for that.
Kindest Regards Pop
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Old 08-25-2010, 09:12 PM
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rodj
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Does this also fix the other common 90 /91 tacho problems?
Slow needle reaction / hanging needle when revs change/ tacho reading having no relevance to actual engine revs etc?
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Old 08-26-2010, 12:04 AM
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Pop Evans
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Originally Posted by rodj View Post

Does this also fix the other common 90 /91 tacho problems?
Slow needle reaction / hanging needle when revs change/ tacho reading having no relevance to actual engine revs etc?
I really can't say.... the fix was for a high reading in my car (90 L98)and that was all I looked at. Mine was pretty steady but at 65 mph it read about 6000 rpm.
Kindest Regards Pop
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Old 08-30-2010, 09:01 AM
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Jim85IROC
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This is a fantastic bit of information for those who prefer to fix this problem on their own, but I do want to add my own $.02:

It may not be obvious from his pictures, but those two terminals that Pop Evans suggested soldering to are electrical equivalents of the two pins on the resistor chip itself, which is actually located below those pins on the board. I went a step further and removed the entire printed board from the cluster in order to gain access to that chip on the other side of the board. The reason for this is that the resistance across those particular pins has changed over time, which is what caused the tach to become inaccurate in the first place. Simply soldering a new resistance in parallel will likely only be a temporary fix because that resistance will continue to drift. You need to cut two pins off of the chip soldered to the board, THEN solder the new resistor into place in order for this to remain reliable.

Once I cut the correct pins off that chip, I determined that 300k is the proper resistance to re-insert into the circuit. Since there's no parallel resistance thanks to cutting the pins, this should be a constant value that will work on everybody's car and won't change over time.

Pics:
This first pic shows the resistor chip on the printed board. It's the white & black chip below the bulb:


The two pins that I have my DMM probes on are the pins across the faulty resistor within:


Once I cut those pins off of the chip, I soldered the resistors right into the same holes:


Ultimately, because I was using the wrong tip on my soldering iron, I managed to burn the copper trace around one of the holes, and had to relocate one end of the resistor to one of the spots that Pop Evans used. To be honest, his spots are better because there's a bigger solder area and there are no nearby pins to deal with, so I would recommend that once you cut the proper pins on that chip, that you solder the 300k resistance across the two pins that he showed.
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Old 08-30-2010, 02:11 PM
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Pop Evans
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You are absolutely correct about cutting the pins.... I did reason that in the interest of simplicity to not do that. Since it was a slow degradation and additional change should be slow. Again... your method is the best. Another thought is to use an exacto knife and scrape the element off of the substraite.
Regards Pop
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Old 09-01-2010, 10:53 PM
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Stu 91 drop top
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Perfect. I have the schematic of the resistor chip if anyone is interested. I cut the pins off as well.

Stu.
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Old 09-02-2010, 08:05 AM
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ddahlgren
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Do you think that chip has all the resistors to calibrate all the gauges. My tch is wrong by about 10% My oil temp gauge is a mile off as well unsure of oil pressure though it reads sensible as does coolant temp and volts.. wondering if changing the resistor chip might fix a couple of problems at the same time. Oil temp sensor is new and reads correctly on scan tool.
Dave
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Old 09-04-2010, 11:20 AM
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Stu 91 drop top
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I'm not sure, but I think the chip is only for the tach. I say this because it is the same sort of chip on similar age camaros, but on the camaros it is on a seperate tach only ciruit board.
Stu
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Old 09-04-2010, 02:46 PM
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On the 91 corvette application a 270k ohm resistor was used. It now reads quite close. 3000 RPM at an actual 3200. When 330K was tried the dash read 2000 at an actual 1700 RPM. That was too high. The actual resistor you need may be different from this one. 300 K ohm as metioned earlier is likely correct. The resistors you buy may have a 5 % or 10% tolerance so keep that in mind. Remember the dash tach is not usually very accurate. (More resistance makes the tach read higher, less resistance makes the tach read lower).

On the table only board 6 is from a corvette. The others are camaros - pins 4 and 10 are the pins in question
Table 1 all readings in Ohms


[IMG][/IMG]

Last edited by Stu 91 drop top; 09-04-2010 at 05:11 PM. Reason: pictures not showing up
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Old 10-18-2010, 07:59 AM
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I did this repair yesterday to my '90 and it worked flawlessly! Thanks guys for your write up, it was easy to follow. I did as Jim85IROC suggested and cut the 2 legs off of the chip and soldered 3 - 100K Ohm resistors in series. The whole operation took about 3 hours. It is the first time since I bought my Vette 5 years ago that the tach has worked correctly. Before, the tach would peg-out at idle.
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Old 10-18-2010, 01:09 PM
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Originally Posted by trulytex View Post
I did this repair yesterday to my '90 and it worked flawlessly! Thanks guys for your write up, it was easy to follow. I did as Jim85IROC suggested and cut the 2 legs off of the chip and soldered 3 - 100K Ohm resistors in series. The whole operation took about 3 hours. It is the first time since I bought my Vette 5 years ago that the tach has worked correctly. Before, the tach would peg-out at idle.
According to the diagram by Stu, it should be 445K. Why did you use 300?
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Old 10-19-2010, 01:11 AM
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Originally Posted by aklim View Post
According to the diagram by Stu, it should be 445K. Why did you use 300?
I was going by Jim85IROC's instructions and he said that he determined that 300K Ohms was the correct value, so that it what I used and now my tach reads dead-on (I checked it against the computer plugged into the OBD port). As to the table saying that the value is 445K, I can't explain that....
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Old 10-20-2010, 08:50 AM
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To get R5 don't you need to measure between pins 8 and 9 not 7 and 9 as that should give you R5+R6?
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Old 11-01-2010, 12:14 PM
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I did this mod on my 90 over the weekend. I cut the pins and install the 445 K ohm and installed. The tach was still reading high; I replaced the 445 with a 300 ohm resistor and now the tach reads correct. I dont know if maybe its a year thing or what but 300 worked best for me too.
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Old 12-18-2011, 10:23 AM
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Default High Tach

You guys are the best. I agree. It took me about 2-3 hours to R&R on my 1990. Cost $3.95 at Radioshack. Saved $300 compaired to rebuild. Tach reads with in 150-200rpm of my Snap-on timming light tach. No more lugging the engine because the radio is on. Thank You TY ...
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Old 12-18-2011, 11:37 AM
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I did this repair 14 months ago and still works perfectly. A welcomed write up and must-do mod for us 1990 Corvette Owners. A Million Thanks to Jim85IROC & Pop Evans for their research & time!
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Old 12-30-2011, 03:21 PM
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Streamline
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Trying to fix the tachometer in my 1990.

Anybody know what watt 300K ohms resistor will work for the fix???
I have 1/4, 1/2 and 1 watt.

Thanks
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