1991 Z tach reads wrong
Thread Starter
Junior Member
Joined: Apr 2010
Posts: 22
Likes: 0
1991 Z tach reads wrong
excuse me if this has been covered before but I'm on a very slow internet connection and I don't have the time to search in depth.
if this has been covered already please link my to the thread. thanks.
now on to my problem.I have a 1991 Z-28 305 tpi. my tach started reading 2500 rpm at idle.
so I bought a used cluster from a 1990 RS 305 tbi car but it pegs out.
what confuses me is the part numbers are an exact match for both clusters.
I took the clusters apart and pluged in the tach circuit board from the old set in the new set and it reads the same.
so i know the guage itself isn't bad. the only difference between the 2 boards is the old one has a giant red resistor and the newer one has a smaller yellow resistor.
my question is, what would cause the tach to read so far off and can it be corrected?
(I cannot post pictures on this internet connection)
if this has been covered already please link my to the thread. thanks.
now on to my problem.I have a 1991 Z-28 305 tpi. my tach started reading 2500 rpm at idle.
so I bought a used cluster from a 1990 RS 305 tbi car but it pegs out.
what confuses me is the part numbers are an exact match for both clusters.
I took the clusters apart and pluged in the tach circuit board from the old set in the new set and it reads the same.
so i know the guage itself isn't bad. the only difference between the 2 boards is the old one has a giant red resistor and the newer one has a smaller yellow resistor.
my question is, what would cause the tach to read so far off and can it be corrected?
(I cannot post pictures on this internet connection)
Supreme Member
Joined: Jan 2006
Posts: 3,341
Likes: 10
From: Mooresville NC
Car: LOWERED ♦ CRIMSON METALFLAKE
Engine: ► 400 KUBES ◄
Transmission: 765R4
Axle/Gears: EATON POSI 4.56
RPM Mayhem
The used one might have gone out of calibration also.
There is circuitry in the dash near the tach.
It’s contained on a small plug-in PCB or integrated in the main board.
On the PCB you’ll see a resistor chip.
Desolder pins 4 & 10.
Use a 500K potentiometer.
Solder it across traces 4 & 10.
Move the potentiometer to half way.
Start the engine and compare the tach reading with a test equipment quality tach.
Dial it in, unsolder the potentiometer, measure it and install a matching resistor.
◙◙◙◙◙◙◙◙◙◙◙◙◙◙◙
Happy Racing!
◙◙◙◙◙◙◙◙◙◙◙◙◙◙◙
I Hit The Water Barrels Because My Car Was Dirty
.
There is circuitry in the dash near the tach.
It’s contained on a small plug-in PCB or integrated in the main board.
On the PCB you’ll see a resistor chip.
Desolder pins 4 & 10.
Use a 500K potentiometer.
Solder it across traces 4 & 10.
Move the potentiometer to half way.
Start the engine and compare the tach reading with a test equipment quality tach.
Dial it in, unsolder the potentiometer, measure it and install a matching resistor.
◙◙◙◙◙◙◙◙◙◙◙◙◙◙◙
Happy Racing!
◙◙◙◙◙◙◙◙◙◙◙◙◙◙◙
I Hit The Water Barrels Because My Car Was Dirty
.
Thread Starter
Junior Member
Joined: Apr 2010
Posts: 22
Likes: 0
Re: 1991 Z tach reads wrong
hey thanks NINA.
I found the thread about putting the 1 megaohm potentiometer on the circuit for the tach and it worked for me.
I set it to the correct RPM via a engine analyzer.
now she's all back to normal.
I left the poteniometer in and wrapped it in electrical tape so it doesn't touch anything and short.
my other problem I need to find is why the speedo doesn't read for a few miles then it does.
I figured the old one was going bad but the newer guages do it too.
I guess you'll start having these problems on a car with 240k miles.
that's no typo. it's been the best car I've ever had.
I found the thread about putting the 1 megaohm potentiometer on the circuit for the tach and it worked for me.
I set it to the correct RPM via a engine analyzer.
now she's all back to normal.
I left the poteniometer in and wrapped it in electrical tape so it doesn't touch anything and short.
my other problem I need to find is why the speedo doesn't read for a few miles then it does.
I figured the old one was going bad but the newer guages do it too.
I guess you'll start having these problems on a car with 240k miles.
that's no typo. it's been the best car I've ever had. Last edited by serpentracer; Apr 21, 2010 at 06:35 PM.
Supreme Member
Joined: Jan 2006
Posts: 3,341
Likes: 10
From: Mooresville NC
Car: LOWERED ♦ CRIMSON METALFLAKE
Engine: ► 400 KUBES ◄
Transmission: 765R4
Axle/Gears: EATON POSI 4.56
Dyno-Mite
The speedometer problem might be the VSS.
If you have cruise control, and it doesn’t function when the speedometer doesn’t function, it’s the VSS.
If you don’t have CC you can monitor the output of the VSS with an audio amplifier to make sure it’s putting out a steady signal.
Connect the amp’s input to the output of the VSS as you would a CD player.
Ground to ground, the signal wire to the inner wire of the audio amplifier wire.
When driving, or running on a dyno, you will hear a hum which increases in pitch when the wheels are turning faster, if the VSS is good.
◙◙◙◙◙◙◙◙◙◙◙◙◙◙◙
Happy Racing!
◙◙◙◙◙◙◙◙◙◙◙◙◙◙◙
I Hit The Water Barrels Because My Car Was Dirty
.
If you have cruise control, and it doesn’t function when the speedometer doesn’t function, it’s the VSS.
If you don’t have CC you can monitor the output of the VSS with an audio amplifier to make sure it’s putting out a steady signal.
Connect the amp’s input to the output of the VSS as you would a CD player.
Ground to ground, the signal wire to the inner wire of the audio amplifier wire.
When driving, or running on a dyno, you will hear a hum which increases in pitch when the wheels are turning faster, if the VSS is good.
◙◙◙◙◙◙◙◙◙◙◙◙◙◙◙
Happy Racing!
◙◙◙◙◙◙◙◙◙◙◙◙◙◙◙
I Hit The Water Barrels Because My Car Was Dirty
.
Thread
Thread Starter
Forum
Replies
Last Post