Installed new coil, now the tach acts funny
- Thread starter Righty Tighty
- Start date
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TheDuke
Member
I have the exact same problem you’re having with my tach. I switched coils and had horrible issues with my tach directly after. Everything seemed to read right but I got erratic readings after 2000 rpm. I’m no expert but I did solve the problem.
I saw a video auto meter put out about needing to put a 10k ohm 0.5W resistor in the wire that connects to the negative side of the coil. I did this and it immediately made my tach work perfectly!
NOTE! The 10k resistor is not the be all and end all of solutions. I think different resistors should be used for different coils of different power capabilities. I tried to hook up a Blaster 2 coil to this system and it only worked up to 2500rpm and read erratically. A different resistor needs to be used.
EDIT: Okay, so the reason the 10k ohm resistor worked for me was, the secondary resistance for my coil is 10k ohm. I found this out by visiting Jegs.com, locating my coil and finding what the “secondary resistance” rating was on the product description was.
I saw a video auto meter put out about needing to put a 10k ohm 0.5W resistor in the wire that connects to the negative side of the coil. I did this and it immediately made my tach work perfectly!
NOTE! The 10k resistor is not the be all and end all of solutions. I think different resistors should be used for different coils of different power capabilities. I tried to hook up a Blaster 2 coil to this system and it only worked up to 2500rpm and read erratically. A different resistor needs to be used.
EDIT: Okay, so the reason the 10k ohm resistor worked for me was, the secondary resistance for my coil is 10k ohm. I found this out by visiting Jegs.com, locating my coil and finding what the “secondary resistance” rating was on the product description was.
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TheDuke
Member
I don’t know either but it seems to work.
Did you accidentally bump the 4 6 8
Cylinder selector on the tach ?
Cylinder selector on the tach ?
Dave999
Well-Known Member
if you have exhausted all possibilities
try undoing cleaning up and redoing the ground strap between motor and chassis
one sure fire way to make all of your instruments go weird is to run the car with no earth strap or one so corroded it doesn't connect anymore
motor then earths through throttle cable, speedo cable into the dash if the cable tube it uses isn't nylon lined .
through the coolant and radiator
none of the devices that use the ground at the engine can see a "true" ground. so they can not measure anything against true ground, their ground varies with every spark.
and depending on if your ignition switches the coil off and on by grounding or by pushing the negative side to the same 13+V as the positive the ignition module may know what ture ground is but that's worthless if the engine isn't at true ground, really depends what the ignition is bolted to.
temperature reads wrong
oil pressure reads wrong
tach needle just buzzes at some random level
you appear to have either way more or way less (depending on year) fuel
car runs kinda ok but is hard to start.
often easier to start if you press the throttle which puts tension in the throttle cable making for a better ground connection between it an the metal ferrules on the throttle cable clamp and the bulkhead clip.
probably wrong but you never know...
Dave
try undoing cleaning up and redoing the ground strap between motor and chassis
one sure fire way to make all of your instruments go weird is to run the car with no earth strap or one so corroded it doesn't connect anymore
motor then earths through throttle cable, speedo cable into the dash if the cable tube it uses isn't nylon lined .
through the coolant and radiator
none of the devices that use the ground at the engine can see a "true" ground. so they can not measure anything against true ground, their ground varies with every spark.
and depending on if your ignition switches the coil off and on by grounding or by pushing the negative side to the same 13+V as the positive the ignition module may know what ture ground is but that's worthless if the engine isn't at true ground, really depends what the ignition is bolted to.
temperature reads wrong
oil pressure reads wrong
tach needle just buzzes at some random level
you appear to have either way more or way less (depending on year) fuel
car runs kinda ok but is hard to start.
often easier to start if you press the throttle which puts tension in the throttle cable making for a better ground connection between it an the metal ferrules on the throttle cable clamp and the bulkhead clip.
probably wrong but you never know...
Dave
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The GM HEI had to have a resistor of sorts in the distributor tach lead for the factory tachs to read correctly. You can still get the GM part number. I had to get one for my Ford truck when I installed the Skip White GM style big cap HEI. The tach was pretty erratic without it. Sounds like something similar is going on here.
Dave999
Well-Known Member
anything that keeps to back emf in the 300-400V range protects that tacho
GM used a condenser and a resistor i think. plug in loom on a plastic bracket with a radio suppression cap
jaguar and Lucas used a zener diode as per the original motorola product sheet circuit diagram for the original stand alone ignition chip that the module is based on.
both achieved the outcome of limiting what voltage was seen on the primary after the spark was triggered.
GM used a condenser and a resistor i think. plug in loom on a plastic bracket with a radio suppression cap
jaguar and Lucas used a zener diode as per the original motorola product sheet circuit diagram for the original stand alone ignition chip that the module is based on.
both achieved the outcome of limiting what voltage was seen on the primary after the spark was triggered.
KitCarlson
Well-Known Member
The 10k resistor attenuates tach signal, such that secondary spark triggers are reduced or eliminated. A spark event has initial high voltage peak, followed by decaying oscillating peaks. A tach should just read first peak. Additional peaks add to indicated RPM.
Viewing coil primary with scope and appropriate voltage rated probe may reveal other issues such as plug cables. Each cylinder fire should have similar signal.
Viewing coil primary with scope and appropriate voltage rated probe may reveal other issues such as plug cables. Each cylinder fire should have similar signal.
67Dart273
Well-Known Member
Man Dave, good to see you!!! Another thing I was thinking is that IF he does not have a good radio suppression cap on the coil + that might be an idea. It will help keep the coil power from spiking due to coil feedback.
KitCarlson
Well-Known Member
Del,
Good to hear from you. Radio suppression cap is good idea.
I have used a tach buffer cct that I got from Peter Serio, a fellow in the US that repairs car instruments & gauges & knows his electronics. I have used it with HEI dists that are used with cars that originally had points. It is a bit more than a resistor....but easy to consruct for a few bucks. It has 4 res, a diode, transistor & a cap.
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