Power to Ground Short - What did I break?

[GAJon]

So of course my first post is because I screwed something up on trying to install some parts on my (new to me) 1976 911S Targa 3.0 conversion. The engine is from a 1983 911SC 3.0.

A little backstory: I bought this car sight unseen from BAT a few weeks back. Overall I've been pretty happy with the condition of the car and have been doing some routine maintenance items to set a baseline for things that need replacing regularly.

One of the more cosmetic items was replacing the bulbs in the dash instruments, which I opted to go with the LED kit from CarMagic. When installing one of the bulbs in the speedometer, the small positive wire got bent a little to the side and was contacting the bulb ground which is the same as the speedometer housing ground.

I reconnected power, turned the ignition on (car starts), and pull the light switch. The lights come on but very shortly see some smoke from the back of the speedo. Quickly turn off car and disconnect battery.

Found the offending light socket and fix it. Also verify the other lights are fine. Reconnect the battery and turn ignition on (but don't start car). All lights works and install looks great.

Clean up tools and reassemble dash. Go to start the car and fuel pump starts and starter motor works fine, but no ignition. All electronics seem to be working (lights, radio, flashers). There is nothing that I can tell is wrong from the start up from before the bulb update to after. It sounds the same minus the engine not actually firing.

So I start the diagnostics. Swapped relays around and verified they all work. Checked all fuses and have no blown fuses. CDI box has high pitched constant whine that seems to indicate the HV side is working. Check the coil and shows the correct resistances. Pulled a BMW spark plug from another car and tried it in one of wires. I get a spark but it is yellowish red, not blue. I'm going to try and get a stock plug tomorrow and see if the spark changes with proper type and gap.

So my main question is, even with the whine and yellow spark, could the CDI box still be bad? Is it possible power was applied to the CDI Signal to Tachometer wire? What other systems could have been killed by applying power to ground?

This car is very basic. Everything is manual, no AC, no power brakes, no power windows. It does have the emissions pump, but that looks all mechanical. So a car not starting after an electrical accident seems to only point to the CDI system to me.

I appreciate the help and the forum (and host). Obligatory first post pic from a pizza run a few nights back.

[timmy2]

Welcome aboard, car looks good.
What color is your coil? Silver? If so, change it to a good one. (Search silver Bosch coils)
How charged is your battery?
Very unlikely that anything you did with the Gauge lights affected the ignition system.
Completely different systems.

[JSV798]

Check for spark by unplugging a plug lead, inserting spare plug, rest on engine (for earth) and get assistant to crank engine. No spark on plug means problem in ignition.
Do you have the Bentley manual? Check for a pulse from the distributor as described on page 280-9.

[GAJon]

Originally Posted by timmy2

Welcome aboard, car looks good.
What color is your coil? Silver? If so, change it to a good one. (Search silver Bosch coils)
How charged is your battery?
Very unlikely that anything you did with the Gauge lights affected the ignition system.
Completely different systems.

The coil is black. I checked the resistances on it and they seemed within spec. ~600Ohms between two small leads, ~0.6Ohm between center lead and outside lead.

Battery is sitting at about 13.5V no load, put it on the charger over night as I drained it some trying to get the car to crank. Voltage drop when normally connected is only ~0.2V or so. So don't think there are any more shorts.

Originally Posted by JSV798

Check for spark by unplugging a plug lead, inserting spare plug, rest on engine (for earth) and get assistant to crank engine. No spark on plug means problem in ignition.

I tried this and got a yellowish red spark. Seemed sort of weak, but it was still sparking. I don't have the Bentley manual yet, but will need to get one.

[Mark Salvetti]

I’m not going to be much help on your electrical issues, but I find it interesting that your car has the emissions air pump. A 1983 SC didn’t use one, unless you have a Euro motor.

Mark

[yelcab1]

I have gotten a bad CDI box for no apparent reason. Take it out, disassemble the cover to see if there is anything burnt inside. It will be obvious.

[GAJon]

Originally Posted by Mark Salvetti

I’m not going to be much help on your electrical issues, but I find it interesting that your car has the emissions air pump. A 1983 SC didn’t use one, unless you have a Euro motor.

Mark

I got my years mixed up. Just checked the engine case and its serial is 6580546. So a 1978 3.0 motor specced for California market.

[GAJon]

I pulled the CDI box and didn't see any obvious area where the smoke came out. But using the link below I did a quick check and comparison and found that D1 is shorted out. It also doesn't look like D6 and D7, so maybe a rebuilt unit.

I know enough about circuits to understand the components, but not enough to understand the impact of having this part shorted. I can only guess that T1 is not behaving as it should.

All other diodes checked out OK and the resistors all look fine.

I guess best thing to do is try swapping in a known good CDI and hope for the best.

6-pin SC/Turbo CDI unit repair documentation

[fintstone]

How did you determine that D1 was shorted? Did you unsolder it and take it out of the circuit?

I would suggest that your CDI is likely just fine and it has something to do with what you were doing under the dash.

I suggest that you look under your dash at the round, six pin connectors plugged into the bulkhead directly behind your ignition switch. Check to see if one is pulled or pulling out. If not, unplug them all, one at a time and plug them back in. and then give it another try.

If not, check all your fuses again. Not by looking at them or swapping them, but by measuring the voltage on the terminals at either side of the fuse.

[timmy2]

What color wire was the offending bulb at the speedo attached to?
Black/Blue or White/Green?

[Dpmulvan]

Just curious how did you test D1?

[GAJon]

Originally Posted by fintstone

How did you determine that D1 was shorted? Did you unsolder it and take it out of the circuit?

I would suggest that your CDI is likely just fine and it has something to do with what you were doing under the dash.

I suggest that you look under your dash at the round, six pin connectors plugged into the bulkhead directly behind your ignition switch. Check to see if one is pulled or pulling out. If not, unplug them all, one at a time and plug them back in. and then give it another try.

If not, check all your fuses again. Not by looking at them or swapping them, but by measuring the voltage on the terminals at either side of the fuse.

I've been tinkering away today to try and knock out issues. I checked D1 with a diode check on my DMM on the board, haven't desoldered anything yet. D1, D6, and D7 should all have same specs, however D6 and D7 show a consistent 0.6V voltage drop and do not conduct when reverse biased. D1 behaves like a short, conducting both ways and no voltage drop. Also D1 is about half the size of D6 and D7, so it's probably something someone replaced. D1 being dead would also support a weak (yellow) spark from the spark test because it's basically shorting the boost converter. So Vbe will remain 0 and the transistor won't oscillate.



I checked the fuses for continuity but haven't done much of a deep dive on that panel. The only circuit that isn't working as it should is the fog lights. That's on the project list but not a high priority right now. When the car was running, they were still broke, so I'm not overly concerned about it now.

Originally Posted by timmy2

What color wire was the offending bulb at the speedo attached to?
Black/Blue or White/Green?

The black/blue wire for the instrument lights initially. I pulled the ground wire off of all of the gauges to see where else it was shorted and I found a few where the insulation had melted away. I separated each of the wires back to where the wire is bundled together and measured resistance to ground for each dash wire. Nothing was under ~2 Ohms. Also checked the dash wire ground to another ground and it measured at around ~0.4 Ohms, so I think the dash ground wire is at least isolated from the other dash circuits.


Given all of that, I did try to use the CDI again after making sure the dash wires were not shorted and the car still behaves the same. Starter cranks, fuel pump runs, HV portion of CDI hums, but no ignition.

I ordered a replacement diode for D1 and will try replacing that next before I try a new CDI unit. I'm reasonably certain that D1 is the issue, but am still figuring this car out some.

[GAJon]

So I got impatient and tried a new CDI. I was planning on using a CDI+ from Classic Retrofit in the future, so I swiped the credit card and got one a little faster than I expected.

I checked the wiring again for any issues and everything appears as normal. Plugged the new unit in and still not firing. Error checks continue:

-New CDI+ does a tacho sweep from 0-7k-0 on key on. So it appears the TD and tachometer are responding as expected

-CDI+ has status LED at key on and during ignition that flashes green, so it appears to be working.

-Tacho goes to around 1k and bounces softly to 3k which seems odd, but not sure it would affect the starting of the car

-Pulled the center lead off of the distributor and connected it to a BMW plug. Fired that and got a decently healthy spark. Brighter and more blue than the old spark, so I'm pretty confident the ignition system up to the coil is working fine.

-Pulled plugs 4 and 5. Can smell gas a little, but difficult to tell if it's just residual or not. Plugs look a little rich, but otherwise seem OK.

My next error checks are:

-Check each plug wire separately with a timing light to see if each are triggering. I've also ordered new plugs, new wires, new cap, and new rotor. Don't have a service record on these, so good to replace them regardless.

-Start diagnosing the fuel system. Verify that it is working.

Any others ideas of what I could be checking?

[GAJon]

I started looking through the fueling systems and figured the switches and sensors should be the first culprits. Figured out where these all were and started with the TTS.

Found that the bottom wire (terminal G I think) was disconnected completely and was loose. Tested the TTS and got ~24 Ohms W-G; 0 Ohms W-Ground; ~24 Ohms G to Ground. This fits within the specs of the TTS, so figured it must be good.

Reconnected the TTS, cranked for about 10s on a full battery and it started to sputter! Finally kicked over and came to life.

It just started getting a bit cooler in Georgia, so I think this all may have been a coincidence. I know the TTS is connected to the CSV and other fuel system sensors. So maybe having this plug loose in a cool condition led to a no-start condition. Luckily it happened at home and not while I was out driving.

May have not had a major ignition issue, but I got a new CDI+ plus a better understanding of how the car works, so a win-win all around.

Hopefully I'm not celebrating prematurely, but I appreciate the help!

[fintstone]

Glad to hear. Was pretty sure that you could not test D1 in the circuit as with the leads one way you bias the diode and the other, the PN junction of the transistor (acts like a diode).