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Tacho jumping, lights dimming - '78 911SC
Hi All,
Just a quick one.... The Tacho started jumping around with no change in actual revs??? At the same time, the dash lights dim and then get brighter (not drastically). All this happens for about the first 15mins from a cold start and then settles into SOP (if that makes any difference?) Alternator ready to pack it in???? Cheers
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'65 Porsche 912 (painted dash) #451232 '65 Porsche 912 (painted dash) #451425 - 13 days younger '78 Porsche 911SC - Resto garage queen |
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05-09-2011, 08:48 PM
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Put a meter on the battery terminals......AFTER fully charging the battery....
12.5-12.8 at rest 13 at idle 13.5-14 at 2500 rpms
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1983/3.6, backdate to long hood 2012 ML350 3.0 Turbo Diesel |
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05-09-2011, 09:58 PM
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Location: Southern California
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1984 Targa |
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05-09-2011, 10:04 PM
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Quote:
Quote:
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'65 Porsche 912 (painted dash) #451232 '65 Porsche 912 (painted dash) #451425 - 13 days younger '78 Porsche 911SC - Resto garage queen |
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05-09-2011, 10:13 PM
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Probably the alternator but the regulator is a LOT easier to get to and change out. Then on the other hand if you end up having to change out the alternator the "new" (Ebay) one will likely have the regulator built in.... If you use a voltmeter to check for voltage variations the best place to do that would be back near the engine. Across the battery you will get only average system voltage. The CDI design flaw is the source of the tack jumping and it's responding to the LOCAL system voltage, voltage in the engine compartment. |
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05-10-2011, 07:03 AM
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Registered
Join Date: Apr 2006
Location: La Canada, CA.
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Immediately prior to my cdi unit going out the tach was jumping. The occurrence was on a 1971 914-6. I had the cdi unit rebuilt all problems solved.
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67 275/4 63 B coupe 64 SC sunroof 912 coupe (current) 914/6 (current) |
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05-10-2011, 07:07 AM
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Tach jumping is the result of "self-firing" of the SCR (Silicon Controlled Rectifier) within the CDI, which is the result of the CDI capacitor charge exceeding the breakdown voltage of the SCR. DC-DC upconverter, 12 volt to ~300, is unregulated so 12 volt system going significantly over-voltage will result in tach jumping. |
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05-10-2011, 07:49 AM
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Insert Tag Line HERE.....
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a LOT more common for the regulator to go out....
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05-10-2011, 01:18 PM
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muck-raker
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STONE '88 Cabriolet, using EP Slick 20w50 partial synthetic Snake Oil...just as Rommel intended.  Deny Everything; Admit Nothing; and Always Make Counter-accusations 
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05-10-2011, 05:21 PM
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After my experience with our '78 Targa I pretty sure that an intermittent short within the alternator stator windings was the BASE problem. In addition to the tach jumping if/when we hit a good solid bump the engine would just quit.
The bump would cause the windings to short, maybe only a single phase of the three, the alternator output voltage would drop accordingly, the regulator would then increase the rotor current, possible substantially so, to compensate. The short would clear almost immediately and the alternator would run for the next few (10-100-200 milliseconds) with a severely "pumped up" rotor current. The SCR begins avalanching, self-firing, due to the momentary 16-18 volt supply, the tach jumps into the over-rev range, the rev-limiter opens the fuel pump relay circuit and then latches up itself due to intolerance to the over-voltage. The engine QUITS. The only way to "clear" the latched up rev-limiter is to turn the ignition off for 4-10 seconds. |
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05-10-2011, 05:58 PM
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Thanks for all the advise guys, very much appreciated!!!
I'll start the problem solving (conections, grounds etc etc) and work through as per all the suggestions. Hope to have some good news, quickly ??? Cheers again!
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'65 Porsche 912 (painted dash) #451232 '65 Porsche 912 (painted dash) #451425 - 13 days younger '78 Porsche 911SC - Resto garage queen |
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05-10-2011, 09:18 PM
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At Rest - @ Engine 12.84V / @ Battery 12.71V At Idle - @ Engine 14.03V / @ Battery 13.72V At 2500rpm - @ Engine 14.08V / @ Battery 13.84V Also, what is the box under the voltage regulator as a lead comes from it to the regulator, and is there any 'visual' signs the regulator is toast? 
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'65 Porsche 912 (painted dash) #451232 '65 Porsche 912 (painted dash) #451425 - 13 days younger '78 Porsche 911SC - Resto garage queen |
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05-11-2011, 03:48 PM
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Some good readings.....must be something else......was the tach bouncing when you got these reads?
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1983/3.6, backdate to long hood 2012 ML350 3.0 Turbo Diesel |
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05-11-2011, 03:56 PM
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No it wasn't Joe Bob, the Tacho bouncing pretty much only happens on cold start (or when the car has sat for a while) and lasts about 15mins. Unfortunately I had driven the car already.
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'65 Porsche 912 (painted dash) #451232 '65 Porsche 912 (painted dash) #451425 - 13 days younger '78 Porsche 911SC - Resto garage queen |
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05-11-2011, 03:59 PM
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I would temporarily "bridge" a small motorcycle or lawn tractor battery into the 12 volt alternator output circuit in the engine compartment to check and see if that abates the tach jumping. Your measurements mean NOTHING absent knowing the level of voltage ripple riding atop the base DC voltage. You might try a voltage reading using the AC scale provided the measuring device, DVM, DMM, etc, uses capacitor coupling for that function. |
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05-11-2011, 05:56 PM
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Ok, so after 2 cold starts the 'Tacho jumping' thing hasn't happened?
After further investigation with a Multimeter, I've found that when the heater blower (engine compartment) is activated by the floor levers, the voltage drops from 14.03 to 13.5ish? After trying all the other electrical items in the car (other fans, radio, wipers, defroster etc etc) this seems to be the only thing that interfers with the voltage. Could this have something to do with the jumping (which as I said hasn't happened now for 2 days)? Also, should the heater fan motor take a few seconds before activating (the relay I'm guessing) after the levers are raised? Hmmmm? 
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'65 Porsche 912 (painted dash) #451232 '65 Porsche 912 (painted dash) #451425 - 13 days younger '78 Porsche 911SC - Resto garage queen |
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05-12-2011, 11:37 PM
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05-13-2011, 07:44 AM
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I wouldn't consider it abnormal for that amount of voltage drop when loaded....different fans will pull different currents.. CDI is off my list of ANY expertise or experience (sorry). Maybe something IN the tach... BEST! Doyle
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Recording Engineer, Administrator and Entrepeneur Designer of Fine Studios, Tube Amplifier Guru 1989 Porsche 911 Carrera Coupe 25th Anniversary Special Edition Middle Georgia |
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05-13-2011, 12:44 PM
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Join Date: Sep 2008
Location: Madrid, Spain
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It seems it is the external voltage regulator.
Have you tested at higher revs than 2500? Anything above 14v is dangerous, and the CDI can easily fail due to overvoltage. |
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05-13-2011, 03:34 PM
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Ok, so Update...
This morning (damn crazy cold ass Melbourne morning mind you!) My Tacho decided to start the 'jumping' scenario, being the first time in about a week it has happened (weather related?) I quickly grabbed my multimeter and tested the voltage at the engine (whilst on idle). 16.90V  Switched ignition off and restarted, all good. Voltage Regulator Kaputzki or on the way out????
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'65 Porsche 912 (painted dash) #451232 '65 Porsche 912 (painted dash) #451425 - 13 days younger '78 Porsche 911SC - Resto garage queen |
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05-17-2011, 04:17 PM
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2011 Porsche Cayenne Diesel
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