karl ruiter

I'm bringing the 79 back to life after a couple of years and going through the usual fun with fuel lines, FD, WUR, inlet filter etc. Gradually getting there but not quite there yet. One thing I'm wondering about is whether my spark is as hot as it should be. I have a document that says the minimum voltage on the positive side of the coil is 5v and the maximum on the low side is 2v. I'm at 4v and 1.7v
My 4v comes from the following:
12.4 at the battery
12.14 at the jump post
11.77 at panel (where the three big wires from the jump post come in)
11.68 going to the switch (30 on A1, A2)
11.28 from the switch (15 on A4,A5)
10.76 going to the coil at the panel (on O6)
10.0 at the ballast resistors
If my ballast resistors are 1 ohm and my coil is 0.5 ohms (hard to get good measures this low) and the negative side of the coil is 1.7v, I should see 4.4v. I am actually seeing 4.0 so I am guessing the coil or ballast resistors are different than what I have above.
But even at 1.0 and 0.5 for the ballast and coil resistance, with the coil negative at 1.7 I would need to have at least 11.6v going into the ballast resistors to get 5v at the coil and my tired old wiring is giving 10.0 which is not all that close.
If I was losing voltage at one specific connection in the path I could attack that, but I am loosing it pretty much everywhere.
Rather than tearing up the whole wiring buss I would probably put in a relay that switches off the existing ballast resistor voltage signal and relays the voltage direct from the jump post to the ballast resistors.
My main question is: do I need to bother with any of this? If other CIS cars run well with coil voltages of 4v ( or perhaps, more importantly pos side to neg side differences of 2.3) or less I can just let it be and go back to fiddling with my control pressures.
But I do have a nagging feeling that this car is more sensitive to timing and mixture than it should be and that a weak spark might be the reason.

jpitman2

Pet says the ballast resistor is 0.6 Ohm. Have you cleaned all the grounds, and checked the battery earth lead for corrosion ? They can look ok, but be bad inside. The 14pin connector at the hot post is also worth a good clean.

Mine is CIS, but ignition has never been a problem, so I have never check for volts at the coil.
jp 83 Euro S AT 56k

WallyP

The standard system bypasses one of the two ballast resistors at start-up.

Life would be better if you did a systematic and thorough cleaning of all of the electrical connections, including all fuse connections and ground point connections. Here is an old post on the subject.

Sometimes it seems as if the 928 is susceptible to a lot of electrical problems. You can forestall many of these problems by doing preventative maintenance on the electrical system. While I refer to it as "annual" maintenance, once every five years will probably take care of most problems.

1) Open the hatch and remove the tool panel. Remove the carpet and spare tire cover. Remove the spare, tools, etc. Open the battery box.
2) Disconnect the ground cable from the chassis. Remove the negative battery terminal. Remove the positive battery terminal. Remove the battery hold-down. This is a curved piece of sheet metal, held by one bolt. One end holds down the lip at the bottom of the battery. Remove the battery. Use extreme caution to avoid getting battery acid or residue from the top of the battery or cables in your eyes. Goggles are recommended. If the battery brushes against your clothes, you can end up with holes.
3) Clean the battery terminals until they are bright and shiny. Dull gray won't do. If it is possible to check the fluid level in the battery, do so. Fill to the rings. Wash the battery. Put it on a charger if possible.
4) Clean the battery box, removing all corrosion. Wash the battery box - baking soda neutralizes the battery acid. After it dries, touch up the finish as required.
5) Clean the battery cable terminals (all three) until the connection area is clean and shiny. Dull gray won't do. Clean the connection area on the chassis. Carefully check and clean any other connections on the positive battery terminal. On later cars, these are very critical power supplies for injectors, fans, etc.
6) Clean the electrical connector in the right forward corner of the spare tire well. Depending upon the model year and transmission, this connects speedometer drive, neutral start switch, gear indicator lights, etc., etc. This is an important connector.
7) Raise the hood (bonnet). On the right fender panel, find the jump start terminal. Remove the protective cap from the jump start terminal and pull the protective plastic cover from the terminal. If you are missing the cover or the cap, replace them - they are critical items. If there is no cover, water will enter the connector and corrode the connectors and wires, causing serious electrical problems. There should also be a short piece of plastic trim clipped to the edge of the fender flange just above the terminal to divert wash water and rain water from the connections. If the connections are clean and tight, replace the cover and cap. If the connections show any trace of corrosion or any looseness, disassemble, clean and reassemble. Much of the electrical power for the car flows thru these terminal connections.
8) Find the fourteen-terminal connector near the jump start terminal. This is a common trouble spot. While I suggest that you disassemble and clean these connections, be aware that the wires can and will fly in all directions if you are not very careful. I would suggest digital photos before and during disassembly! Clean all of the connections, and check for corrosion running up inside the wire insulation. Reassemble the connector. (In an emergency, you can jump power from the jump start terminal to the yellow wire in the corner of the connector to operate the starter. Be aware that the starter will operate with the car in gear, and the car will run over your foot.)
9) Jack the right front of the car and support it so that you can SAFELY work underneath.
10) Remove the ground cable that runs from the right side of the engine to the chassis. Clean both ends of the cable, and check for corrosion in the cable. Clean the connection points, and replace the cable.
11) Check the heavy cable connection on the starter solenoid for damage, looseness or corrosion.
12) Check the moderately heavy wires on the same connection for corrosion and breaks inside the insulation. These carry the charging current from the alternator and from this point to the jump start terminal.
13) Check the small yellow (may look brown, cream, etc.) wire connection for corrosion and tightness. This wire triggers the starter solenoid.
14) Lower the car. Fold back the carpet and open the central power panel cover.
15) Check the main power connections at the top of the panel for tightness and corrosion.
16) Check the ground connections above the panel for tightness and corrosion.
17) If you do not have a diagram of the fuses and relays, go to the Tips section of our web site ( www.928gt.com )and print the correct chart for your car. If you have round fuses, carefully clean the fuse clips, and make sure that the fuses fit tightly. If you have the newer plastic fuses, remove each in turn and make sure that the fuse connectors fit tightly in the connectors. Check each fuse for the proper value, and examine them visually for damage or corrosion. Any trace of overheating of the fuse or its connector is a definite indication of a bad connection, and must be corrected. NEVER use cheap fuses! Cheap imported fuses have failed to blow under overload conditions, causing damage and fires. While you can test fuses with an Ohmmeter, it is easier to test them with power applied. Later cars have a fuse tester built into one of the relays. You can touch the tiny metal bits on the face of the plastic fuses with one probe of a voltmeter/multimeter (other probe grounded) - you should have power on both bits or on neither bit, depending upon whether the circuit is powered. You can do the same test on the older round fuses by touching each end in turn with the ungrounded probe. Again, both ends or neither end should have power.
18) If you feel that it is necessary, remove and reinsert each relay. This won't be easy, as they fit very tightly in some cases. Porsche sells a relay removal tool, but I am not impressed. It is basically a pair of pliers with square jaws, and will pull the relay apart. One tool that works well is a paint can opener, usually available free at a paint store. This is a small tool made of bent steel wire, with a loop on one end and the other end having the last 1/8" bent over at a right angle. You can put this angle under alternate bottom edges of the relay and lift it out - IF the battery is disconnected!
19) Close the panel and replace the carpet.
20) Replace the battery and install the hold-down. Install the positive cable. Install the negative cable. Put a drop of oil on each nut. Connect the chassis end of the ground (negative) cable. Lubricate the battery box latch and close the box.
21) Lubricate the wiper drive shaft and hatch latch mechanism.
22) Clean the spare tire, jack and tools. Put a couple of large heavy plastic garbage bags in the spare tire well for emergencies, along with a tube of hand cleaner and some paper towels. Nitrile gloves are really nice to have. Install the cover and carpet.
23) Lubricate the jack, and put it inside the tool panel. Clean and lubricate the tools. Reinstall the tool and tool panel.
24) Reset the clock and radio presets.