1978 SC won't start

[911 wide]

HI, I have just purchased a 1978 SC project non runner.
I have cleaned the fuel tank,changed the fuel pump/filter and relay and now have fuel to the injectors. I can hear the CDI unit but there is no spark. Can anyone help?

[theiceman]

could be the coil . is it Silver ? if it is it is definitely the coil.

[yelcab1]

just because it buzzes, does not mean it works. Figure out the CDI first.

[Bob Kontak]

Originally Posted by yelcab1

just because it buzzes, does not mean it works. Figure out the CDI first.

There is some economic logic to just dropping in a new coil, even if it's not the answer.

Borrow one from the neighbor's 67 Chevy sort of logic.

It's a shortcut, surely. But not the worst "throwing parts at a car" error given the meager cost.

[T77911S]

no, shot gunning a coil is not always a bad idea. if they are both good, then you have a spare to carry with you because it eventually will fail.

you can get a MSD for about $60.

[ischmitz]

A humming CDI could still be bad. As a first measure check the coil's primary and secondary resistance. I don't have the exact numbers at the top of my head but it's something around 1.0 Ohm (give or take) for the primary, and 1 - 1.5 kOhm for the secondary. If one is open it's the coil.

Next, it could be the trigger wire from the distributor to the harness. This is a COAX cable and if there is any fraying chances are it's bad. Unfortunately, testing the CDI box is not really straight forward unless you want to build small extension cables for the 6-pin connector. In that case you need three wires (GND, +12V and COIL) to go between the 6-pin harness connector and the box. Then connect another set of wires to the second GND and trigger input of the box. Finally connect a spare spark plug to the coil output and ground it to the engine.

With power to the box you can trigger the box with an AA battery:

+ of battery to GND of box
touch - of battery to TRIGGER of box needs to release a spark

A word of caution. A damaged box is able to put out up to 450V onto the coil primary (if the coil primary is open). So once the system is humming it CAN be live. That's the reason for the yellow warning sticker on the box.

Ingo

[G450X]

Confirm that it is a no spark issue, I thought it was a no spark issue, I swapped coils, etc., with no luck on my ‘82 SC . I looked in low light and saw a little spark.

It turned out to be a blown out pop off valve (old hard glue/epoxy) after a very mild backfire - so mild I didn’t think it could be the pop off valve, and I had a pop off valve which is supposed to prevent this issue...

[SalParadise]

In many instances in my life, it's been as simple as the coil wire - either corrosion or breaking down (or not stuck in all the way).

[JSV798]

Six months ago my '78SC refused to start. So first confirmed no spark by using a spark plug resting on the engine. Then followed CDI checking procedure in Bently Manual; photo of relevant page:



My understanding of spark generation on this car is as follows: the distributor generates a small voltage pulse which you can check by following above. The CDI transforms this to 400 volts which is then relayed to the coil which ups it to 20,000 volts. There could be a problem anywhere on this route. For example there is a connector to the green wire behind the distributor so you can also check there for the pulse if you don't get it at the CDI end of the cable. In my case it was the green wire: Bentley suggests distributor is faulty in absence of pulse but in my case it was the green wire, confirmed by pulling the distributor and bench testing: distributor gave a pulse when rotated by hand.

Roy