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914World.com _ 914World Garage _ 914 Tachometer not working
Posted by: vin man Feb 7 2017, 03:20 PM
I've done some searching and have not yet found a thread that I'm sure has been covered at some point.
The tach in my 72 does not work. The car was converted from fuel injection to carbs long ago. I believe the reason the tach does not work is because it is no longer getting a signal from the computer (no longer have an ECU in the car).
How does one use the factory tach when I no longer have FI? In other car projects, I just ran a line from negative side of the coil to the back of the tach. But I don't think that will work here, will it?
Posted by: timothy_nd28 Feb 7 2017, 04:19 PM
There is already a black with purple stripe wire that runs from the tach to the negative terminal of the ignition coil. All 914 tachometers read the tach signal from the ignition coil, not the computer.
So back to your troubleshooting, there is only 3 wires needed to make this tachometer work. The two outside terminals on the tach should have a brown wire while the other wire should be red with white stripe. These are the power wires. You should measure 12vdc between these two wires with the ignition switch is set to on. The middle wire (black with purple stripe) is the signal wire. You will want to ohm this wire from the negative terminal of the ignition coil to the middle terminal of your tachometer. You should get 0 ohms.
If all that fails, you may have a bad tachometer.
Posted by: The Cabinetmaker Feb 7 2017, 04:28 PM
The djet cars have a large black power wire, a small black with purple stripe (tach), and a green wire from the condenser. Condenser and tach go on the same side.
Posted by: vin man Feb 7 2017, 04:28 PM
Thanks. On my way over to test as you suggest.
Posted by: vin man Feb 7 2017, 07:06 PM
According the repair manual I have, the wire from the back of the tach (black with purple) goes to a "ballast resistor". I've read in other threads that in later models, the resistor is internal to the tach. Or maybe the ballast resistor only used on the 914 /6.
Anyways, when I tracked the black/purple wire back, it went to the relay board in the engine compartment. There was not a black with purple wire coming off the negative side of the coil, but instead a green with red. I disconnected the green with red and spliced a wire right into the black with purple wire.
Tach is now working! Thanks everyone.
Posted by: The Cabinetmaker Feb 7 2017, 07:52 PM
Green with red is the oil pressure sender.
Posted by: vin man Feb 7 2017, 08:02 PM
That's right. Maybe the wire for the tech was green and black. I don't remember for sure. But nonetheless it works now
Posted by: timothy_nd28 Feb 7 2017, 08:39 PM
I'm just curious, where was the black and purple wire plugged into?
Posted by: rdauenhauer Feb 7 2017, 10:02 PM
Tim is there a way to simulate the signals to "bench" test the tach?
Posted by: vin man Feb 7 2017, 10:09 PM
The black with purple stripe wire came to this plug on the relay board. You can also see the gray wire I spliced into it and ran over to the coil.
http://s304.photobucket.com/user/e065206/media/Our%20Cars/1972%20Porsche%20914/wire2.jpg.html
http://s304.photobucket.com/user/e065206/media/Our%20Cars/1972%20Porsche%20914/wire1.jpg.html
Posted by: timothy_nd28 Feb 7 2017, 10:18 PM
20 bucks in Radio Shack parts and a Iphone (or similar), yes you can. I built my own signal amplifier a few years back, and it still works great.
You need to amplify these following frequencies: 33.3, 66.6, 100, 133, 166, 200hz
Each frequency correlates to engine rpm. 33.3 hz is 1000 rpm, 66.6hz is 2000 rpm, 100hz is 3000 rpm and so on.
The iphone has many good apps that you can download which will simulate these frequencies, but the amplitude of these frequencies is the problem as you will only get 1.4 volts from your phone's headphone jack. The tachometer needs to see atleast 30 volts before it will start moving. Buying a few transformers and a very standard JAN2222a transistor, you can build a circuit that will amplify the 1.4 volts to around 60 volts, enough to drive the tachometer.
If you are interested, I can get you a schematic.
Posted by: timothy_nd28 Feb 7 2017, 10:24 PM
The black/purple stripe wire should enter on the 14 pin connector at pin 7. It continues thru the relay board and exits pin 5 of the 12 pin connector.
You should check the 12 pin connector on the relay board, if you take a peek you will see a black/purple stripe wire. Follow it, see where it goes. 
Posted by: vin man Feb 8 2017, 09:32 AM
The black/purple stripe wire should enter on the 14 pin connector at pin 7. It continues thru the relay board and exits pin 5 of the 12 pin connector.
You should check the 12 pin connector on the relay board, if you take a peek you will see a black/purple stripe wire. Follow it, see where it goes.
Is the 12 pin the one closer to the firewall? That is the one that I spliced on to and ran to the coil.
If not, I will look to see if I find a black/purple wire on the other connector and see where it goes. It definitely wasn't on the coil.
Posted by: timothy_nd28 Feb 8 2017, 10:01 AM
12 pin connector is not the one by the firewall
Posted by: vin man Feb 8 2017, 10:15 AM
12 pin connector is not the one by the firewall
Ok, thanks. When I go over to my workshop tonight after work, I will trace the black/purple wire out of the 12 pin connector and see where it goes.
Posted by: vin man Feb 8 2017, 10:18 AM
12 pin connector is not the one by the firewall
Ok, thanks. When I go over to my workshop tonight after work, I will trace the black/purple wire out of the 12 pin connector and see where it goes.
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