another bouncing tach thread
03-17-2008, 12:35 PM
#1
Rennlist Member
Thread Starter
another bouncing tach thread
A few times lately when I'm just driving down the road the tach will bounce around wildly for maybe 20 seconds then go back to normal. I have not noticed any dimming lights or problems with any other gauges or electrical components. I've done TONS of reading on this subject and realize now it could be the: voltage regulator, alternator going bad, poor ground somewhere, loose wire on tach, faulty tach, CDI box, bad battery, etc...
Here's what I've done so far.
Battery terminals are clean with no leaking.
Battery voltage at rest: 12V
Battery voltage at idle: 13.4V
Battery voltage at 3000rpm: 14.3V
Took the coil wire off the distributor and had someone crank the car while I held the wire near metal to test for spark and had a nice blue spark.
Took the tach out of the dash and didn't see any loose wires.
Checked to make sure the CDI box was plugged in tight.
I hear the CDI box buzzing with the key in the ON position but it doesn't seem as loud as it used to be.
Checked the transmission ground strap. One end is perfectly clean, the other is a little dirty but is tight.
Checked all wires going to the starter and they're tight and clean.
A local Porsche mechanic I talked to thinks it's the CDI box. I talked to an alternator repair guy and he says it's not the alternator but that he could test it if I wanted him to. He also says every Porsche alternator he's tested puts out 13.6 volts and no more and therefore thinks I have an aftermarket voltage regulator and that it sounds like it's working fine. He also thinks it's the CDI box.
So just about everyone here thinks it's the VR, the wrenches I've talked to think it's the CDI. What are my options at this point? Just start replacing things in acsending order of cost? Try and rig something up to the cigarette lighter to monitor the voltage as the tach is bouncing? Take out the alternator and have it tested? I could try and swap out a friend's CDI and go for a drive but there's no guarantee the tach would bounce even if it were my CDI since it doesn't act up very often. Is there any way to test the CDI other than the spark test I already did?
Any and all opinions welcome.
thanks
Here's what I've done so far.
Battery terminals are clean with no leaking.
Battery voltage at rest: 12V
Battery voltage at idle: 13.4V
Battery voltage at 3000rpm: 14.3V
Took the coil wire off the distributor and had someone crank the car while I held the wire near metal to test for spark and had a nice blue spark.
Took the tach out of the dash and didn't see any loose wires.
Checked to make sure the CDI box was plugged in tight.
I hear the CDI box buzzing with the key in the ON position but it doesn't seem as loud as it used to be.
Checked the transmission ground strap. One end is perfectly clean, the other is a little dirty but is tight.
Checked all wires going to the starter and they're tight and clean.
A local Porsche mechanic I talked to thinks it's the CDI box. I talked to an alternator repair guy and he says it's not the alternator but that he could test it if I wanted him to. He also says every Porsche alternator he's tested puts out 13.6 volts and no more and therefore thinks I have an aftermarket voltage regulator and that it sounds like it's working fine. He also thinks it's the CDI box.
So just about everyone here thinks it's the VR, the wrenches I've talked to think it's the CDI. What are my options at this point? Just start replacing things in acsending order of cost? Try and rig something up to the cigarette lighter to monitor the voltage as the tach is bouncing? Take out the alternator and have it tested? I could try and swap out a friend's CDI and go for a drive but there's no guarantee the tach would bounce even if it were my CDI since it doesn't act up very often. Is there any way to test the CDI other than the spark test I already did?
Any and all opinions welcome.
thanks
03-17-2008, 02:42 PM
#2
RL Technical Advisor
Chris,
How many miles on your car????
You are correct about the source; it could be any of those things.
I would R&R the transmission ground strap and make sure everything is clean, shiny and bright before reassembly. I'd also have good look at the alternator wiring and ground behind that unit.
If you have a DVOM, you could monitor system voltage when the tach acts up but I would remove the alternator/VR to have it tested before spending anything toward the CDI.
How many miles on your car????
You are correct about the source; it could be any of those things.
I would R&R the transmission ground strap and make sure everything is clean, shiny and bright before reassembly. I'd also have good look at the alternator wiring and ground behind that unit.
If you have a DVOM, you could monitor system voltage when the tach acts up but I would remove the alternator/VR to have it tested before spending anything toward the CDI.
03-17-2008, 02:56 PM
#4
"I would remove the alternator/VR to have it tested"
Most likely the internal voltage regulator alternator (Paris-Rhone) is intermittently
overcharging which is common for that alternator, i.e. a bad regulator.
Overvoltage spikes will cause the tach to exhibit an erratic behavior.
A bad CDI will cause an erratic engine running condition and an erratic tach operation.
It's unlikely that the CDI just has a bad tach output.
So, since the alternator is likely intermittent, as voltage checks usually indicate a proper voltage,
having the alternator tested externally will probably indicate it's O.K. too.
The end solution will probably be to replace the regulator or the complete alternator.
Most likely the internal voltage regulator alternator (Paris-Rhone) is intermittently
overcharging which is common for that alternator, i.e. a bad regulator.
Overvoltage spikes will cause the tach to exhibit an erratic behavior.
A bad CDI will cause an erratic engine running condition and an erratic tach operation.
It's unlikely that the CDI just has a bad tach output.
So, since the alternator is likely intermittent, as voltage checks usually indicate a proper voltage,
having the alternator tested externally will probably indicate it's O.K. too.
The end solution will probably be to replace the regulator or the complete alternator.
03-17-2008, 03:08 PM
#5
Team Owner
I agree with Loren on this one ... the chances of the thing acting up when it is actually being tested is slim. Add to that he would need a scope allmost to see the fast spikes in the voltage .. and i am not sure if they use scopes .
03-17-2008, 03:31 PM
#6
Rennlist Member
I have a $15 cigarette lighter volt gauge. You could use something like this because your problem is intermittent and when acting up, the volt gauge would clearly indicate a different response than 'normal,' and you'd be able to see it.
03-17-2008, 03:38 PM
#7
Rennlist Member
Thread Starter
Thanks for the input everyone. The car has 107K and judging from the condition of many other parts on the car it could very well have the original alternator but I have few service records from the PO. The battery looks fairly new but I don't drive it much (waiting for the tracks to thaw out around here) and it recently sat for a month due to my thermostat adventure.
As an update I had my battery tested and it read 443 CCA, is rated for 650 CCA, and the tester said REPLACE. Could it be something this simple? During my research I did read about a few people who said a bad battery was the cause of their bouncing tach.
If I do go for a whole new alternator do you guys recommend a new one, having mine rebuilt, or buying a rebuilt one?
As an update I had my battery tested and it read 443 CCA, is rated for 650 CCA, and the tester said REPLACE. Could it be something this simple? During my research I did read about a few people who said a bad battery was the cause of their bouncing tach.
If I do go for a whole new alternator do you guys recommend a new one, having mine rebuilt, or buying a rebuilt one?
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03-17-2008, 04:32 PM
#8
Rennlist Member
Thanks for the input everyone. The car has 107K and judging from the condition of many other parts on the car it could very well have the original alternator but I have few service records from the PO. The battery looks fairly new but I don't drive it much (waiting for the tracks to thaw out around here) and it recently sat for a month due to my thermostat adventure.
As an update I had my battery tested and it read 443 CCA, is rated for 650 CCA, and the tester said REPLACE. Could it be something this simple? During my research I did read about a few people who said a bad battery was the cause of their bouncing tach.
If I do go for a whole new alternator do you guys recommend a new one, having mine rebuilt, or buying a rebuilt one?
As an update I had my battery tested and it read 443 CCA, is rated for 650 CCA, and the tester said REPLACE. Could it be something this simple? During my research I did read about a few people who said a bad battery was the cause of their bouncing tach.
If I do go for a whole new alternator do you guys recommend a new one, having mine rebuilt, or buying a rebuilt one?
Last edited by Mike Murphy; 03-20-2008 at 01:31 PM.
03-19-2008, 03:57 PM
#9
This is a timely thread. My tach bounced just the other day. Also, the seatbelt light lit at the same time. It just did it for a few seconds and then back to normal. At least the alternator replacement looks to be a bit easier on this car than my 964 which I did just a month ago. Time to get the meter out, again.
03-19-2008, 09:48 PM
#10
Rennlist Member
Thread Starter
I'm shopping for a new battery and will try that first. I also ordered a voltage regulator and if the battery doesn't do the trick I'll replace the VR and go from there.
deep_uv I've not had a seat belt light come on in the SC but did have the airbag/seat belt light fiasco in the 964 but that's another story.
My first post in this thread pretty much sums up the things you can check for on the SC. The only thing I didn't do was rig up a volt meter to the cigarette lighter and try to see what it's doing while the tach is actually acting up. Good luck.
deep_uv I've not had a seat belt light come on in the SC but did have the airbag/seat belt light fiasco in the 964 but that's another story.
My first post in this thread pretty much sums up the things you can check for on the SC. The only thing I didn't do was rig up a volt meter to the cigarette lighter and try to see what it's doing while the tach is actually acting up. Good luck.
03-19-2008, 10:55 PM
#11
I'm shopping for a new battery and will try that first. I also ordered a voltage regulator and if the battery doesn't do the trick I'll replace the VR and go from there.
deep_uv I've not had a seat belt light come on in the SC but did have the airbag/seat belt light fiasco in the 964 but that's another story.
My first post in this thread pretty much sums up the things you can check for on the SC. The only thing I didn't do was rig up a volt meter to the cigarette lighter and try to see what it's doing while the tach is actually acting up. Good luck.
deep_uv I've not had a seat belt light come on in the SC but did have the airbag/seat belt light fiasco in the 964 but that's another story.
My first post in this thread pretty much sums up the things you can check for on the SC. The only thing I didn't do was rig up a volt meter to the cigarette lighter and try to see what it's doing while the tach is actually acting up. Good luck.
03-20-2008, 01:33 PM
#12
Rennlist Member
04-02-2008, 12:26 PM
#15
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Having similar issues. However, I replaced the VR and it helped the tach issue. Is there an internal regulator on the alternator as well? Btw, I had it rebuilt last year when it started whining.
My problem is still a mis or crossfire. Could a bad VR have damaged the CDI unit? It runs ok, but not like it should. Had some issues with intermittent stalling which left me stranded a time or two. Cold starts are fine, but an ocassional hot start is hard.
Gonna take the book, a smarter person and some better tools to it tonight.
Justin
My problem is still a mis or crossfire. Could a bad VR have damaged the CDI unit? It runs ok, but not like it should. Had some issues with intermittent stalling which left me stranded a time or two. Cold starts are fine, but an ocassional hot start is hard.
Gonna take the book, a smarter person and some better tools to it tonight.
Justin