Tach reading is suspicious
Thread Starter
Intermediate
Joined: Aug 2025
Posts: 26
Likes: 11
From: Germany, Bielefeld
Tach reading is suspicious
Hi to all,
: C3, 350cui HEI-iginition.
while checking the ignition-timing, I realized a difference between the rpm-measuring shown on the strobe and dash-tach.
The difference is (depending on the rpms) from 0 to 1500 to low.. Meaning, th tach shows lower values than the stroblight.
This is dangerous because it may destroy the engine if you rely on the dash-tach.
What I did:
Disassembled the dash and brought tach to my lab.
Using a rpm-meter while the tach was driven by a sqarewave-generator, there was only a small diffence of about 200rpms compared to the needle.
I adjusted the tach to meet the frequeny generated by the squarewave-generator.
This are the current values (intermidiate values not shown):
Frequenzy 100 -> 1500 rpm
Frequency 200 -> 3000 rpm
Frequency 300 -> 4500 rpm
Frequency 400 -> 6000 rpm
So far, I think the tach is adjusted very well.
Of course, I may install it to the car and try to adjust the tach to meet the stroblight-rpms.
But:
In case there is something wrong at the distibutor or other components, the tach will be 'unaligned* if the root cause is eliminated. For example, replacing the distributor may now work fine, but then the tach shows a wrong value.
I have no documentation for the HEI.
What are your recommendataions for troubleshooting?
It's a Delco-Remy HEI.
The cable from distributor to tach straight forward, no resistors, nothing. Just a wire...
Thanks in advance
Flashjack
: C3, 350cui HEI-iginition.
while checking the ignition-timing, I realized a difference between the rpm-measuring shown on the strobe and dash-tach.
The difference is (depending on the rpms) from 0 to 1500 to low.. Meaning, th tach shows lower values than the stroblight.
This is dangerous because it may destroy the engine if you rely on the dash-tach.
What I did:
Disassembled the dash and brought tach to my lab.
Using a rpm-meter while the tach was driven by a sqarewave-generator, there was only a small diffence of about 200rpms compared to the needle.
I adjusted the tach to meet the frequeny generated by the squarewave-generator.
This are the current values (intermidiate values not shown):
Frequenzy 100 -> 1500 rpm
Frequency 200 -> 3000 rpm
Frequency 300 -> 4500 rpm
Frequency 400 -> 6000 rpm
So far, I think the tach is adjusted very well.
Of course, I may install it to the car and try to adjust the tach to meet the stroblight-rpms.
But:
In case there is something wrong at the distibutor or other components, the tach will be 'unaligned* if the root cause is eliminated. For example, replacing the distributor may now work fine, but then the tach shows a wrong value.
I have no documentation for the HEI.
What are your recommendataions for troubleshooting?
It's a Delco-Remy HEI.
The cable from distributor to tach straight forward, no resistors, nothing. Just a wire...
Thanks in advance
Flashjack
Race Director
Joined: May 2006
Posts: 18,655
Likes: 2,561
Hi to all,
: C3, 350cui HEI-iginition.
while checking the ignition-timing, I realized a difference between the rpm-measuring shown on the strobe and dash-tach.
The difference is (depending on the rpms) from 0 to 1500 to low.. Meaning, th tach shows lower values than the stroblight.
This is dangerous because it may destroy the engine if you rely on the dash-tach.
What I did:
Disassembled the dash and brought tach to my lab.
Using a rpm-meter while the tach was driven by a sqarewave-generator, there was only a small diffence of about 200rpms compared to the needle.
I adjusted the tach to meet the frequeny generated by the squarewave-generator.
This are the current values (intermidiate values not shown):
Frequenzy 100 -> 1500 rpm
Frequency 200 -> 3000 rpm
Frequency 300 -> 4500 rpm
Frequency 400 -> 6000 rpm
So far, I think the tach is adjusted very well.
Of course, I may install it to the car and try to adjust the tach to meet the stroblight-rpms.
But:
In case there is something wrong at the distibutor or other components, the tach will be 'unaligned* if the root cause is eliminated. For example, replacing the distributor may now work fine, but then the tach shows a wrong value.
I have no documentation for the HEI.
What are your recommendataions for troubleshooting?
It's a Delco-Remy HEI.
The cable from distributor to tach straight forward, no resistors, nothing. Just a wire...
Thanks in advance
Flashjack
: C3, 350cui HEI-iginition.
while checking the ignition-timing, I realized a difference between the rpm-measuring shown on the strobe and dash-tach.
The difference is (depending on the rpms) from 0 to 1500 to low.. Meaning, th tach shows lower values than the stroblight.
This is dangerous because it may destroy the engine if you rely on the dash-tach.
What I did:
Disassembled the dash and brought tach to my lab.
Using a rpm-meter while the tach was driven by a sqarewave-generator, there was only a small diffence of about 200rpms compared to the needle.
I adjusted the tach to meet the frequeny generated by the squarewave-generator.
This are the current values (intermidiate values not shown):
Frequenzy 100 -> 1500 rpm
Frequency 200 -> 3000 rpm
Frequency 300 -> 4500 rpm
Frequency 400 -> 6000 rpm
So far, I think the tach is adjusted very well.
Of course, I may install it to the car and try to adjust the tach to meet the stroblight-rpms.
But:
In case there is something wrong at the distibutor or other components, the tach will be 'unaligned* if the root cause is eliminated. For example, replacing the distributor may now work fine, but then the tach shows a wrong value.
I have no documentation for the HEI.
What are your recommendataions for troubleshooting?
It's a Delco-Remy HEI.
The cable from distributor to tach straight forward, no resistors, nothing. Just a wire...
Thanks in advance
Flashjack
Capacitor
Le Mans Master
Joined: Oct 2005
Posts: 7,257
Likes: 431
From: Texas Where God intended me to be.
2023 Corvette of the Year Finalist - Unmodified
2022 C5 of the Year Winner - Unmodified
Cruise-In X Veteran
St. Jude Donor '08 thru '11,'22 thru '26
Factory had a noise suppression capacitor bolted to intake.
Capacitor
Capacitor
Just be careful when buying one. I picked up one from a big Corvette supplier when I thought mine was bad. iI was made in China and instead of having a bracket to ground it it had a wire. That thing jumped worse than the original. Put the original one back on and made sure the ground was good and clean. worked like a champ.
