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Detritus Maximus
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3,105 Posts
Discussion Starter · #1 ·
Bouncing Tach

Well, at really low rpm 6-700 or less, sometimes a little more, pretty much when idling, the tach in my GT starts to 'bounce'. I'm not talking about a couple hundred rpm, but thousands. It will swing around to redline, drop back down, swing up to 4k, drop back to 2k, and then back up. If I put my foot on the accelerator and bring the revs a little or alot, it acts normal.

Sound like a current flow problem? Grounds?
 

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rust + magic = gold!
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169 Posts
Hi
I wonder if a combination of low idle vibrations and a poor connection between the distributor and the tach are causing this problem? Check the green 20g wire that runs from dist. -> coil -> tach.
Hope this helps,
Bob
 

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Detritus Maximus
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3,105 Posts
Discussion Starter · #4 ·
I'll check the wire. I had the distributor apart Saturday night checked and lubed everything. The screw for the ball bearing is tight. I'm about to install a Pertronix set up, so I'll see if it changes anything (or I find more strange things going on!).
Thanks for the repsonses..:)
 

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Member
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Help! Erratic Tach

Hi,

I have a 2.4FI. And recently, it started running really rough. But then it would clear up and be fine. I haven't had time to tend to it. Then yesterday, I had to use the car and I noticed the rpms just dropped to ZERO! But the car was running fine. And then the rpms would jump up and back to zero and up again. Then it settled back to normal. But I was home and didn't have time to deal with it.

Any guesses?

Thanks,

Manny
 

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OPEL-LESS!!!
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2,116 Posts
mine did the same thing.....altho i didnt have a 2.4. it was a bad ground, its still not right but atleast it works
 

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OPEL-LESS!!!
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2,116 Posts
i dont know exactly how it went. when it come to electrical i'm completely lost. my dad took my car to work last week and he got annoyed by the tach and got under the dash to check it out. he said there was a loose ground wire and he just tightened it up with some little channel locks he always carries around. that fixed it. its sort of bouncy but it seems to read right as long as you arent picking up R's too fast
 

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My tach has been wrong since i've had the car. When u turn on accesories the rpm goes to 3000 and it says it idles at a little under 4000 (which should be right). But theni think it dosn't keep proportions the rest of the way up, cause on the freeway i go 80-90 and the needle points to 6-7000 rpm's, which then should be 3000 rpm, and that can't be right can it?
 

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OPEL-LESS!!!
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2,116 Posts
my GT does 4200 at 80 or 90. its still got the 1.9 with a 4 speed and i believe stock wheel size.(stock rims) its a 70
 

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oh then maybe it is right, but then why do i top out at around 102? If the rpms don't max their rev then shouldn't keep goin higher even if slow? Also anyone know rev limit on opel gt?
 

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Opeler
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30 Posts
When I last drove my GT (last fall), I noticed that the tach takes a long time to settle when the engine RPM changes. It appears to be accurate but, for instance, if you rapidly change from 2000rpm to 3000rpm, it will overshoot to 3500, then back to 2600, 3300, 2800, 3100, 2950, then finially settle at 3000. Once the engine RPM stays the same, the needle stays steady.

Is this normal? The GT was new to me last fall so I don't know if it should be doing that. How much do the tach needles on other GT's bounce when the engine RPM changes?

I've taken the tach apart and checked out the circuitry and mechanicals, and can't find anything wrong. Adding a capacitor across the meter (about 2,000uf seems to work well), helps stablize the needle a lot, but it still bounces a bit. The capacitor basically slows down the change in voltage to the meter a bit so it does not overshoot by as much. It also increases the reading a bit, so the tach will need to be re-calibrated if I permatently install the capacitor.

I also tried to add a bit of silicone grease to the rear bearing for the needle to help damp it. This helped a lot, but the needle does not always settle at exactly the correct speed with the grease in the bearing so I don't really like that solution a lot. My only hope with the grease is that the normal engine vibration in the GT will help the needle settle in that case.

As far as I can see, the only damping in the tach would have to to have been some sort of mechanical thing in the bearings that has either worn out or dried up over the years.

I've seen the info around on building new circuitry for the tach, but I do not think that will help as the meter bounces around when I drive it from a power supply with a changing voltage.

Any comments or thoughts from the group?
 

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I had the same problem with my stock tach, it bounces around. I just went full custom so I don't have the problem anymore. BTW, I went from stock points and coil to a pertronix ignitor and coil and the problem persisted.
 

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OPEL-LESS!!!
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my car does the exact same thing. its really bad on free-revs too. the tach will jump from an idle to redline, and its really only doing say 3500-4000. jumps alot when shifting also.
 

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OPEL-LESS!!!
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2,116 Posts
i've never seen fuses like the ones in my GT....where would you get some?
 

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OPEL-LESS!!!
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i never tried looking for them yet, i just assumed they were hard to get since i never seen any before
 

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Opeler
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138 Posts
Bouncing Tach

Back to the unsettled tach topic.

Mine has been doing the same for some time now. An Opel GT expert / friend of mine was visiting this past weekend and said this happens when the coil is not grounded properly.

I will take this advice and attempt to correct it this weekend. Thought you guys would want to hear about this opinion.
 
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