912 Electrical Problems

From: Patrick Van Asbroeck; Sun Jan 14, 2001; p.asbroeck@uk.betalasermike.com;

I had my share of electrical problems to solve...

The battery was always a bit low at 12.5V. I replaced the battery. No improvement. Then I checked the dynamo:

1. Remove both wires on the dynamo, check the brushes and their springs.
2. Make a link between both electrical studs on the dynamo
3. Connect a voltage meter between the link and chassis
4. Run the engine. The voltmeter should indicate a voltage well above 14V. If I remember right it went up to 20V. If it doesn't get to this voltage then maybe the field coil is dead. To check:
5. Measure the resistance between the DF stud (field contact) and chassis. (link removed) I measured 6 ohms. Replace the coil if you measure a too high resistance.

It's also possible the field became demagnetized or reversed. To get the field right again leave the link in place and make momentarily a contact between the link and the plus from the battery. The dynamo will now work as motor AND should rotate CW. If it rotates CCW you need to swap the field wires.

Field coils are easily replaced once you get their mounting screws loose with an impact screwdriver Note I got the VW style Bosch dynamo as fitted to late 912s. Part number 0101 302 111. (14V 30A 420W)

The dynamo was OK.

Then I checked the regulator. The regulator was not original, had no brand name on it, was black painted and looked identical to the original Bosch one. It was only marked "made in USA". The field resistor was 8 ohms.

It's easy to check the regulator.

1. Refit the wires to the dynamo from the previous test
2. Remove the BIG crimp connector (B+) on the regulator. This wire goes straight to the battery so make sure it doesn't touch anything while loose.
3. Connect a voltmeter to this B+
4. Run the engine. You should measure a voltage around 14,5V regardless of rpm. If not replace it. It's not worth trying to adjust these regulators. They are not expensive around 50$.

Part number 0 190 350 068. The Bosch field resistor is only 2 ohms!

Always note your dynamo type for the Bosch guys so they can give you the correct regulator. The Bosch technician told me these mechanical regulators don't last any longer then 50000 km.

The battery charge was OK now.

The first time I drove with this new regulator it became quite hot! This was normal as we had to charge the battery from a low 12,5V. Charge current must have been high. BUT still she wouldn't start as should be. At first try that is! The problem was found in a bad contact (what else?) in the grey connectors under the dashboard. About 1V loss! It can make a difference. I fitted a wire straight from the fuse panel to the key switch passing the grey connector. Problem solved, finally...

One way to find bad contacts in the ignition circuit from battery to ignition coil:

• Disconnect the battery + wire and connect an 12V , 1A power supply instead to the + wire. Your car is now fed by the power supply. An old PC power supply is just fine for this. Don't use anything else but a 12V supply or you risk burning a few things!
• Disconnect the ignition coil's red wire and fit a 15 ohm 5watt resistor between the red wire and chassis. These resistors are cheap and available at any electronic part shop. Solder a wire at each side of the resistor with a crocodile connector to ease installation.
• Take a voltmeter. Make one 5 meter long test wire. Fit this wire between the + of the 12V power supply and the + input of your multimeter. With another test wire fitted to the - input on your voltmeter we now measure each point in the chain and can see the actual voltage loss they cause. The 5 meter wire makes it easy to go to any point around the car you wanna check. For the ignition circuit you will pass the fuse panel, ignition switch and grey connectors under the dash board. You should get about all of the supply voltage to the + wire from the ignition coil. I wouldn't accept more then 0,5V total loss measured between the battery + and the + wire at the coil.

This technique can be used to fault find ANY circuit in your Porsche. To find bad contacts, current must flow through it to create a voltage drop. No current, no voltage drop. Just like water flowing through a tube. This method works better then measuring the actual resistance.

I like the Fluke 10 series meters for this work. Not too expensive and more then accurate enough for this task.

Also ALWAYS solder all crimp connectors. I've seen too many bad contacts on crimped connectors! I cut off the plastic from the crimp connectors, then make the soldering and put a heat shrink or rubber sleeve around it. Frequently the solder will not flow nicely on the copper of the wire because it's oxidized. If the copper has a green coating on it then that's the oxidize. Its normal to have this when copper comes in contact with moisture but luckily it doesn't affect the electrical conduction. It only makes soldering difficult.

To get rid of it cut off an inch or so and strip it, if your lucky you'll get back to nice clean copper. Mostly this isn't the case. What else can we expect if we got the original sixties wiring in place. File off the oxidize to get rid of it.

On the ignition coil the connectors can fall off as they hang upside down. To prevent this I use crimp connectors with a push-in lock inside them. Don't know their part number.

The connectors for the reverse gear switch on the gearbox were another problem. The AMP ones are just too small in diameter to fit right. Instead I found the connectors used on LUCAS regulators the right ones. These regulators were used on most of the English motorcycles in the fifties.

The mechanical ignition points were replaced by a Petronix unit part number 1847V. It works fine and is easily installed.

The spark plug cables (copper) were replaced by resistive ones from SUPERLEAD part number 1KS490S. Actually this is the set for a VW. The plugs were changed to BOSCH. I did this on the advice from the local dynoman and he is good! Three of the first four cars arriving in a Belgian 24 hours race were tuned by him. Say no more...

The resistive spark plug cables help to get a higher voltage which helps to prevent plug fouling. The only problem with them are the spark plug connectors. They are DIFFICULT to pull off . I cut them off and replaced them with the bakelite Porsche style ones. Works fine.

The only spark plugs the dynoman found worth using are Bosch and NKG.

While on ignition, check the timing marks on your pulley! I had a pulley (VW?) where only the OT mark was correct! I filed marks every 5 degrees to the right hand side of the OT with a degree rule.

The clock still worked but ran too slow. They are adjustable from the backside. You must remove them to get to the adjustment screw but one advice: DON'T pull off the supply connector without supporting the white plastic cover! The clock is fitted on three anti-vibration rubbers with E-type circlips. They can slip off if you pull too hard. Then the circlip falls in the clock housing and can't be removed without taking off the clock front panel. This front panel is "crimped " onto the clock body which makes is VERY hard to remove. Something I don't wanna do yet as I'm afraid of destroying it. Instead I installed a quartz clock found at a Porsche autojumble. It looks identical except it has quartz written on it. This clock is always on time.

Being a technician I'd like to solve technical problems but my 912 sure has given me more then enough things to think about... Hopefully my experiences can save you 912ers some search time.