dimented24x7

iTrader: (2)

Edit: See Part 2 below for the standard repair method for tachs that are non-functional or way off. The first part of the thread is good for quickly trimming tachs that are off less than 30% or is a V6 tach that needs to be recalibrated for a V8 car.

I have a spare tach for my '88 that I want to calibrate, but there doesn't seem to be much info on how to fix these.

I've seen some people saying to replace the 68 nF mylar cap, but having a look Im not so sure. On my meter the 6.8 nF and 68 nF both come out to roughly what they should be, and they fall well within the 10% tolerance on the cap.

When I look at the tach PCB on the input side I see a cermic IC with two resistors in it. One without a laser etch (~ 5kOhm) is for the zener diode voltage clamp for the coil input, and the other with the laser etch, presumably for balancing/calibrating the tach (reading 255 kOhm).

What usually causes these to be off? The resistor on the ceramic substraight?

dimented24x7

iTrader: (2)

Ok so taking a closer look, it looks like this is sort of a balanced bridge type meter. The 255 kOhm etched resistor zero's the meter against another resistor by the meter, so that's good to go since they both measure the same. The two mylar caps are also good, and look to be just DC blocking caps. The last cap is a tantalum one that conditions the incoming 12V, and that's reading what looks to be printed on the cap as well.

The IC itself looks to just be an amplifier for the bridge, with some form of driver circuit that converts the frequency to a voltage to drive the meter movement.

The tach is reading exactly 1.33 x what it should be, so I don't know if its a V6 tach, or a broken V8 one. The gauge cluster was for a V8 car, but the history is unknown. So, any ideas on what to check/change? Im thinking it might be a jumper on the ceramic chip that will change the gain of the IC?

dimented24x7

iTrader: (2)

Success! I found the divider network that sets the gain on the ceramic chip. Paralleling one of the resistances (two pins on the ceramic chip) changes the output. 0 ohms = nothing for all frequencies, increasing resistance scales the RPM output. Not linear since its parallel to unknown network, but its probably good enough to fix it.

dimented24x7

iTrader: (2)

I recalibrated the tach with a function generator. I was able to get it to within +/- 3% of what it should read across the range (0 - 7000 RPM), which is a lot better than the random RPM it had before. Plus side is that this is easy to do. Minus is that you need a function generator or something that can output a square-wave of known frequency.

Here's how to recalibrate it: On the back of the tach (picture), there are two pins circled. The pins are on the ceramic chip with the two film resistors. On the other pins there are additional resistors. One of these is one side of the divider network (~ 7 kOhms) that sets the multiplier for converting the tach signal to RPMs. Less resistance = lower multiplier for the RPM scale.

To see what resistance I needed, I soldered two alligator clips to a 10 kOhm potentiometer with a 10 kOhm resistor in series to boost the resistance. Next I connected each clip to one of the circled pins, taking care that it wasn't shorting to anything. I then set the function generator for 400 Hz (6000 RPM), and adjusted the pot until the tach read 6000 RPM. Then I measured the resistance (on mine it was 12.8 kOhms), and soldered the equivalent fixed resistors to the two pins to finish the calibration.

As for how to hook up the tach, the PCB on the tach has all the terminals labeled, so its pretty clear what needs to go where. +B, battery, ground, and Coil (+ signal to coil with - signal going to ground). On the generator, make sure you have at least 10V or so of amplitude on the waveform, and use a square wave. The voltage needs to be above the voltage that the voltage clamp kicks in at. If not, the tach readings will be off.

One word of caution: The IC in the tach is a high gain amplifier, like VERY HIGH GAIN. The leads on the pot (and fixed resistors) must be kept as short as possible, and nothing including your fingers should be touching the pot when your taking a reading. If the leads are too long they pick up noise and the needle flies all over the place. Same goes for touching anything on the tach, the needle shoots to one extreme or the other. Its really touchy (literally). Also make sure your power supply can supply at least 1 amp at 12V as the oil pressure gauge draws a lot of current.

Quick update: I looked at the other thread, and both these and the 90+ use the same circuit. What I did was parallel the divider network on pins 7 and 9, which seems to do the trick of changing the scalar for the RPMs. See post below for how this works.

dimented24x7

iTrader: (2)

Mods: Consider maybe making this a sticky? Also, could you please change the title to "Pre-90..." or something similar? I typed 89 by mistake.

dimented24x7

iTrader: (2)

I figured out how this works. Basically what I did was feed back the 8.5V reference voltage back to the non-inverting input of the Norton op-amp, which is the charge pump that is part of the conditioning and driving circuit. The onboard transistor is tied to pin 8 of the ceramic IC, and produces a clean square wave from the input that switches from 0V to 8.5V. By feeding some voltage back, the low voltage swing is reduced, which reduces the gain of the op amp. The advantage of this method is that it can be done with off the shelf resistors (I used an assortment from radio shack) and does not require any modification to the existing circuit.

The standard method is to trim the trim resistor R4, which requires either a large resistance (> 500 kOhms) or cutting pins on the ceramic package and adding a new resistor of fairly large value (~ 190 kOhms). This adjusts the feedback resistance on the inverting input of the Norton op-amp thereby restoring the tach.

Both the methods outlined here and in the 90-02 tach fix thread do the same thing, so take your pick as to which you want to use. I attached a pic of the equivalent circuit of what I have as well as pics of the finished calibration.

BigBadLou

iTrader: (3)

If the Rreg and R1 were replaced (bypassed) with the originally intended 3.3k and 4.7k resistors, would that yield an equal fix?

dimented24x7

iTrader: (2)

I think the resistor that is bad in all cases is R4, the trimmer resistor. This resistor is used to set whether it works as a V6 or V8 tach, and is set at the factory with a laser to what is needed, but after a while they all seem to read high. Both of my V8 tachs work as though they are for a V6 car, so I assume that trimming resistor by now is totally gone (infinite resistance).

Rreg (R6 on ceramic IC) and R1 (R5 on ceramic IC) are still good on my chip, but R4 is likely bad. What Im doing is feeding back some voltage from the Vreg output to the output of the switching transistor/pull up resistor (R5). This pulls the 0V of the switching waveform away from ground, and reduces the gain, thereby restoring the proper calibration. Its a round about way to fix it. The way in the 90-92 tach repair thread is to actually replace R4 with a new resistor to restore the tach to its original operation. This requires that pins be cut on the ceramic IC, which is not that easy to access on the older tachs. My method is to just solder a resistor on the back of the board.

Edit: Hopefully this doesn't muddy the waters too much, but I decided to stick with it since the board is harder to access on the older tachs.

Chevy86 IROC-Z

iTrader: (17)

I'm looking forward to repairing my tach too. By any chance, where is the circuit board that is the main focus?

Here is what my tach reads on my 86 IROC.

Chevy86 IROC-Z

iTrader: (17)

Nevermind dimented24x7. I just read that it's on the tach itself. So by following your formula, and by what you see in the video, what value of a resister should I solder in?

dimented24x7

iTrader: (2)

If you dont mind taking the board off the back of the tach, you can cut the pins on R4 as shown in the "90-92 Repair" thread and replace that resistor. Before cutting the leads, measure R4's resistance, and the new resistance needed is that for R4 x (500 RPM / 2700 RPM), which is the difference in the two readings at idle. It will be approximate, but it should be close enough. If you want to get closer, take the readings at several points in the RPM range and average out the calculation for R4. Yours is so far off that R4 is likely nearly an open circuit, so direct replacement is probably best.

If you want to use my method, temporarily solder a 100 kOhm potentiometer with wire leads (radioshack sells them) to the pins 7 and 9, and adjust until the RPMs are where they should be. Then desolder it and measure the resistance of the pot, taking care not to move it away from where you set it. Finally, solder an equivalent amount of resistance in the form of resistors (again radio shack should have an assortment of 5% tolerance 1/2 W reistors) to pins 7 and 9 and the tach should be good to go. As it looks like R4 is failing, over time if you dont replace it the tach may drift back out of calibration.

Chevy86 IROC-Z

iTrader: (17)

So what's the deal with pins 4 & 10 in the other thread?

dimented24x7

iTrader: (2)

Pins 4 and 10 are on that white ceramic chip that has resistors on it. Pins 4 and 10 contain R4, which is the feedback resistor for the Norton amplifier for the input signal. Resistor R4 sets the ratio for the RPM reading to the tach signal. The higher R4s resistance, the further the tach needle will swing for a given engine RPM. The norton op-amp is a low impedance current amplifier, and when R4 starts to fail, the amp looses feedback and the tach reads higher and higher. In the schematic of a typical circuit, R4 is circled. To fix R4 requires that you take the board off, cut the pins 4 and/or 10 for it, and solder a new resistor to the board in its place to fix the tach. Probably you will need something like 190-200 kOhm resistor to fix it.

I have another tach that needs fixing, so over the weekend I can take some pics of how to do it. Its a little more work than my method is when R4 is just off, but from your vid, it looks like its almost totally gone on yours. A norton needs some current feedback, so if R4 fails completely, it will go to 7k RPM and just sit there regardless.

Chevy86 IROC-Z

iTrader: (17)

Very good info man. Thank you for taking the time to explain. And yes. That would be great if you can take some pictures. So I relation to my tach reading, I would need 2 - 100 ohm pots to get an accurate reading correct when I do my testing correct?

dimented24x7

iTrader: (2)

The pots actually need to be 100,000 ohms since the feedback resistor R4 is very large (~192,000 ohms for V8, 256,000 ohms for V6). You will also need to solder an additional 150 kOhms of resistance to the pot to allow it to go from 150-250 kOhms for calibration.

Chevy86 IROC-Z

iTrader: (17)

dimented24x7

iTrader: (2)

Ok heres the standard way to repair these tachs when they have a failing resistor. The previous tach I had was possibly a V6 unit, but this is a V8 one.

To do this you will need:

• Solder iron
• 60/40 or RoHS lead free solder (60/40 preferred)
• De-solder wick or desolder iron
• Function generator (optional)
• 250 kOhm potentiometer (optional)
• Assortment of 1/4 watt resistors

If you dont have a function generator, the following resistor values can be used directly:

V6 tach = 256 kOhms
V8 tach = 192 kOhms

To properly do this, the PCB will need to be removed. Start by carefully unscrewing the spring nuts that secure the large ceramic resistor to the back of the oil pressure gauge. Then remove the three nuts on the movements for the oil pressure and tach (6 total). Once removed, the PCB can be separated from the tach face. If you have a steady hand, its a good idea to touch up the pads that the nuts seat on to make electrical contact to restore their condition (they will be gouged when you remove the nuts). To do it, put a small amount of solder on the tip of the iron and with one continuous circular motion, run the tip around the pad to reflow the solder.

Next pins 4 and 10 need to be desoldered and removed (refer to post #4 in this thread for circuit diagram of ceramic IC that will be repaired). Remove the solder from the pads to expose the pins, and cut them with a small pair of wire cutters. Next, remove the remainder of the pins on the ceramic resistor IC by carefully twisting them off with fine-point needle nose pliers. This will prevent them from contacting the leads for the replacement resistors. (see pic).

If you will be calibrating the tach with a function generator and potentiometer, the following steps are done, if not, skip to the next post. In place of pins 4 & 10, solder two 1" solid wire leads out the back of the tach. These will allow you to hook up the pot. Solder wires and two alligator clips to the pot so you can hook it to the tach (see pic).

dimented24x7

iTrader: (2)

With the pins removed (and leads connected if calibrating the tach), reassemble the tach. Replace the 6 nuts on the posts for the movements and then put the large resistor back and secure it with the remaining two nuts.

If calibrating the tach, solder two alligator clips to the pot and connect it to the flying leads. Power up the tach and set the generator for 400 Hz (300 Hz for V6 apps). Trim the pot until the tach reads 6000 RPM. Disconnect the pot, and measure its resistance. This will be the value you need. Remove the leads afterwards and desolder the pads. On mine, I needed 195 kOhms, but 200 kOhms was as close as I could get.

To finish the repair, solder the resistor or resistors that make up the resistance you need to the pads that pins 4 & 10 where located. Hook the tach up to the function generator (or put it back in the cluster) and verify it works. Finally, put some epoxy on the resistor to keep it in place (see pics)

Annihilate

iTrader: (1)

Soo, if I mailed my tach out to you, how much would you charge me to fix it?

antman89iroc

Ditto Lol!!! Do you wanna go into business?

Now I'm not trying to shoot myself in the foot here but just yesterday I contacted Gentry at autoinstruments01@gmail.com and he will replace the "board" and calibrate our tachs for $175. If you would do a "repair" for a good price I am in for sure. Seriously man, there are a lot of us who would love to have a calibrated meter and there just aren't a lot of people offering service. Not that I have been able to find anyway.

SolarGoldRaptor

So most of the time, a 195-200k ohms resistor should do the trick on a V8 tack ?

I had made the repair last year to my '84 HO tach (8k rpm) and it just goes straight to 1800rpm, and revs to 2000rpm, no more :/ I'm not sure which resistor I put exactly now, I'm gonna have to take it apart again and make sure about the ohms.

Annihilate

iTrader: (1)

Lol, I know someone doing it for $50 But I am looking for something a wee bit cheaper. Lol.

Chevy86 IROC-Z

iTrader: (17)

Why don't you get a ~$10 multimeter, ~$10 soldering iron kit and a few resistors? Done deal. I'd say that's the most economic solution. Anyone can do it. Just don't get intimidated. My 9 year old son soldered a battery tray to a circuit board for a school project with my guidance. Of course we used gloves and lead free solder. But if my 9 year old can do it, so can you.

Annihilate

iTrader: (1)

I have a soldering iron and a multimeter. Thats not the issue. I just need a REALLY good picture filled guide. And links to the right resistors and so on and so fourth,

antman89iroc

Geez so you're gonna shame me into doing it. ~ Er uh oh I mean thanks for the encouragement.

Yeah I have attempted the repair in the past and it did seem to help- for a while anyway. But that was before this thread so maybe I will attempt it again.

The other thing I was hoping to achieve with a "professional" repair is to update the electronic to something more responsive. It seems to me the stock tach is a bit sluggish and other tachs like Autometers are a lot quicker to respond. I mean it's not like I'm racing everyday or anything but it would be nice to have a better gauge.

And who will do it for $50? I have done everything on my car from front to back top to bottom myself as long as I have the tools I generally do it. But for some reason this one has been tough for me. Ok ok I will give it another try. Especially if a 9 year old can lol. Hey wait, maybe that's the problem. I'm too damn old!

Chevy86 IROC-Z

iTrader: (17)

Well my son didn't do nothing tough. All he did was solder some lead wires from the battery box terminals to some wire. Nothing special. But it really up to you man. Sorry if I made you feel degraded. Wasn't my intention.

antman89iroc

Lol all in good fun, no worries.

Chevy86 IROC-Z

iTrader: (17)

antman89iroc

Ok Dimented, I just completed this repair and it worked perfectly. I just took your value of ~192k ohm. I used 2 100k in series, checked a few to find ones low in the tolerance so I ended up at 198k. Just soldered them in place and it is verrry close to my datalog. I think the tach is more accurate than it's ever been. Man I appreciate you taking the time to show your work. Saved me a few $$ and feel good about doing it. A+!

midias

Please forgive me if I missed this somewhere in this thread. Here are the freq values that should correlate to a specific RPM for a V8 tach. I found them nice to have as I swept the range to check the tach

Scamo-2.8

iTrader: (16)

Is any of this thread in English? lol STICKY material for sure!!!

gcj280

iTrader: (15)

I repair tachs for 82-89 camaros for $69 including shipping. I would like your old one as a core if possible. I replace the circuit board with a new one and then calibrate it with a signal generator. Lifetime warranty on the circuit boards. I sell the circuit boards separately too.
https://www.ebay.com/itm/142495943416
Let me know if anyone is interested.

Thanks, Greg
~ Cajun Tach Shop

Annihilate

iTrader: (1)

I am very interested. And would love to have mine working properly again.
PM me please.

larry88ttops

Greg - I am also interested in one of your refurbished tach's. Mine started acting stupid a week or so ago and now it's just dead. I have a 1988 305 TBI 5 spd car. I can get you the redline on the tach if you need it. Let me know what the next step would be on this. Thanks.

Larry

gcj280

iTrader: (15)

I repair these tachs for $65 or I can send you a brand new circuit board (pre-calibrated) for $50.

Thanks, Greg

dmm3369

I spoke with you back in 2017 about fixing a tach then I had major shoulder surgery and was down for a year with car playing. I am back at it again. Do you have anymore of these boards for 50.00 pre calibrated for a 89? I will send you the money today with paypal. I just need the board. Mine works reading high.

gcj280

iTrader: (15)

I have a few ready to go. paypal is: hottbubb4@yahoo.com
Please include your full name and mailing address.

thanks! Greg

dmm3369

I got it off ebay from you. David Martin

gordfc

Do you ship to canada?

gcj280

iTrader: (15)

Sorry, I just saw this. I can ship to canada, but shipping will be on the buyer.

Thanks! Greg

chip5409

Old 07-16-2013, 01:34 AM
#3
@dimented24x7
I know this is an old thread but we just completed the fix you gave us and it worked flawlessly! What a genius. Thank you

antman89iroc

Third gen is awesome-huh?