Tach jumping around - Team Camaro Tech
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post #1 of 12 (permalink) Old Dec 7th, 02, 03:52 PM Thread Starter
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I have a ZZ4 engine with a HEI distributor. It seems like my engine runs fine, but at idle my tachometer bounces around 700 RPM's to around 2500 RPM's. The engine is still idling when the tach is jumping around. I checked my electrical connections, but they seem fine. I am not to familiar with an HEI distributor. If the cap on the HEI is cracked would this cause the tach to jump around, but wouldn't this also cause the car to idle rough? We have an Autometer street tach.
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post #2 of 12 (permalink) Old Dec 8th, 02, 04:35 AM
 
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I dropped the 350HO in my 69 and had the same exact problem. This may sound lame but, after tuning the car, eliminating various vacuum leaks and setting the timing to the GM recommended 10 BTDC and 36 total my AutoMeter street tach quit jumping all over the place. Perhaps this doesn't answer your specific question but, at least you know that it might not be a serious problem. I don't believe that your cap is cracked. Let us know if you have already tuned the car as mentioned above. Maybe some of the electrical wizards in here would be able to shed some light on the problem if you have already tuned the motor.

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post #3 of 12 (permalink) Old Dec 8th, 02, 10:12 AM Thread Starter
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I just put on new ignition wires and new spark plugs, but it is still jumping around. But, when I had the car running and I turned the lights on, the tach quits jumping, and also when I touch the brakes it stops. So I am pretty sure it is an electrical problem. We checked the ground on the tach and it is okay. We checked the battery ground and it is okay. We checked the negative cable to the block and that is fine. Any ideas on what might be causing this?
Thank you
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post #4 of 12 (permalink) Old Dec 8th, 02, 11:03 AM
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ground ok from engine to firewall? Is Tach grounded to frame? If so, frame ground strap tight? Just a thought.
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post #5 of 12 (permalink) Old Dec 8th, 02, 12:38 PM Thread Starter
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All of the grounds are okay. I think. Could this problem have anything to do with the starter. We turned the car on and off a couple of times to check electrical connections and see if what we did helped the problem but after a couple times of doing this, my starter started making really bad sounds and the car wouldn't start. I am not sure if my starter is dying or if something is causing it to do this?
Thanks for the help
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post #6 of 12 (permalink) Old Dec 8th, 02, 08:42 PM
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If you're using an external voltage regulator the points settings may need adjustment. Hook up a test light with the car idling and see if the light flickers. If it does it's caused by the points opening and closing repeatedly.

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[This message has been edited by 1 2RUN (edited 12-08-2002).]
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post #7 of 12 (permalink) Old Dec 9th, 02, 06:09 PM Thread Starter
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I am not using an external voltage regulator. Thanks for all of the help. I am pretty sur it has got to be a ground. When I hit the brakes or the lights, the tach quits but also the dome light blinks. Wouldn't this be caused by a bad ground?
Thanks for all of the help,
Kyle
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post #8 of 12 (permalink) Old Dec 9th, 02, 09:20 PM
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The dome light works with an isolated ground. It has two wires, one hot all the time (orange), and one that's grounded by the door jam and headlight switches (white). There should be a braided ground strap on the firewall to the frame or engine somewhere. Make sure it has a clean connection.

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post #9 of 12 (permalink) Old Dec 10th, 02, 01:00 AM
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I think that you have some sort of voltage supply problem. I used to have a similar problem when my car had an externally regulated alternator. When I would turn on the signal lights, my dash lights would get dimmer, then brighter when the signal light blinked. A higher output, internally regulated alternator along with cleaning all of the connections and grounds, cured the problem. My car now has a constant 14v, no matter what.

If you already have an internally regulated alternator, try cleaning the connections on the horn relay. The power wire that feeds the horn relay, also feeds the rest of the car. Also, verify that the ground straps that go from the valve covers to the firewall are in place and have good connections. If that isn't it, you need to verify that the voltage regulator (either internal or external) is functioning properly. I'll bet that the voltage supply that the tach is getting is not contant. You need to check this with a multimeter, preferrably an analog one, while the tach needle is bouncing. If the tach supply voltage is bouncing around, but the supply voltage that is coming directly off of the alternator is constant, you have some sort of connection problem somewhere in the system.

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post #10 of 12 (permalink) Old Oct 18th, 14, 03:18 PM
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Re: Tach jumping around

I found this old post on a Google search and wanted to share how I fixed this exact same issue on my 69 last week. I had purchased my tach from classic industries and am running a MSD HEI with a 1 wire alt. My tach bounced and would act normal if I turned on anything that drew power (lights, blinker etc...) Alt was new and I had just added 4 new ground straps but the fix was installing a tach filter out of a late 70's early 80's gm camaro that I found in a junk yard. You can buy them off eBay $55 or make your own. Msd and autozone also sell them but are around $80. Hope that helps the next person that has this same issue in the future.



Last edited by Aaronm822; Oct 18th, 14 at 03:30 PM. Reason: Add pic.
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post #11 of 12 (permalink) Old Oct 18th, 14, 04:22 PM
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Re: Tach jumping around

I installed an aftermarket Autometer tachometer in my '69 Camaro (stock, non-HEI ignition) and the readings were erratic and spiked frequently. I contacted Autometer and they suggested I install a 10K, 1/2 watt, resistor in the green tachometer wire.

Not sure if that helps... it's an easy thing to try though, and less than a $5 fix.

John

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post #12 of 12 (permalink) Old Oct 18th, 14, 08:32 PM
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Re: Tach jumping around

Yes you can purchase a Tachometer filter
http://rodneydickman.com/catalog/pro...roducts_id=265
http://rodneydickman.com/catalog/index.php?cPath=56

or make your own. Here is our site page instructions.
https://www.camaros.net/techref/elect...lter/index.htm


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