Please help: Smoking 308GT4 ballast resistor !

It sounds like the resistor in question has been wired inbetween +12V from the ignition and ground.

In the factory wiring, a yellow wire from the ignition lock is wired to the left terminal of both resistors. A red wire from the resistors right terminal (marked 'D') then goes to the ignition coil right terminal 'B+'.

There is a starter ignition relay which directly supplies +12V to the coil as long as the starter is activated, to help with hot and cold starts. Sometimes this setup is disabled by someone working on the car in the past, which in general is not a good idea There should be two red wires coming from the wire trunk, of which each one is wired to one of the right resistor terminals 'D'.

The brown wire from the wire trunk is coming from the tach and wired to the negative '-' terminal of one of the coils. Not sure about the blue wire, could be an old wire coming from one distributor terminal which has been discarded.

The gray sensor (points) wires were black and azurro/black in the factory wiring.

Both distributors have two points sets each, a standard and a retarded one for the emission control. The retarded points sets usually get discarded, so that would explain why one of the condensers on each dist is not connected (respectively the wire has been cut/broke)

See below for a quick chart with the wire colors. Would recommend that you check all of the ignition wiring and maybe replace the coils/ballast/condensers to have a clean setup to start with.


Good luck,
Adrian
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+1 -- looks like some serious mis-wiring/re-wiring has gone on based on the insulation wire colors in the OP's photos (those red wires don't look like the stock type of wire to me) and the OP's description (amazing that it runs). OP -- Please give your exact year and version 308GT4. If you can sketch up your existing wiring in a manner similar to the figure that Adrian posted, it will probably show you what's wrong.

Each distributor should have 2 condensors -- one with a single wire at one end and one with two wires (i.e., a wire at both ends) if you have a stock R1/R2 point system in place. Do you still have a throttle microswitch present at the 1-2 carb? Also, have you ever looked inside the distributor caps to see if there are point(s) still present or something aftermarket (e.g. Pertronix)?

 

I can confirm that after purchasing an expensive set of NOS/OEM Marelli coils from a reputable dealer quite some time ago, those red wires are original... Even the original coils on my car before that had them. They are solid wire (not stranded).

The Bosch Blue coils would be a better choice as they have built-in resistance.

 

Peter, the Bosch Blue coils won't support the factory wiring for the stronger spark when starting as they don't have an external resistor which can be bypassed by the starter relay. The resistor bypass mechanism compensates for the lower available voltage due to the load the starter motor puts on the battery, that ensures that you still get a healthy spark even when the battery output goes down to 8-9 volt while the starter motor turns.

My personal recommendation for the 308 is the Red coil, as the Blue coils only have about 60% of the spark output voltage the Red coils have.

Best,
Adrian

 

I've had a look at the '75 GT4 NA owners manual, your car did not have the starter relay setup per factory, so ignore the two red wires for now.

The '75 GT4 NA wiring has the two yellow wires going to the coil's B+ terminal, so this is correct.

Correct, blue with a black stripe is Azurro/Nero, in the factory wiring this wire went from one of the points sets to the '-' terminal of the coil supplying bank 1-4 (rear of car), together with the brown tacho wire.

-> Try reconnecting the brown wire to one of the two coil '-' terminal to see if your tacho starts working again.

I don't have that wire on my EU '75 GT4, not sure what it is. Steve would know

The blue wire coming from the dist and going to terminal 'D' looks correct for the US GT4 wiring. I wonder why the grey wire coming from the other resistor terminal 'D' first goes into the wire trunk and then reappears (?) on the other side, going to the other dist.

-> Try removing that 'mystery' wire and wire the resistor 'D' terminal directly to the distributor terminal, where the mystery wire was connected before (similar to the blue wire in the other dist/coil setup). Any change?

 

p.s. wiring diagram for the NA GT4 ignition copied from the '75 owners manual (the tacho wire is not shown for some reason).

R1 is the standard points set. R2 retards the timing via the (F) carb switch as part of the emission control setup when the engine idles. Many cars had their R2 setup removed, this might have been done (partially at least) with your GT4 too.
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I can only add that with no throttle microswitch = no R2 points function = no need for the condensors with 2 wires. You might want to have a look inside the distributor caps. It seems very odd to have any of the wiring between the coils and the distributors go back into the harness (and odd that the two distributor + coils seem wired differently). If you've only got R1 points inside the distributor, your system should be wired:

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PS Note how F's figure recycled something from the 206/246 and (wrongly) shows a six-lobe distributor cam

 

here's the one from the 308 GT4 WSM
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Ignore the Azurro/Nero wires for now and try the second test i mentioned:

Your wiring setup should then be identical to the chart Steve posted.
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I missed that, well spotted

 

from an electrical standpoint the condensers (aka capacitor) form a tuned circuit with the primary coil, and as a spark arrester, not having them properly connected will effect the circuit adversely, and lower the output current voltage to the plug. the coil will provide 22kv~30kv at 12v on the primary.

 

If you turn the ignition switch to the "run" position it would be expected that the resistor would get too hot to touch after 30 seconds or so and definitely smoke after 3 mins if the points happen to be closed at that time. The other resistor probably is not getting hot because the points are open.
If you immediately start the engine (ie not leaving the key in run for a long time first) is one still heating up more than the other? This would indeed not be expected. One cause could be a leaky (ie partially shorted) condenser on that side.

That grey wire, does it look factory, part of the harness? If not it could be a "bodge" install of an immobiliser, which would affect one bank only!

 

the brits are so higbrow dry, we just call 'em hacks


Do you have a flag?....

 

Ok, then we can rule out the mystery wire as the culprit for now.

Another test, this one will check if the points wiring for the disabled (no microswitch) R2 retarded points setup is faulty:
-> On both distributors, please remove all wires going to the R1 terminal except:
- The wire going to the coil '-' terminal
- The wire going to the single-wire-condenser mounted next to the R1 terminal.

See the wiring chart in post #14 as reference for the now still connected wires.

The R1 label is stamped into the body next to the R1 terminal, this is usually the terminal where the single-wire-condenser is mounted.

When you now switch on the ignition, does the resistor still gets hot? Does the engine run as it did before?

 

If the wire which should have gone to one end of the 2-wire condenser is instead connected to the body, that sounds like the problem if the body is grounded via the mounting clip. The result would depend on which end of the condenser it is.

 

I'll get more details posted in a few minutes, but is anyone familiar with pulse forming networks? The coil and points along with the condensers form such a circuit, there are few circuits posted above that don't utilize the condenser 'charge' for the inductive spike on the coil.

 

See here for all the tech info you could want on the dinoplex system, some of the above diagrams seem to be adaptations of this.

Magneti Marelli Dinoplex Repair and Restoration Documents

didn't want this link to get lost in a long post so it's separate for those who may refer back in the future.