My 3.0 engine is installed and running.
I have some tuning and details to be worked out.
My tach (stock 6) does not work.
My dizzy is a 3.0 SC that has been recurved and is wired to a standard 6 pin CD unit
that is powered through the relay board.
My relay board has the standard 6 transducer.
Can this combo work ?
Haven't spent any time with it.....maybe someone here already has.....
Anyone ?
Full Version: 3.0 to 914 6 wiring questions
This might work for you, it's for a Motec system, but I don't know why it wouldn't work;
Click to view attachment
Click to view attachment
Either Tach Adapt or have JBell on Pelican install a new drive (no transducer needed).
You shouldn't need the -6 "transducer", the 6 pin CDI has a dedicated tach output, which should be able to drive the tach correctly.
The pulse transformer was introduced (I believe) for the 3 pin CDI units, which didn't produce a signal the tachs at the time were happy with. The 6 pin CDI fixes that.
Don't think you should need the tach adapt..?
- Tony
The pulse transformer was introduced (I believe) for the 3 pin CDI units, which didn't produce a signal the tachs at the time were happy with. The 6 pin CDI fixes that.
Don't think you should need the tach adapt..?
- Tony
I'm assuming you kept the wiring harness from the 3.0? The CIS harness has a black and purple wire in it. That will feed your tach. IF your tach is not an SC tach it won't work. Carrera might, I can't remember the chart. 2.7 will not. When I transplanted the 3.0 into my 911 I needed to replace the 77 2.7 tach with one from an SC.
QUOTE(Retroracer @ Feb 13 2022, 12:24 AM) 
You shouldn't need the -6 "transducer", the 6 pin CDI has a dedicated tach output, which should be able to drive the tach correctly.
The pulse transformer was introduced (I believe) for the 3 pin CDI units, which didn't produce a signal the tachs at the time were happy with. The 6 pin CDI fixes that.
Don't think you should need the tach adapt..?
- Tony
Tony this is what I was hoping to hear.
I think you are saying that I can jump across the relay board,
go from the black/purple of the engine harness to the tach feed (black/purple too.
I thnk) of the chassis harness.
That will drive old points type tach ?
No SC tach necessary ?
QUOTE(nditiz1 @ Feb 13 2022, 01:25 AM)
I'm assuming you kept the wiring harness from the 3.0? The CIS harness has a black and purple wire in it. That will feed your tach. IF your tach is not an SC tach it won't work. Carrera might, I can't remember the chart. 2.7 will not. When I transplanted the 3.0 into my 911 I needed to replace the 77 2.7 tach with one from an SC.
Actually I made a copy of the SC engine harness to suit the 914 6 chassis harness.
Many years agoe I installed a 3.0 into '76 911s and have no recollection of inop tach. Wish me luck.
Fingers crossed.
The tach in a 70 914-6 is designed for a high voltage input, meaning a standard Kettering (points, condenser, coil) input.
https://en.wikipedia.org/wiki/Delco_ignition_system
The tach output from a CD box is measured in millivolts, which is not enough to drive the tach for a Kettering ignition. When Porsche designed the 911 engine, they found the plugs would foul out all the time. To prevent this, they went to the CD ignition. But they already had tachometers in the pipeline for the Kettering ignition. So they built a "Tach Amplifier" to increase the signal from the CD box to a voltage enough to drive the tach. Basically, it was a small coil like an ignition coil. When the 914-6 was created, they used the same tach guts as the 911, so they designed in the tach amplifier on the relay board. In 1971, Porsche started using redesigned tachometer guts that would read the low voltage signal directly from the CD box. The same tach guts circuit was used until the end of the air cooled models, as it could be driven by the Motronic ECU just like it was driven by the CD box.
So to solve your problem, you have to get the tach from a 71 or newer 911. I fixed our 70 914-6 tach (it was already broken) by gutting a 911 SC tach and putting the electronics in the 914-6 tach. Then I took the dead tach amplifier, opened it up, and put in a jumper from the CD box input to the tach output. So it looks stock. Considering the tach amplifiers are now hundreds of dollar to buy, if you can actually find one, it is a really good permanent solution.
Or if you don't care about appearances, you can buy a tach amplifier and wire it into the relay board.
This is the one I used on my six conversion before going to the 4.0 and Motronic.
https://en.wikipedia.org/wiki/Delco_ignition_system
The tach output from a CD box is measured in millivolts, which is not enough to drive the tach for a Kettering ignition. When Porsche designed the 911 engine, they found the plugs would foul out all the time. To prevent this, they went to the CD ignition. But they already had tachometers in the pipeline for the Kettering ignition. So they built a "Tach Amplifier" to increase the signal from the CD box to a voltage enough to drive the tach. Basically, it was a small coil like an ignition coil. When the 914-6 was created, they used the same tach guts as the 911, so they designed in the tach amplifier on the relay board. In 1971, Porsche started using redesigned tachometer guts that would read the low voltage signal directly from the CD box. The same tach guts circuit was used until the end of the air cooled models, as it could be driven by the Motronic ECU just like it was driven by the CD box.
So to solve your problem, you have to get the tach from a 71 or newer 911. I fixed our 70 914-6 tach (it was already broken) by gutting a 911 SC tach and putting the electronics in the 914-6 tach. Then I took the dead tach amplifier, opened it up, and put in a jumper from the CD box input to the tach output. So it looks stock. Considering the tach amplifiers are now hundreds of dollar to buy, if you can actually find one, it is a really good permanent solution.
Or if you don't care about appearances, you can buy a tach amplifier and wire it into the relay board.
This is the one I used on my six conversion before going to the 4.0 and Motronic.
Pulled from Pelican.
1. 1965-1968 SWB Tachs. These may have VDO number 333.230/041/002, according to VDO, "this is a old version Tachometer with 24 Volt Square Wave signal from Ignition KL.1."
KL is German for "klemme" or contact, and #1 is the points contact, so this is a 24-volt square wave signal that comes off the coil. In another thread, Pelican 914GT tested it and found that the coil output was way more than 24 volts, but the spec came from VDO. Tach Thread These also say "12V 14000 Imp." which I think is the impedance value to make them work? I think all 914 tachs fit into this category as well.
2. 1969-1970 LWB Tachs. These have VDO numbers 333.230/041/007 to 333.230/041/010. According to VDO, these are driven by an "11 volt square wave." Now, we know that up until around 1971 the tachs were driven from an "intermediate unit" which was a silver can with Bosch part number 0 227 990 001. We know that this contains an inductor that is probably used to simulate the original coil to drive the low-impedance tach. Some of these say "12V 24000 Imp."
3. 1971-1973 Tachs. These appear the same externally but have been altered internally to be driven off the high-impedance circuit between the points and the Bosch CDI box. In the early 3-pin CDI boxes, there is a 33 ohm pullup resistor in the trigger circuit that would allow only around 425 mA to flow in the circuit. The tach must be sensitive to 11 volts and 425mA, which is why the later CDI won't drive the earlier tach off this circuit-- the earlier tach needs higher voltage.
4. 1974-1977 Tachs. I know these were driven off the CDI box, but I don't know when the CDI box changed from an "001" to an "008" or when the cutover from a 33 ohm pullup resistor to a 120 ohm occurred. With 120 ohms the input current is only 116mA so the tach must have changed somewhere along the line to match. If you send your tach to NH speedo to be recalibrated they want to know what CDI box you have, so this must affect the input value to the tach.
5. 1978-1983 Tachs. These are driven from the "TD" tachometer drive output from the six-pin CDI box because the points were eliminated in favor of a magnetic reluctor. According to the following thread, the SC tach signal is the same as the later Motronic:SC Thread
6. 1984-1989 Tachs. These are driven from the Motronic box. Chris Bennett posted the Motronic signal here: Motronic Thread
1. 1965-1968 SWB Tachs. These may have VDO number 333.230/041/002, according to VDO, "this is a old version Tachometer with 24 Volt Square Wave signal from Ignition KL.1."
KL is German for "klemme" or contact, and #1 is the points contact, so this is a 24-volt square wave signal that comes off the coil. In another thread, Pelican 914GT tested it and found that the coil output was way more than 24 volts, but the spec came from VDO. Tach Thread These also say "12V 14000 Imp." which I think is the impedance value to make them work? I think all 914 tachs fit into this category as well.
2. 1969-1970 LWB Tachs. These have VDO numbers 333.230/041/007 to 333.230/041/010. According to VDO, these are driven by an "11 volt square wave." Now, we know that up until around 1971 the tachs were driven from an "intermediate unit" which was a silver can with Bosch part number 0 227 990 001. We know that this contains an inductor that is probably used to simulate the original coil to drive the low-impedance tach. Some of these say "12V 24000 Imp."
3. 1971-1973 Tachs. These appear the same externally but have been altered internally to be driven off the high-impedance circuit between the points and the Bosch CDI box. In the early 3-pin CDI boxes, there is a 33 ohm pullup resistor in the trigger circuit that would allow only around 425 mA to flow in the circuit. The tach must be sensitive to 11 volts and 425mA, which is why the later CDI won't drive the earlier tach off this circuit-- the earlier tach needs higher voltage.
4. 1974-1977 Tachs. I know these were driven off the CDI box, but I don't know when the CDI box changed from an "001" to an "008" or when the cutover from a 33 ohm pullup resistor to a 120 ohm occurred. With 120 ohms the input current is only 116mA so the tach must have changed somewhere along the line to match. If you send your tach to NH speedo to be recalibrated they want to know what CDI box you have, so this must affect the input value to the tach.
5. 1978-1983 Tachs. These are driven from the "TD" tachometer drive output from the six-pin CDI box because the points were eliminated in favor of a magnetic reluctor. According to the following thread, the SC tach signal is the same as the later Motronic:SC Thread
6. 1984-1989 Tachs. These are driven from the Motronic box. Chris Bennett posted the Motronic signal here: Motronic Thread
The "Imp"s refered to are Impulses, essentially the number of cylinders firing per minute.
A 30,000 Imp tach is rated for 10,000 RPM max (the racing tachometers) for a 6 cyl
A 24,000 Imp tach is rated for 8,000 RPM max for a 6 cyl
A 14,000 Imp tach is rated for 7,000 RPM max for a 4 cyl
A 30,000 Imp tach is rated for 10,000 RPM max (the racing tachometers) for a 6 cyl
A 24,000 Imp tach is rated for 8,000 RPM max for a 6 cyl
A 14,000 Imp tach is rated for 7,000 RPM max for a 4 cyl
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