Need help fast!!
 

1979 Porsche 928- rebuilt motor, everything has either been repaired or replaced. The engine has been installed and everything is connected. The motor turns over, but no ignition. I have discovered that the 0.4 ohm ballast resistor to the coil has burned out. All other ignition components have been tested and function normally. Fuel system is serviceable.

1).The resistor pulls about 1.1 ohms. Shouldn't it be 0 ohms if it has gone bad?

2). What might cause the resistor to burn out? Could there be a problem elsewhere, or do they just go bad with age/usage?

3). If i used a 0.6 ohm resistor in its place, would this damage the ignition system?

4). Does the positive power lead from the battery, and the alternator lead connect to the same post on the starter (as shown on the wiring diagram)? I've been told this is wrong, though I can't see it working any other way.

Thanks
Down to the wire on this project...

Taking a swing from the hip on this, but .... don't most cars have TWO ignition circuits, full power during cranking, and thru a resistor for normal running?

Yeah, from pin 15 of the transistor ignition module there is a 0.4 ohm resistor in series with a 0.6 ohm, and during cranking the starter pin 16 supplies power directly to the 0.6 ohm resistor.

I suspect the signal wire from the distributor back to the ignition module pin 7 is bad.

As always check relays and voltages the easy stuff first.

I've checked the voltage between terminal 16 on the starter and the resistor. Voltage is adequate. I don't suspect a continuity issue between pin 15 on the ignition module and the resistor, due to the heat generated by the resistor when the ignition switch is on. I'll check it anyway to be sure.

The distributor has been rebuilt, using a new harness to the module. I did check it, however. Resistance falls within spec.

Fuses and relays function normally.

I've tried to cover everything. I just worry that a replacement resistor will burn out like the last, and i won't have time to re-order it. Sometimes it helps to have a fresh perspective on the problem. Thanks for your quick response.

Check and re-check all the simple stuff.

In big projects a LOT of times something simple is usually the culprit.

Maybe its the coil? This is a very simple circuit, not much different from points and condenser except the transistor ignition module should be a more reliable switch than points. The current flow is from the 12v pos side of the battery, through a resistor, into the low voltage side of the coil, out the low voltage side of the coil to some kind of switch. When the switch is grounded current flows, and when the switch is opened (just like points) it makes a big voltage on the high voltage side of the coil.

Swap in a known good coil, and check all the voltages, also check for a spark.

The battery, alternator & starter must be connected together... there should be 2 connections to the starter big terminal - battery & alternator. The alternator big terminal should have this connection to the starter & also a connection to the jump post. The jump post connects to the Central Electric (Fuse/Relay) panel.

Get a replacement resistor of the right value (why screw with something else?) and yes when it fails its likely to go open circuit or higher resistance rather than short circuit. 1.1 ohms seems an add final value though.. maybe the wrong resistor was installed there already? or are you measuring both resistors?

Alan

I measured both resistors while they were disconnected from the circuit. The 0.4 ohm did not check out. I've replaced both and the coil to be safe. What I am still finding is that the .4 ohm gets extremely hot when the ignition switch is on. I've checked for spark using a timing light- no problem there. When I turn the motor over, it rumbles as if it were going to fire but stops after releasing the key. Could the ignition control module be shot?

If a timing light is showing a good spark, maybe its NOT a spark issue?

Verify the big three, spark, fuel, and compression. Pull a plug and see how it looks, and visually check the spark with the plug out.

Whats the voltage drop across the 0.4 resistor? actually measure the voltage at each end relative to ground and tell us that.

On this model (C.I.S. injection), if you're getting any kind of spark across the plugs, but still no ignition, the most common culprits seem to be, in decending order:

Your fuel-pump relay is defective
One or both fuel pumps is not spinning
Your "Warm Up Regulator" is clogged (controls mixture)
Your timing belt jumped a couple of teeth or was installed 180 deg off

Fuel pump relay has been replaced. Fuel delivery rate and pressure were checked and both were found to be in spec. Warm up regulator was cleaned, inspected and bench tested- no problems. Compression check showed between 160-163lbs. on all cylinders. Valve timing has been checked many times, I don't suspect an issue there. However, I was wondering about how the distributor is installed.

Number 1 piston must be at TDC, as indicated by the alignment of the pointer with the mark on the vibration damper. Then the distributor, with the rotor pointed toward #1 can be slid into the cam assembly. I did this with the the cam sprockets aligned with their timing marks. Is this correct, or should the crack be rotated one full turn so that the cams are 180 degrees from their timing marks?

I'm not with the car at the moment and don't expect to carry on with it for at least a month. I'm just responding to keep this thread current. Thanks for the responses. I'll let you know what happens.