Tach way far out!!!!

[1963korvette]

Hey? My tach is sooo far out. When I'm at idle according to my snap on scanner it reads 675 rpm. The tach reads 2300. Aside from the obvious what other problems could this cause? I've read something about a rev limiter? Is this is real does it redad from the tach or the computer?
Thanks
Rich

[Paul Workman]

Hey? My tach is sooo far out. When I'm at idle according to my snap on scanner it reads 675 rpm. The tach reads 2300. Aside from the obvious what other problems could this cause? I've read something about a rev limiter? Is this is real does it redad from the tach or the computer?
Thanks
Rich

The rev limiter is independant of the tach, so there's no problem there.

As for DIY tach calibration, it isn't a big deal, if you can solder a few resistors. But, I must say an error of over 300%...is quite a bit! Usually the error is much lower. Mine was off 20+%, and I've heard of a couple a bit more (e.g, near 30%).

Usually the error increases over time; a function of deterioration of a printed resistor circuit. But, in your case, I wonder if the ground on the voltage divider isn't lifted (or the like).

I can dig up the calibration procedure, if you want to try it yourself. Who knows...the calibration process might just fix a lifted ground problem too.

P.

[1963korvette]

I'm game for any ideas. If you can find the fix I would appreciate it.
Thanks
Rich

[Paul Workman]

ZR-1 Tachometer Calibration

Special tools:

A small set of side cutters
A 40Watt solder iron (pencil type), flux core (22 gauge) solder
1/4" drive socket set and extensions
15mm socket and wrench
Phillips screwdriver
10mm socket


1.Disconnect the battery!

2.Remove the ALDL connector and then the carpet hush panel under the steering column (two screws on right side, accessible only with the door open, plus 3 along the upper length of the panel and 3 (or was it 4?) along the lower length of the panel.)

3.Remove the metal support panel; 2 10mm bolts on the left side, and two more on the right side, snuggled up against the console carpeting.

4.Drop the steering column by unscrewing the tilt lever and inserting the key and turning it vertical (as needed). Then remove the two 15mm bolts holding the support plate and steering column in place and lower the wheel to the drivers seat cushion. Note: Do not fool with the yellow airbag wiring attached to the steering column!

5.Remove the bezel: There are 4 screws holding it in place

6.Remove the cluster: There are (4) 7mm screws holding the cluster in place. Once they are out, you can wiggle the cluster free of the connector on the bottom left side of the cluster.

7.Remove the PC board: Remove only the screws necessary to remove the PC board (You don’t have to take the cluster apart, unless you also want to clean the lens, for example.) Be careful when lifting the PC board off of the gauges pin connectors – do it evenly so the PC board remains parallel to the cluster; e.g., don’t just lift it up from one end, in other words! Take care to avoid bending the pins on the digital read out panel.

8.Clip leads #4 and #10 on the resistor pack: Here’s where you can use the small electronics side cutter. Be careful not to cut the wrong pins. (For reference, you can look at the other side, locate the ends of the capacitor located above the resistor pack and trace those traces to the resistor pack. Those are the leads to be clipped.)

http://i185.photobucket.com/albums/x220/6PPC_bucket/tech%20files/cut4and10-2.jpg

9.Attach the potentiometer across C4: The resistance that will ultimately calibrate the tach varies. The top of the bell curve ranges between 140k to 220k (according to Marc Haibeck as I recall, but mine came in at 240k) Note: I suggest getting a 100k Ω pot and a couple 100k Ω resistors. In some combination you will be able to range between 100k Ω and 300k Ω. In the picture you can see two 100k Ω resistors in series, coupled with the pot for a total variable resistance range of 200k Ω to 300k Ω.

http://i185.photobucket.com/albums/x220/6PPC_bucket/tech%20files/1kpotwithpair100kresistors.jpg

The calibration jig attached across C4 on the "green side" of the PC board...

http://i185.photobucket.com/albums/x220/6PPC_bucket/tech%20files/greenside-potattachmentshown.jpg

10.Reinstall cluster: Reinstall the PC board to the cluster and reinstall the cluster into the dash, making sure the cluster connector has fully engaged the cluster. Let the pot hang on the wiring where you’ll have access to it for calibration adjustment.

11.Set calibration: Connect your scanner (e.g. to the ALDL connector), reconnect the battery, and start the motor. Compare the scanner to the tach and adjust the pot so the tach matches the scanner at about the 2000 or 3000 rpm range. Note: Sometimes the tach linearity is such that the tach may not track perfectly. That being the case, you can calibrate the tach to be accurate at the crucial (higher) rpm, but to do so you might have to reassemble the steering column etc in order to do a road test. However, according to those that have done this procedure some, the tracking error is not significant in most cases. And, because of lag – especially in the lower gears – a shift light is the way to get accurate shift rpm indication. It may be necessary to adjust the total resistance of the jig to get the tach into calibration.

12.Install the proper resistor in place of the pot: Disconnect the battery again. Remove cluster and the jig, taking care not to disturb the position of the pot stem, and read the resistance across the jig. Then select a resistor or combination of (series/parallel) resistors as needed to get as close as possible to the resistance measured across the jig and install the resistor(s) on the PC board, replacing the jig across C4. Use electrical tape to insulate the resistor wires from the PC board and arrange the resistor(s) to prevent the wires from touching anything else.

13.Reverse the disassembly procedure to reassemble the cluster, install it and return the dash and steering column to original configuration.

Credits go to Marc Haibeck with whom I consulted when I performed this procedure on my car.


Hope this helps!

P.

copyright 2012 All rights reserved

[ghlkal]

My tach is also way off (maybe not 2K at idle, but not close to being accurate)

I had read that ~300K ohms was needed. Paul's suggestion of a pot is a good one. Thanks for the procedure too, Paul

[VetteMed]

My tach is also way off (maybe not 2K at idle, but not close to being accurate)

I had read that ~300K ohms was needed. Paul's suggestion of a pot is a good one. Thanks for the procedure too, Paul

On my cluster, 300K was not correct. 200K was close, but 220K was about as close as I think I can get. I think 300K may be more correct for base C4s with the 6K tach.

[cvette98pacecar]

ZR-1 Tachometer Calibration

Special tools:

A small set of side cutters
A 40Watt solder iron (pencil type), flux core (22 gauge) solder
1/4" drive socket set and extensions
15mm socket and wrench
Phillips screwdriver
10mm socket


1.Disconnect the battery!

2.Remove the ALDL connector and then the carpet hush panel under the steering column (two screws on right side, accessible only with the door open, plus 3 along the upper length of the panel and 3 (or was it 4?) along the lower length of the panel.)

3.Remove the metal support panel; 2 10mm bolts on the left side, and two more on the right side, snuggled up against the console carpeting.

4.Drop the steering column by unscrewing the tilt lever and inserting the key and turning it vertical (as needed). Then remove the two 15mm bolts holding the support plate and steering column in place and lower the wheel to the drivers seat cushion. Note: Do not fool with the yellow airbag wiring attached to the steering column!

5.Remove the bezel: There are 4 screws holding it in place

6.Remove the cluster: There are (4) 7mm screws holding the cluster in place. Once they are out, you can wiggle the cluster free of the connector on the bottom left side of the cluster.

7.Remove the PC board: Remove only the screws necessary to remove the PC board (You don’t have to take the cluster apart, unless you also want to clean the lens, for example.) Be careful when lifting the PC board off of the gauges pin connectors – do it evenly so the PC board remains parallel to the cluster; e.g., don’t just lift it up from one end, in other words! Take care to avoid bending the pins on the digital read out panel.

8.Clip leads #4 and #10 on the resistor pack: Here’s where you can use the small electronics side cutter. Be careful not to cut the wrong pins. (For reference, you can look at the other side, locate the ends of the capacitor located above the resistor pack and trace those traces to the resistor pack. Those are the leads to be clipped.)

http://i185.photobucket.com/albums/x220/6PPC_bucket/tech%20files/cut4and10-2.jpg

9.Attach the potentiometer across C4: The resistance that will ultimately calibrate the tach varies. The top of the bell curve ranges between 140k to 220k (according to Marc Haibeck as I recall, but mine came in at 240k) Note: I suggest getting a 100k Ω pot and a couple 100k Ω resistors. In some combination you will be able to range between 100k Ω and 300k Ω. In the picture you can see two 100k Ω resistors in series, coupled with the pot for a total variable resistance range of 200k Ω to 300k Ω.

http://i185.photobucket.com/albums/x220/6PPC_bucket/tech%20files/1kpotwithpair100kresistors.jpg

The calibration jig attached across C4 on the "green side" of the PC board...

http://i185.photobucket.com/albums/x220/6PPC_bucket/tech%20files/greenside-potattachmentshown.jpg

10.Reinstall cluster: Reinstall the PC board to the cluster and reinstall the cluster into the dash, making sure the cluster connector has fully engaged the cluster. Let the pot hang on the wiring where you’ll have access to it for calibration adjustment.

11.Set calibration: Connect your scanner (e.g. to the ALDL connector), reconnect the battery, and start the motor. Compare the scanner to the tach and adjust the pot so the tach matches the scanner at about the 2000 or 3000 rpm range. Note: Sometimes the tach linearity is such that the tach may not track perfectly. That being the case, you can calibrate the tach to be accurate at the crucial (higher) rpm, but to do so you might have to reassemble the steering column etc in order to do a road test. However, according to those that have done this procedure some, the tracking error is not significant in most cases. And, because of lag – especially in the lower gears – a shift light is the way to get accurate shift rpm indication. It may be necessary to adjust the total resistance of the jig to get the tach into calibration.

12.Install the proper resistor in place of the pot: Disconnect the battery again. Remove cluster and the jig, taking care not to disturb the position of the pot stem, and read the resistance across the jig. Then select a resistor or combination of (series/parallel) resistors as needed to get as close as possible to the resistance measured across the jig and install the resistor(s) on the PC board, replacing the jig across C4. Use electrical tape to insulate the resistor wires from the PC board and arrange the resistor(s) to prevent the wires from touching anything else.

13.Reverse the disassembly procedure to reassemble the cluster, install it and return the dash and steering column to original configuration.

Hope this helps!

P.

copyright 2012 All rights reserved


When you get the resistors make sure they are plus or minus 1%. There are a lot of company's that sell resistors plus or minus 5%. There is an phone app "ElectroDriod that tells you the ohms and tolerance levels.

Nice Write up Paul.

[Paul Workman]

My tach is also way off (maybe not 2K at idle, but not close to being accurate)

I had read that ~300K ohms was needed. Paul's suggestion of a pot is a good one. Thanks for the procedure too, Paul

Yes, I studied that too, and determined the deteriorating resistor in the pack was not removed, thus left to continue to affect the calibration again as it continues to change; sorta like painting over rust!

Clipping the leads on the offending resistor pack removes at least that resistor from ever affecting the calibration again. As for the discrepancy between the 300k reference and the 140-220k (this procedure) is because resistors in parallel result in less resistance across them than the lowest value. The combination of the failing resistor and an arbitrary value like 300k Ω could result in exactly the value needed...or miss, depending on what the true value should be.

Rt = 1/((1/r1)+(1/r2)...) where (parallel) Resistance total can be determined for different variables of two or many parallel resistor values. Example: a 300k in parallel with another 300k would result = 1/((1/300)+(1/300)) = 150k R(total) This little formula and possibly adding resistors in series can be used to hone in on the exact value needed.
P.

[1963korvette]

Thanks Paul, I will attempt this operation this weekend.
Thanks
Rich

[RICHARD TILL]

Thanks from me also for this article as i have a 90 Z and a 91 tuned port and both tachometers are incorrect.

[ghlkal]

On my cluster, 300K was not correct. 200K was close, but 220K was about as close as I think I can get. I think 300K may be more correct for base C4s with the 6K tach.

VetteMed, I bet you are correct ... 300K for the 6K tach.

Paul, thanks again for the write up and follow up. After Bloomington, I'm pulling the cluster and fix this.

[Paul Workman]

Let us know how it works out. Yours is the largest error I've heard of, short of additional problems, e.g., a pegged tach...(don't ask!)

P.

[ghlkal]

My tach is also way off (maybe not 2K at idle, but not close to being accurate)


Just to follow up now that I can read the RPM from the HVAC display. Here's how far off my tach is:

tach reads / actual RPM

1000 / 650
3000 / 1750
4000 / 2325
4500 / 2700
5000 / 2975
6000 / 3500

This is up to a 70% error :eek:

[Paul Workman]

Just to follow up now that I can read the RPM from the HVAC display. Here's how far off my tach is:

tach reads / actual RPM

1000 / 650
3000 / 1750
4000 / 2325
4500 / 2700
5000 / 2975
6000 / 3500

This is up to a 70% error :eek:

Well, it sounds like waaaay more than the run of the mill drift of resistor values in the voltage divider: a connection on the board, or (I hope not) the dual op amp chip.

I'd try the cal approach first and see where we are after that.

P.

[XfireZ51]

Hard for me to believe that 3500rpm could be mistaken for 6k. I would double check using a scan tool to confirm rpms.

[ghlkal]

I would double check using a scan tool to confirm rpms.

Yup ... I really want to. The first tech 1 I purchased was defective and I'm on the lookout for another.


I'd try the cal approach first and see where we are after that.


Yes, hopefully soon, but I would rather have the scan tool to be sure first. And, Paul, I hope it's just the resistor too :neutral:

[ghlkal]

I just want to bring closure to my tach story …

I did replace the pot with 220K ohm worth of resistors. I used one 220K plus three 1K ohm resistors in series. Because they are +- 5%, the 223K stated value was actually 220K. The tach works great now!


For future reference, you might want to check out CCM diagnostics http://www.zr1.net/forum/showthread.php?t=17846 (http://www.zr1.net/forum/showthread.php?t=17846)

Using your HVAC as a tach
http://www.zr1.net/forum/showthread.php?t=17799 (http://www.zr1.net/forum/showthread.php?t=17799)

And, of course, check out -Solutions-
http://www.zr1.net/forum/showthread.php?t=16778 (http://www.zr1.net/forum/showthread.php?t=16778)

Thanks for everyone’s input!

Gary

[Dynomite]

Great write up Paul....but I do have a question ;)

I think Paul replaced the suspect resistor on the Dash Circuit Board (Resister across Capacitor C4) with a 240K Ohm resistor to get the Tachometer Calibration correct. Some say 220 K Ohm is the standard resistance in that chip (#4 to #10). It appears Paul was correcting a circuitry that did Drift with Time and the Drift in Resistance possibly do to deteriorating original chip resistance (#4 to #10) and capacitor deterioriation.

What was the resistor in the chip (#4 to #10) that came on the board in the first place? Was it 220 K Ohms?

It seems that one might get very close to a correct Tachometer reading just replacing that chip resistor (#4 to #10) with a resister of 220K Ohms in parallel with the Capacitor C4 (soldering on either side of the circuit board)?

ZR-1 Tachometer Calibration

http://i185.photobucket.com/albums/x220/6PPC_bucket/tech%20files/cut4and10-2.jpg
Hope this helps!

P.

copyright 2012 All rights reserved

It appears that Paul was correcting a deterioration error in the original Chip Resistor (chip #4 to #10) and Capacitor C4 deterioration and ghikal was correcting a major deterioration in the standard Chip Resistor (#4 to #10) only.

Just to follow up now that I can read the RPM from the HVAC display. Here's how far off my tach is:

tach reads / actual RPM

1000 / 650
3000 / 1750
4000 / 2325
4500 / 2700
5000 / 2975
6000 / 3500

This is up to a 70% error :eek:

[We Gone]

So Dynamite, did you add this to your solutions for quick access. Thanks Paul for the write up.

[Dynomite]

So Dynomite, did you add this to your solutions for quick access. Thanks Paul for the write up.

Absolutely....I added a Link to Paul's post (Post number 4 above) regarding the details for tachometer correction in the third post of -Solutions- shortly after Paul created the post. The Link was added in the third post of -Solutions- under the Heading KEYLESS ENTRY, TACHOMETER, CRUISE :thumbsup:

I ran into a major tachometer error recently over the whole range of tachometer read out on the order of ghlkal's error which ghlkal apparently corrected as Paul suggests by clipping OUT the original (#4 to #10) chip Resistor and replacing with 220K Ohms Resistor. The chip Resistance (#4 to #10) does in fact deteriorate over time as in ghlkal's case maybe not having anything to do with the associated C4 Capacitor :p

[Paul Workman]

*... The C4 resistor does in fact deteriorate over time as in ghikal's case and NOT the associated printed circuit :p

Nope. Just appears that way...

"C4" in this case designates "capacitor #4", and is used in this case to suppress unwanted voltage noise spikes. C4 (per se') is not a resistor at all.

I chose to place the replacement resistance across the leads of C4 only out of practical convenience (easy access on the circuit board). This is made possible due to the capacitor's leads are directly connected to the ends of the (failing) printed resistor (between pins #4 and #10 on the white "chip"), i.e., the ends of C4 are electrically the same as #s 4 & 10.

Two ways to "skin a cat" or calibrate the tach using a selected resistor(s):



By placing another select resistor in parallel with the failing resistor bringing the combination of resistance in compliance with that value needed to calibrate the tach.

By removing the failing resistor (clipping the #4 and #10 leads) from the circuit and substituting the proper value in its place.


Of the two, the second is the preferred due to the fact the deteriorating resistor will continue to do so, resulting in a temporary "fix". So, the better solution is eliminating the failing resistor entirely and replacing it with a more stable solution.

[Dynomite]

Nope. Just appears that way...

"C4" in this case designates "capacitor #4", and is used in this case to suppress unwanted voltage noise spikes. C4 (per se') is not a resistor at all.

I chose to place the replacement resistance across the leads of C4 only out of practical convenience (easy access on the circuit board). This is made possible due to the capacitor's leads are directly connected to the ends of the (failing) printed resistor (between pins #4 and #10 on the white "chip"), i.e., the ends of C4 are electrically the same as #s 4 & 10.

Two ways to "skin a cat" or calibrate the tach using a selected resistor(s):


By placing another select resistor in parallel with the failing resistor bringing the combination of resistance in compliance with that value needed to calibrate the tach.


By removing the failing resistor (clipping the #4 and #10 leads) from the circuit and substituting the proper value in its place.

Of the two, the second is the preferred due to the fact the deteriorating resistor will continue to do so, resulting in a temporary "fix". So, the better solution is eliminating the failing resistor entirely and replacing it with a more stable solution.

OK....thanks for that explanation....I will clean up my posts so as not to confuse folks ;)

I was thinking maybe the capacitor C4 was also failing........here I go again thinking that a possibly failing C4 Capacitor actually changed the resistance in the circuit and may continue to progress in failure without replacement. But.....that does not explain the addition of basically a new 220K Ohm Resistor as the only fix required in ghlkal's case :D

[Paul Workman]

OK....thanks for that explanation....I will clean up my posts so as not to confuse folks ;)

I was thinking maybe the capacitor C4 was also failing........here I go again thinking that a possibly failing C4 Capacitor actually changed the resistance in the circuit and may continue to progress in failure without replacement. But.....that does not explain the addition of basically a new 220K Ohm Resistor as the only fix required in ghlkal's case :D

Well, who knows zackly what failed in his case, except to say these particular printed resistors value tends to increase as it deteriorates, resulting in rpm values too high.

Electrolytic "caps" do fail too. But, when they fail, (typically) I find the DC resistance (respective of polarity) across them falls or even shorts out as opposed to resistance increasing - the exact opposite failure mode.

An unusually large error of 300% could be the result of the printed resistor perhaps opening or at least having become extremely high. In which case a 220 Ω across C4 might work quite well. Too many variables to say "this or that" is an absolute fix, I guess is the point. But, placing an arbitrary value resistor across C4 without eliminating the printed value (between pin 4 and 10) is iffy at best, and a temporary (Bandaid).

Example...

Given:

Rt (Resistance total) = 220k Ω = correct calibration
R1 = unknown (degraded) resistance value
R2 = the parallel value required to bring the tach into calibration = 300k Ω

Traditional formula to determine total resistance of parallel resistors:

Rt = 1/(1/R2 + 1/R1)

Solving for R1:

R1 = 1/(1/Rt - 1/R2)

inserting known values for Rt and R2:

R1 = 1/(1/220k - 1/300k)

R1 = 825 Ω

825 Ω?? Well, who knows what the actual value of the printed circuit degrades to for any given case without measuring it. Even then, the calibration value tends to fall within a range of several thousand ohms (welcome to analog!!). So, considering all the blanks, it is anybody's guess what parallel value will actually result in achieving reasonable calibration. Furthermore, the printed resistor just continues to degrade.

SO! Nuts to all that nonsense, sez me. I say just clip out the failing resistor, and good ridence. Then insert a (temporary) variable one and achieve calibration. Then measure the resulting variable resistance value and replace it with the proper fixed resistor of the same (or close) value (across C4) and call it good!:dancing

And, of course, the tach lags actual rpm changes; worst in the low gears especially. So, the tach is useful for troubleshooting, etc, when rpm is relatively constant. But for drag racing, for example, and especially in the lower gears where rpm values are changing more rapidly, a digitally triggered shift light is ever so much better than a tach anyway (to the point I don't even pay attention to the tach when rowing the gears while drag racing).

Paul.