Spark only when cranking - 260Z

[Jeff G 78]

Here's the background. My barn find LeMons car is a 74.5 260Z that hasn't run in 15+ years. I swapped the carbs to SUs, cleaned the fuel system, tore the engine down and installed new gaskets. It fired right up and ran great (no tuning yet). That was Monday night. Tuesday night, after touching nothing, we went to work on the car and cranked it over. It fired right up, but died as soon as we stopped cranking. I got out the FSM and traced every wire back. I found that the black/blue wire is hot when cranking which bypasses the secondary resistor and gives the coil more voltage. That all checked out. Here are the specifics from testing:

Battery +12V

Black/white wire at (+) end of resistor +10.5V with ign on

Ballast resistor 1.4 ohms end to end - FSM says 1.2 to 1.4 ohms

Ballast resistor 1.0 ohms secondary ((+) end to center)

Ballast resistor 0.4 ohms primary (center to (-) end)

Black/blue wire (center post of resistor and hot wire during crank) +3V ign on

Black/green wire ((-) end of resistor) +3V ign on

Coil (+) terminal +3V ign on

Coil (+) terminal +5.5V while cranking though tough to get steady reading in short time during crank

+3V and +~5V seem very low. I thought these were supposed to be +6V and +12V respectively??? If so, why are they so low?

The resistor info is lumped with the coil info on one simple page in the FSM and is states that the Primary resistance range is 0.45 - 0.55 ohms and the secondary resistance range is 8.5 - 12.7 ohms. Where are these measured. I couldn't tell. Is this the coil from the (+) to the center terminal and the (-) to the center terminal? That confused me and it never shows how to test or where.

FYI, the resistor gets very hot, very quickly with ign on. Is this normal? I know it will get warm, but it seemed extreme.

The car fires right up during crank, but dies immediately when not cranking. The FSM says next to nothing about testing the coil, resistor, etc. We are in a huge time crunch and need to fix this ASAP.

Can someone tell me where to look next? The FSM listed the resistor at 1.4 ohms which it is. The car does fire, so the transistor ignition unit must be working and the coil must also be good for the same reason. That leaves wiring and the resistor, but the resistor tests what the FSM says. The wiring is a disater in the car, but we traced everything ignition-wise back and it is wired per the FSM. From the ignition switch, we have power going to the resistor and transistor ignition, and while in the start position, we have power going to the starter and center terminal on the resistor. What am I missing???

If the problem is the resistor, can I fix it with autostore or Radio Shack parts? I don't have time to wait for new replacement parts to arrive. The race is in a week and we have a hundred other things to finish as well.

What exactly does the resistor protect? Is it the coil, or the transistor ignition unit? It's frustrating since it worked one night and then didn't the next.

[geezer]

Jeff, I have the Ignition Systems manual for the 260Z. Here are a few pages that cover the Volt-OHM Meter checks that may help.

[geezer]

Highlighted diagrams. Sorry I can't offer a solution. I vaguely remember having the same symptoms on a GM car that had lost the ignition sense signal.

[Jeff G 78]

Thanks Ron. I will take a look at them and see if anything doesn't check out. The 74 FSM doesn't have any diagnosis in it. What manual are these from?

[sblake01]

That appears to be from the 260-Z Diagnostic Procedures manual. There were a couple other 260Z suppliment type manuals out there also. A 260Z carb adjustment manual and a fuel system modification plus program manual IIRC. I think the three of them are on my other computer.

[geezer]

Jeff, if it comes down to replacing the transistor unit, you could PM beezee to see if he still has an extra one available. If he does and you need it, I can help get it to you on time.

http://www.classiczcars.com/forums/showpost.php?p=286438&postcount=8

[Jeff G 78]

Update: I checked everything per the sheets Geezer sent and everything is within range. If I only follow the manual, I'm fat dumb and happy, but in reality all is not well. I am pretty sure I have a back feed from somewhere because I figured out how to make it run, but when I do, there are consequences.

If I unhook the wire going between the "start" position at the ignition switch and the center terminal of the resistor, I go from 10.3V at the resistor (<3V at the coil) to 12.5V and the car starts and runs. When I do this, however, The resistor does nothing and I get 12.5V at all three terminals as well as the coil. I tried every combination of removing wires to find the culprit and it didn't matter. I still get 12.5V at the coil. As soon as I hook the ignition-to-resistor wire back up to the center terminal, the voltage drops to 10.3 at the input terminal of the resistor and 2.5V after the resistor and at the coil. I'm completely stumped why this is happening, but it has to be back feeding from somewhere. Why would the ballast resistor read 1.4ohms (1.0ohms+.4ohms) across the terminals, yet not do anything if the wire is not connected between the center terminal and the start position of the ignition (note that I don't have an ignition cylinder, I am using a HD toggle switch and pushbutton starter and the wiring has all be checked per the FSM).

I can leave that wire off and the car runs, but then I am running 12.5V through the coil at all times. Do I really need to run the ballast resistor? My car is a late 260 without points. What does the resitor protect? I have heard of people running without them if they don't have points and others say the coil will burn out quickly. I have also heard that a new, high power coil doesn't need a resistor. I have a Fireball coil on my '78 280. If needed, can I run that so I don't have coil problems? What about the transistor ignition unit and pickup coil in the distributor? Is there any risk to them if I run without a resistor?

I don't have time to tear the harnesses apart and check every wire, I just need it to run and not crap out halfway through a 24hour race. Chasing problems in the middle of the night doesn't sound like fun.