Shorted out coil on 911t?

Bench-Racing · 04-30-2006, 07:47 PM

This might have been previously posted but I did not find the answer. Vehicle in question is a 1970 911t. I accidentally shorted the "15" post on the coil to ground. It popped the middle fuse in the three fuse holder in the engine compartment. I replaced the fuse but it popped again when I turned the ignition on. There seems to be a grounding short now from the 15 terminal on the coil. Did I wreck the coil or the CD ingnition?
Thanks in advance,
Charles

ischmitz · ischmitz's Garage

When exactly does the new fuse blow. Once you switch the ignition to ON or when you cranK?. If it blows when you switch it to ON chances are that you have damaged your CDI box. If it is during cranking it might be the coil. Do a search. There are lots of posts for checking the CDI coils with an ohm-meter.

BTW, PelicanParts sells rebuilt CDI boxes in case you find it's the box.

Cheers,
Ingo

Early_S_Man · 04-30-2006, 08:42 PM

Unless your engine wiring has been altered, the middle fuse on the electrical panel has nothing to do with the CDI unit, which is unfused.

Does your CDI unit still whine with the ignition switch on?

The ignition coil primary is only 0.4 Ohms - 0.6 Ohms, and terminal #1 is connected to ground at all times, so it is perfectly normal for terminal #15 to look like a grounded short during static tests!

I suggest reading the following thread, especially the Bosch Test Instructions document:

History of Bosch CDI ... toubleshooting info, parts list changes, and schematics ...

Bench-Racing · 05-03-2006, 10:22 AM

Trouble shooting results: Middle fuse in three fuse block in engine bay still blows. I can't find anywhere that tells me what these three fuses are for, anyone know? Fuse only blows when engine is cranked. Ignition switch can be turned on without blowing the fuse but after a couple of cranks the fuse pops. Coil was tested and swaped out with new coil, no change. No wires are shorted between coil and CDI. Blue wire w/yellow trace seems to be the affected wire. I felt I have given the distributor a really complete once over and that is not the source of the problem. With center coil wire disconnected it still pops the fuse. Opened CDI box and no signs of burn/melt/damage looks like new. Besides combing over the above mentioned links (which I am working on but they are complicated) can anyone point me in a direction? I cannot seem to find the rebuilt Bosch CDI boxes from Pelican. Does anyone have their part number for this? I really appreciate the above two postings and other members willingness to share their practical experiance.
Regards,
Charles

Early_S_Man · 05-03-2006, 01:38 PM

Charles,

The fuse assignments are:

1. 5 Amp -- Sportomatic [or unused]

2. 8 Amp -- Speed Relay/Emission cut-off solenoid

3. 25 Amp -- Rear Window Defogger

Maybe this diagram will help if you can disregard the MFI components ... it is located on this Pelican Electrical Diagrams page:

http://www.pelicanparts.com/911/911_Parts/911_electrical_diagrams.htm

ischmitz · ischmitz's Garage

Here is the link http://www.pelicanparts.com/catalog/shopcart/911M/POR_911M_ELignt_pg5.htm#item22 The part number is A-602-702-00R

If the middle fuse pops while cranking and your harness has not been changed maybe there is something else wrong with the emission relay or the MFI speed relay (given your car is MFI).

It could also be that someone changed the feed for the CDI box. Try cranking without the CDI box connected and check if that still blows the fuse. The original wiring harness uses a fuse in the trunk on item 84 as shown above

Cheers,
Ingo

Bench-Racing · 05-04-2006, 08:40 AM

The car is carburated. I will try to see if the fuse blows when the CDI is disconnected. There is voltage going into the CDI box. Does voltage only come out when the motor is cranking? The paper label inside the 3 fuse block (engine compartment) says the middle fuse is the "Stop-magnet-valve...Start-valve". Maybe I am chasing the wrong thing and this fuse was blown before I shorted out the coil? Should the #15 side of the coil get voltage with ignition on or only when cranking? Is this amplified voltage that you cannot test with a simple test lamp or will it blow out an amp/voltage meter? Can you simply run 12 volts to a regular coil (bypass the CDI) and start the car?
Hang with me, I am determined to get to the bottom of my self inflicted damage.
Charles

ischmitz · ischmitz's Garage

Your fuse seems to drive the cold-start soleniod and seems not to be related to the coil or CDI box.

Early_S_Man · 05-04-2006, 08:54 AM

Charles,

Please do yourself a favor and read the Bosch test document I posted the link to ... and the testing precautions I detailed! You can test for spark at the coil and manually trigger the CDI unit by disconnecting the Black and Black/Violet wires at the distributor and repeatedly ground that Faston connector like a telegraph key ...

The fuse label you found was intended for 911S and 911E cars, as was the schematic I posted above! As I already suggested ... your blown #2 fuse isn't related to the CDI, and, yes, I suspect it was already blown!

You may be able to test for the presence of the +460 Volt signal from the CDI unit at the coil #15 terminal while you do the 'telegraph key test' with a neon bulb type AC-DC tester available at hardware stores in the electrical section ... but it would need to be in a darkened garage.

Bench-Racing · 05-20-2006, 05:34 PM

Sure enough, when the + terminal of the coil got shorted to ground, it blew something out in the CDI unit. I was able to pick up a used but working CDI box through this forum which instantly solved the problem. The blown fuse was a red herring. I then located Systems Consulting, also through this forum and sent my dead box to them for a rebuild. I understand it is good to have a spare. I am looking for the rubber insulating boot that fits over the coil terminal so no more shorts from the shielded spark plug wires, electrical tape and tie wraps will keep it safe for the present.

Thanks to everyone that makes themselves and their knowledge available in cases like mine. It's easy to call the flatbed but it's better to solve some problems yourself so that you get to understand the machine we hold so dear. Especially if you drive it far from home.

Regards,
Charles