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Rev counter jump on startup

 
Old 01-02-2018, 06:16 AM
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545svk
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Default Rev counter jump on startup

Recently my rev counter started jumping when cranking on the starter motor - similar as in this
. (found the video when searching for answers)
This morning car wouldn't start due to a dead battery - it was fine yesterday.
With a new battery fitted all is well, and the rev counter does not jump anymore.
The question is: Is a jumping rev counter an indication of a dying battery or something else?
Thanks.
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Old 01-02-2018, 07:40 AM
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it could be an indication of lower starting voltage being applied,
I wouldnt use it as a guide, keep the battery charged up as it will reduce the chances of relays being damaged.
Many 928s have had poor or no starts due to low battery conditions

Last edited by Mrmerlin; 01-03-2018 at 01:59 AM.
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Old 01-02-2018, 06:57 PM
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Thanks Stan - The electric side of my car is going to force me to revisit school science, or get the teenager to pay more attention.
Lower starting voltage sound like it can include many points of resistance or failure, including the battery. So if the rev counter start bouncing it's time to start checking.
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Old 01-02-2018, 07:02 PM
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Make sure your negative battery cable is tight. This recently started happening to me with an eventual no start on a very very cold day. Appears the cable connector didn't shrink as much as the pole, tightened it up and all is well. Also fixed some of my other misc gremblins like my random high idle... of course if that isn't it check your other grounds.
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Old 01-02-2018, 07:33 PM
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This also happens to me but when I stop the engine.

Never after a long drive but more often when I just move the car around. Like taking it out of the garage.

The needle can bounce between 2 to 4k.

It started doing this a year or two ago.

Don't know what to look at.
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Old 01-03-2018, 08:30 AM
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Originally Posted by Bertrand Daoust View Post
This also happens to me but when I stop the engine.

Never after a long drive but more often when I just move the car around. Like taking it out of the garage.

The needle can bounce between 2 to 4k.

It started doing this a year or two ago.

Don't know what to look at.
Look out the window and you won't see it, therefore, nothing wrong = nothing to fix = no 928 rabbit hole!
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Old 01-03-2018, 09:18 AM
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Originally Posted by DeWolf View Post
Look out the window and you won't see it, therefore, nothing wrong = nothing to fix = no 928 rabbit hole!
Thanks!
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Old 01-03-2018, 10:31 AM
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Mine is funky as well. When I turn the ignition on, before cranking the starter, my tach bounces randomly. Then it continues doing so after starting the car. Once the car has been running for a while, it settles into normal behavior, sometimes after a minute or so, others it may take 15 minutes. The car runs fine, fairly new CPS, good clean connector. I had the dash out this past year, and sent to tach to Palo Alto, and it checked out fine, although they noticed some swollen diodes, which they replaced. After reinstall with clean pod connections, the issue persists. This has been going on for 3 years. I think my issue is somewhere in the pod connections or the foil itself - although all looked good when I had it apart.

Could weak coils (16V Euro), or other ignition components cause erratic tach behavior? Hadn't thought about the battery or battery connections before reading this thread, but seems to warrant a check.

Here is a video.

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Old 01-03-2018, 03:33 PM
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As I could ascertain after some research there is something about the VDO gauges that relies on a sound earth connection and if that if faulty the gauge can bounce toward full scale deflection. Had such problem on my oil pressure gauge, got rid of the three pin sender and fitted a 2 pin sender- problem solved. I have an issue with my tachometer in that over 5k rpm the tach signal goes bonkers and the needle goes full scale bouncing around somewhat. Whether the root cause of these issues is earth fault related I have no idea but it seems credible.
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Old 01-03-2018, 03:41 PM
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I know I didn't get my ground cable on tightly (the main one by the tool kit) and my tach did that until I stopped and tightened it properly.
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Old 01-04-2018, 09:20 AM
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Originally Posted by 545svk View Post
Thanks Stan - The electric side of my car is going to force me to revisit school science, or get the teenager to pay more attention.
Lower starting voltage sound like it can include many points of resistance or failure, including the battery. So if the rev counter start bouncing it's time to start checking.
Hats off to you if your teen takes direction. Just kidding. You are right the electrics are demanding. If you haven't found it, there is a list of ground points (the main ones) in the New Visitor thread. There are lots more brown wire connections to look for. Stan will recommend with the merits of DeOxit. Also check for corrosion on coil wires. People have found corrosion under the coating on the ground strap. If yours is original, I'd replace it.
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Old 01-05-2018, 02:23 AM
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Thanks Steve - The teenager is a full on aircooled Beetle nut and pretty handy part washer. There is hope.
My main earth straps are about a year old and about half the others has been serviced. The one behind the dash pod is not on that list yet.
I'll have to check the coil wires.
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Old 01-05-2018, 06:45 AM
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I had many annoying issues caused by my foil and replacing it fixed them. It also exited the alt faster as a new resistor was part of it.

If you haven't, replace the ignition switch as it can cause all kinds of weird issues with the gauge cluster.
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