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High Reading Tach Repair

 
Old 12-30-2011, 04:35 PM
  #21  
Pop Evans
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I used a 1/2 watt since it was the first that I picked up out of the box.
Regards Pop
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Old 12-30-2011, 06:36 PM
  #22  
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Originally Posted by Pop Evans View Post
I used a 1/2 watt since it was the first that I picked up out of the box.
Regards Pop

Thank you!
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Old 12-31-2011, 08:15 AM
  #23  
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On my 90 I used 3-100k 1/4 watt (Radioshack P#271-1347). Tach reads +/- 100rpm according to my Snap-on T. light/tach. Thanks again to the guys that first posted the info. It takes a leap of faith to cut the legs on the IC chip. But they're right it work fine.
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Old 12-31-2011, 10:19 AM
  #24  
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Originally Posted by Streamline View Post
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
For the record, the 1/4 watt will not be a problem either.

But, if you're a glutton for punishment and want resistor wattage for future, some Ohm's law for ya:

P (watts) = voltage squared / resistance (ohms)

So, in this case -

Watts = 12^2/300,000 or 144/300,000 = 0.00048 watts "Worst case"

For those that may have access to an assortment of color coded resistors, but don't know the code, here's a link to help.

P.
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Old 01-25-2012, 12:33 PM
  #25  
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Default High Tach Reading

Thanks for the information, it was very easy to do.
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Old 09-09-2012, 06:30 PM
  #26  
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I just tried adding the 300 ohm fix to my '91, but I didn't cut the old pins because I had no reading across them, so I assumed they were already open. But I cracked the glass LED screen trying to pry the red connector off the top of it to remove the circuit board. When I put it all back together, the speedometer didn't show the usual 0mph (of course), and the tach stays a 0 when the engine is running. Is this because the speedo screen isn't working now? I rechecked the tach resistance and it shows 300 ohms across the #4 and #10 legs of the IC.

UPDATE.... Noticed I used 300 ohms instead of the suggested 300K ohms, so I corrected that and it works fine now. Also did have to cut the 2 leads from the IC because there was some resistance showing on the 200K scale, which changed my 300K to 238K. But I found it difficult to plug that speedo LED screen back into the red connector again, what's the trick to this? Anyplace I can buy just the glass LCD unit?

Last edited by cimchazz1; 09-09-2012 at 09:24 PM. Reason: new info
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Old 07-20-2015, 02:04 PM
  #27  
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Fixed my '91 tachometer according to the 1st post. I bought a 500 kOhm potentiometer, cut the two legs, soldered two wires to the potentiometer and the other ends to the suggested points on the circuit board.



Then I adjusted the potentiometer to even 300 kOhm, but that was just a tad off when compared to the scanner readings, so I had to adjust it slightly to get the tachometer readings 'spot on' according to the scanner.

I adjusted it when driving on cruise control, since on the highway there's some load on the engine and the revs don't change significantly. I checked the readings on every 500 rpm from 1 000 rpm up to 5 000 rpm, and on every point the scanner readings were in 100 rpm range from the tachometer readings, i.e. if tacho needle was on 4 000 rpm, the scanner showed 3 950 - 4 050 rpm

I still keep the potentiometer available, if its value should change in the near future.



Kudos to Pop Evans for starting this thread.
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Old 07-20-2015, 06:02 PM
  #28  
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Nice work!
I too was thinking of using a potentiometer to be able to find time. Did you notice any other meters changing reading?

Was it difficult to access? Took long time?

Cheers
Dagge

Originally Posted by Yorcci View Post
Fixed my '91 tachometer according to the 1st post. I bought a 500 kOhm potentiometer, cut the two legs, soldered two wires to the potentiometer and the other ends to the suggested points on the circuit board.



Then I adjusted the potentiometer to even 300 kOhm, but that was just a tad off when compared to the scanner readings, so I had to adjust it slightly to get the tachometer readings 'spot on' according to the scanner.

I adjusted it when driving on cruise control, since on the highway there's some load on the engine and the revs don't change significantly. I checked the readings on every 500 rpm from 1 000 rpm up to 5 000 rpm, and on every point the scanner readings were in 100 rpm range from the tachometer readings, i.e. if tacho needle was on 4 000 rpm, the scanner showed 3 950 - 4 050 rpm

I still keep the potentiometer available, if its value should change in the near future.



Kudos to Pop Evans for starting this thread.

Last edited by Dagge; 07-20-2015 at 06:04 PM. Reason: adding text
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Old 07-21-2015, 03:58 AM
  #29  
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Originally Posted by Dagge View Post
I too was thinking of using a potentiometer to be able to find time. Did you notice any other meters changing reading?
If you mean before-after fixing the tacho, then no.

The oil temp gauge seems to be off (showing quite a bit less than it should) when comparing with the scanner readings.

Originally Posted by Dagge View Post
Was it difficult to access? Took long time?
Depends... I have torn out the instrument cluster plenty of times, so it takes something like 20 minutes, when I have the IC in my hands. For the first time it took over an hour, maybe an hour and a half.



Then the dissassembling of the IC takes some time. There are many Youtube -videos showing 'how to'.
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Old 07-21-2015, 06:45 PM
  #30  
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Hi again,

Thank you for your speedy reply!
I guess removing the IC is described in the FSM as well. If it's not worse than two hours I might give it a shot.
Did you notice any change in reading from any of the other gauges?
R u from Finland? Pratar du svenska?

Rgds
Dagge

Ps. Just read your full answer. Sorry for asking again.

Originally Posted by Yorcci View Post
If you mean before-after fixing the tacho, then no.

The oil temp gauge seems to be off (showing quite a bit less than it should) when comparing with the scanner readings.



Depends... I have torn out the instrument cluster plenty of times, so it takes something like 20 minutes, when I have the IC in my hands. For the first time it took over an hour, maybe an hour and a half.



Then the dissassembling of the IC takes some time. There are many Youtube -videos showing 'how to'.

Last edited by Dagge; 07-21-2015 at 06:47 PM. Reason: adding text
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Old 07-23-2015, 04:00 AM
  #31  
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Originally Posted by Dagge View Post
R u from Finland?
Yes, sir!

Originally Posted by Dagge View Post
Pratar du svenska?
Lite, förstås, det är vårt andra officiella språk

Men det är svårtare än engelska, eftersom mitt ordförråd är begränsat och det behövs så sällan. Också, jag har inte studerat det efter 1989 när jag tog studentexamen.
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Old 06-30-2016, 02:33 PM
  #32  
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Default 1990c4hightac

I would like to thank pop evans and cimchazzi for info on this repair. I have some notes to help some dummies like me! I cut the element below the solder points recommended by pop evans to avoid taking the board off. This removed another circuit needed for the tach. Maybe leaving the element connecting pin 10 alone would be ok. I installed a jumper to reconnect it. I also used a 300 ohm resistor, instead of a 300K resistor by mistake, Duh... My tach now works perfect and I can take my instrument cluster out in 15 minutes! Thanks again.
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Old 07-28-2016, 03:43 PM
  #33  
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Default thanks pop

Hey Pop! Awesome repair, did it yesterday. Been waiting to do it, and said "what the heck, It's time." Easy, works great. Had to buy 10 of the 300k ohm 1 Watt flameproof resistors , so I got 9 left. If anyone wants one, send me a text @ [email protected], and i'll send you one free. My way of paying it forward. Thanks again.
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Old 07-29-2016, 08:24 AM
  #34  
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Default Just to update:

I clipped the leads of the IC module, rather than solder a resistor across the existing (IC) value.

My reasoning was, that the printed resistor on the resistor board was deteriorating, and would continue to do so w/ time, heat, etc. And, eventually, the tach error would reappear at some point.

However, by clipping the leads (effectively removing the printed resistor from the circuit) and installing a discrete resistor of ample wattage, my thought was the fix would be more stable than the printed resistor (in parallel w/ the fix resistor).

Apparently, some 4 years hence, according to comparisons between my scanner and the analog tach, the fix is still "spot-on" - as expected it would be. So, again, kudos to "Pop" for this fix!

Last edited by Paul Workman; 07-29-2016 at 08:26 AM.
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Old 06-06-2019, 09:52 PM
  #35  
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So do i need to test what my tach is reading or can I just pop a 300kohm resistor on my board ? My value at 65 mph is extremely close to 6000 rpm ? Also if someone can provide me with a picture of what to cut that would be excellent thank you

Last edited by Tvette99; 06-06-2019 at 10:11 PM. Reason: Typo
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Old 06-06-2019, 10:20 PM
  #36  
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Originally Posted by Jim85IROC View Post
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.
Can someone provide me with these pictures they do not show up while im looking at this thread 😰
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Old 06-06-2019, 11:45 PM
  #37  
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Last edited by cimchazz1; 06-06-2019 at 11:49 PM. Reason: responded to old wrong post
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Old 06-06-2019, 11:46 PM
  #38  
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Old 06-07-2019, 08:10 AM
  #39  
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Here is the IC (resistor board) you are dealing with. You can cut the #4 and #10 leg and solder the resistor to the two points on the PC board as shown in the previous post.

However, BATEE now provided a replacement IC which eliminates the whole resistor bit. If you can use solder wick and can solder, you can remove and pop in the new part in just a couple minutes (once you access the board, of course!).

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Old 06-08-2019, 12:13 AM
  #40  
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Originally Posted by Paul Workman View Post
Here is the IC (resistor board) you are dealing with. You can cut the #4 and #10 leg and solder the resistor to the two points on the PC board as shown in the previous post.

However, BATEE now provided a replacement IC which eliminates the whole resistor bit. If you can use solder wick and can solder, you can remove and pop in the new part in just a couple minutes (once you access the board, of course!).

Thanks ive never soldered anything in my life ive brazed thousands of times as a hvac tech if i ruin the board are they available ? And if so what are they called how do i order one. Can someone else chime in aswell i am very hesitant to cut things being i literally just bought this car. Its for my wife i always told her id buy her a vette.... Also im in canada so whatever you pay times that by ten its ******* great living up north

Last edited by Tvette99; 06-08-2019 at 12:17 AM.
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