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Tachometer jumps around after 4000 RPM


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Justin @
User

May 4, 2021, 8:13 AM

Post #1 of 21 (558 views)
  post locked   Tachometer jumps around after 4000 RPM  

My tachometer jumps wildly after 4000 RPMs. I’m looking for ideas to start tracing this problem down. The car is a 1980 camaro. Thanks.


Tom Greenleaf
Ultimate Carjunky / Moderator
Tom Greenleaf profile image

May 4, 2021, 9:38 AM

Post #2 of 21 (554 views)
  post locked   Re: Tachometer jumps around after 4000 RPM  

Hi Justin@,
Which engine do you have is going to help. If any 8 cylinder should be GM's HEI distributor - right? If so the impulse is from that can be checked right at it with a now obsolete "dwell-tach" not sure best other tricks to see it there.


Blow dust out of even those distributors. With that tool you can electrically see bearing wear of distributors drop dwell with RPM but only so much or needs a reman now or DIY that if you are up to it.
1st thing is which engine before we go on,


T



Justin @
User

May 4, 2021, 9:43 AM

Post #3 of 21 (552 views)
  post locked   Re: Tachometer jumps around after 4000 RPM  

Thanks Tom. The car is a 1980 Camaro V8. It has about 75k miles and is in like new condition all around. The motor was rebuilt from a previous owner. Not sure if that helps, but thanks for your thoughts.


Tom Greenleaf
Ultimate Carjunky / Moderator
Tom Greenleaf profile image

May 4, 2021, 10:08 AM

Post #4 of 21 (543 views)
  post locked   Re: Tachometer jumps around after 4000 RPM  

Engine rebuilt means different things to people. IMO if 75K is real why did it need to be done. Not the point let's get some info first.
Is it an OE kept car AND is it in fact HEI ignition?
Either way it has a distributor and any can have worn bearings to shaft. It was an issue or not this "Small Block Chevy" or other cars with tachs to dash wasn't as common as you go back into what was popular or not. Camaro would command it IMO so I'm pretty sure it gets the signal like for tuning it up, you could alter several adjustments and need to see RPM while working on it.
So I'm stuck on distributor shaft engine rebuilt or not that might have be kept would be good but low miles isn't the only issue or reason for them to wear. Quickly putting on miles was different than just stored a lot with periodic use or none for some years as it is a car of interest.
Look in back by firewall how many spark plug wires come off distributor cap. That will tell me what distributor you have,


Tom
This may help? See if distributor looks like this with or without that vacuum diaphragm...….




(This post was edited by Tom Greenleaf on May 4, 2021, 10:37 AM)


Justin @
User

May 4, 2021, 12:07 PM

Post #5 of 21 (524 views)
  post locked   Re: Tachometer jumps around after 4000 RPM  

The guy put in a cam and put it up to 400hp about 10 years ago. Five wires and a vacuum. One connector with a brown and orange wire and another connector with three wires, red, brown and black.


Justin @
User

May 4, 2021, 12:13 PM

Post #6 of 21 (518 views)
  post locked   Re: Tachometer jumps around after 4000 RPM  

I tried a picture of the distributor, but the system asks for my email and password. It says my email is not in the database. Maybe it takes overnight for that. Hmmmm.


Hammer Time
Ultimate Carjunky / Moderator
Hammer Time profile image

May 4, 2021, 12:59 PM

Post #7 of 21 (513 views)
  post locked   Re: Tachometer jumps around after 4000 RPM  

If you want to use the imaging feature you have to create a user name and password so other people can't mess with your pictures.



~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

We offer help in answering questions, clarifying things or giving advice but we are not a substitute for an on-site inspection by a professional.



Justin @
User

May 4, 2021, 1:14 PM

Post #8 of 21 (506 views)
  post locked   Re: Tachometer jumps around after 4000 RPM  

Hammertime, I set up a user name and password when I joined. I went to edit profile and my email address and user name are there. I updated my password there.

I press the upload photo and it takes me to a screen to login with my email and password. I try resetting my password from that screen and it gives me this error:


Password Retrieval
The email address you have provided is invalid or does not exist in our database.


Hammer Time
Ultimate Carjunky / Moderator
Hammer Time profile image

May 4, 2021, 1:35 PM

Post #9 of 21 (500 views)
  post locked   Re: Tachometer jumps around after 4000 RPM  

It's not the same credentials.

You need to create an additional one for the imaging software. It can be the same one you use for the forum but you still have to create it within the imaging feature.



~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

We offer help in answering questions, clarifying things or giving advice but we are not a substitute for an on-site inspection by a professional.



(This post was edited by Hammer Time on May 4, 2021, 1:35 PM)


Justin @
User

May 4, 2021, 1:53 PM

Post #10 of 21 (491 views)
  post locked   Re: Tachometer jumps around after 4000 RPM  

Tom, The previous owner put in a cam and put it up to 400hp about 10 years ago. The distributor has Five wires and a vacuum. One connector with a brown and orange wire and another connector with three wires, red, brown and black.


Tom Greenleaf
Ultimate Carjunky / Moderator
Tom Greenleaf profile image

May 4, 2021, 3:26 PM

Post #11 of 21 (474 views)
  post locked   Re: Tachometer jumps around after 4000 RPM  

I meant plug wires but you just stated (plain memory on my part) those wires are for the module inside. I'm pretty sure your signal comes from the brown one can show RPM.
Cam deal? Alone not sure if what was done could cause a problem with RPMs is all you wanted to know why. If this was in front of me I'd see if RPMs also went screwy on my dwell tach. It just reads RPMs when for working on this vintage like set idle speed cold and warmed up and other tests.
If that brown wire also is spliced to another near there it's something to check. OE wiring or who did what? Your colors were right TMK and memory.
Just know HEI stands for "High Energy Ignition" for many GMs just showed what type but is powerful for spark,
Tom



Hammer Time
Ultimate Carjunky / Moderator
Hammer Time profile image

May 4, 2021, 3:40 PM

Post #12 of 21 (472 views)
  post locked   Re: Tachometer jumps around after 4000 RPM  

Does the engine run any differently in response to this problem?



~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

We offer help in answering questions, clarifying things or giving advice but we are not a substitute for an on-site inspection by a professional.



Justin @
User

May 4, 2021, 3:55 PM

Post #13 of 21 (468 views)
  post locked   Re: Tachometer jumps around after 4000 RPM  

The car runs great, just notice when giving it the gun through the gears, the tach goes haywire after 4000. I’m guessing, I had it up near 6000. No issues at all with performance. It’s a 4 speed.


Justin @
User

May 4, 2021, 3:57 PM

Post #14 of 21 (463 views)
  post locked   Re: Tachometer jumps around after 4000 RPM  

Tom, all is OE wiring and the car other than the engine work he did. Thanks for your insight.


Hammer Time
Ultimate Carjunky / Moderator
Hammer Time profile image

May 4, 2021, 3:58 PM

Post #15 of 21 (461 views)
  post locked   Re: Tachometer jumps around after 4000 RPM  

If the engine performance doesn't change when this occurs then I think that is a pretty good indication that the issue is in the tach itself and not the engine.



~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

We offer help in answering questions, clarifying things or giving advice but we are not a substitute for an on-site inspection by a professional.



Justin @
User

May 4, 2021, 4:28 PM

Post #16 of 21 (457 views)
  post locked   Re: Tachometer jumps around after 4000 RPM  

Thanks Hammer Time.


Justin @
User

May 4, 2021, 5:01 PM

Post #17 of 21 (447 views)
  post locked   Re: Tachometer jumps around after 4000 RPM  

I’m wondering if the tach filter has any impact on the bouncing needle over 4K?


Hammer Time
Ultimate Carjunky / Moderator
Hammer Time profile image

May 4, 2021, 5:04 PM

Post #18 of 21 (442 views)
  post locked   Re: Tachometer jumps around after 4000 RPM  

Could be



~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

We offer help in answering questions, clarifying things or giving advice but we are not a substitute for an on-site inspection by a professional.



Justin @
User

Jun 3, 2021, 3:25 PM

Post #19 of 21 (332 views)
  post locked   Re: Tachometer jumps around after 4000 RPM  

The tach filter replacement didn’t help.

SOLUTION: Bouncing tach needle. The connection at the distributor was loose. I disconnected it, cleaned it up and tightened it. The tach runs smooth all the way up in RPMs now.

Thanks for all the support. Best!


Tom Greenleaf
Ultimate Carjunky / Moderator
Tom Greenleaf profile image

Jun 3, 2021, 3:39 PM

Post #20 of 21 (328 views)
  post locked   Re: Tachometer jumps around after 4000 RPM  

Great news. That's where you can unplug and use a tach underhood so maybe it was done ONE too many times? :-) Tom


Hammer Time
Ultimate Carjunky / Moderator
Hammer Time profile image

Jun 3, 2021, 4:05 PM

Post #21 of 21 (318 views)
  post locked   Re: Tachometer jumps around after 4000 RPM  

Closing this now as solved.



~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

We offer help in answering questions, clarifying things or giving advice but we are not a substitute for an on-site inspection by a professional.







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