Starting trouble: MSD unit to blame?

[jake28]

Howdy all,
New guy here, new to Porsches, with a new (old) Porsche 911 in the garage.

The Issue:
I can’t get the 911 to start reliably. The ask: assistance and advice.

Primary resources:
- this forum
- https://www.holley.com/support/troubleshooting_techniques/

The specs:
1975 911s. 70K miles, only driven 159 miles in last 6 years by PO. Largely sat.
Motor: 1984 3.2, built. ~260HP at the crank per dyno sheet
Intake: Weber 46 IDA carbs
Fuel: MSD hi flow pump
Ignition: MSD everything: Timing Computer, Timing Control, Digital Ignition Control, blaster 2 Coil, Helicore wires

The issue/ symptoms
- Just before I bought: owner said that she was having issues with the car stalling out sometimes, but “it would always start back up”
- Transporter:” the car died and then I couldn’t restart it”
- Me on day 1: car started fine cold (first crank) but wouldn’t restart when hot. Had to let it sit for ten minutes in order to restart it. Then ran great.
- Me in day 2: car wouldn’t start at all.
The changing nature of the symptoms gives me pause.

Diagnosis:
Electrical: in day 2, When it wouldn’t start in my garage, I did the following:
1. Checked for fuel pressure at the carbs. Confirmed with fuel pressure gauge on the regulator.
2. Pulled a distributor plug wire and put in a new plug: no spark
3. Checked ground from MSD to motor, it is tight.
4. Put a plug on the coil wire: no spark
5. Replaced the coil wire which was cracked: no change.
6. Cleaned the distributor cap contacts: no change / no spark
7. Confirmed the distributor top rotates when cranking
8. Measured resistance across coil and confirmed primary and secondary resistance are in spec.
9. Measured for 12V at wires coming OUT of MSD computer and in to coil with ignition on, and found 0V. As I understand it, because The MSD unit is a CDI, that is normal. https://www.holley.com/support/troubleshooting_techniques/
10. Confirmed there is power going to the MSD unit and measured 12V at fuse bay in motor compartment.
11. Pulled red and green trigger wires and shorted them with plug in the coil wire, per Holley instructions. Confirmed no spark

Hypothesis: MSD computer is bad and needs to be replaced.

Does anyone have sage advice they can offer and other things to check?

I spent plenty of time readily on these forums and rules out the likely suspects like the DMF relay, since, AFAIK, this car doesn’t have one after the carb and MSD conversion.

I’m more than happy to get a new MSD unit, and would be delighted if that’s as the issue, I just want to see if I’m missing anything else obvious first.

[jake28]

And, fuel to the fire of confusion.
In a last ditch effort before pulling the existing MSD, I tried starting the car today and it turned on first crank.

Color me confused

[eastbay]

Originally Posted by jake28

only driven 159 miles in last 6 years

IMHO, pull and clean the carbs, hope they aren't too far gone, check fuel pressure also.

[Zuffenwerker]

Sounds like you have a bad msd box. If theres good power and ground going to the unit and you perform the trigger wire test w no spark I think the box is to blame.

Daniel

[john walker's workshop]

Check the rotor for the hell of it. MSDs can roast the resistor.

[jake28]

Originally Posted by john walker's workshop

Check the rotor for the hell of it. MSDs can roast the resistor.

Will do, thanks for the response and advice.

[Peter Bat]

I have just been through a year of misery with starting issues on my 911 Carrera, G15 g/box, 1985.
Turns out that the throw out clutch on the starter motor was worn. Finally manifested itself by self destructing. New recon starter fitted, new ring gear and assorted small bits of metal removed from the flywheel area. Fortunately nothing else hurt. Now starts first time everytime.
So If you have a high mileage car (mine has 135K on the clock) it is worth while having the throwout clutch sorted. That would have saved considerable grief, new ign switch, new terminals on the battery leads, a relay for the starter feed from the ign sw. Mind you I can get a starter off in about 15 minutes now!

[jake28]

Update: still no luck.
-I’ve replaced the whole MSD unit to a brand new 6AL, and exactly the same symptoms as before. Recap, I’ve replaced the coil, coil wire and MSD ignition, still no start.

Any other ideas appreciated.

[gamin]

May have missed it, but no mention of fuel filter(s).

[75 911s]

You can test the MSD box by grounding the wire off the coil with a spark plug in it (I think you can do it without too, but easier with plug as you can rest it on the chassis bolt ground), (I think you tested this on your first MSD box)

You should get spark this way. So that's your first test. If you get spark, put the dizzy together and then test one of the leads by putting a spark plug into say #4 and grounding it on the engine carrier bolt to chassis.

If you get spark there, you are probably good inside the dizzy in terms of rotor, cap, etc.

I wouldn't dive into fuel unless you are sure your spark is happening.

MSD needs a solid power supply both switched and constant as you know. Are you going directly to the starter or battery for the power lead or to the rear fuse panel? If you go to the rear fuse panel, make sure you are getting good power there. The msd will also blink when producing spark, Not sure on the AL but the streetfire has an initial blink on switched power, and then a blink on each spark fire, - the sparks happen so quickly it is confused for a steady light fyi.


EDIT: Be careful not to shock yourself doing this test! The spark plug should be laying on the chassis grounded out on the carrier bolt (Not your body!)

[jake28]

Thank you @75 911s. All great tips.
I installed the new box last night, and, as it would stand to reason since it was very late, managed to shock the bejeezus out of myself holding the spark plug and grounding the MSD trigger wire.

So, I'll repeat, minus the shock, to see if I'm getting spark.

Thanks for aiding in the misadventures,
Jake

[911 SLANT]

Love this thread. Had that happen a few times. Lol

[rwest]

Did you check your rotor like John Walker suggested? MSD is notorious for melting the internal resistor. Check to see if you have continuity between the top of the rotor and the edge.

Without juice flowing through the rotor, you won’t get spark through the distributor to the plugs.

Best of luck, electrical stuff is so frustrating.

[jake28]

Update,
Ive got the new MSD 6AL hooked up, powered on, and working, based on the light on the box.

Issue:
- when I put a fresh spark plug in the coil cable, and ground out the trigger cable, I get spark, and blinking light on the MSD confirming it is firing.

- when I put a plug in one of the plug wires, and ground out the trigger wire, I get no spark.

Next up will be to test and replace the rotor.

Anything else I can/ should check out?
Thanks so much, I feel like I’m getting close.

Jake

[75 911s]

Originally Posted by jake28

Thank you @75 911s. All great tips.
I installed the new box last night, and, as it would stand to reason since it was very late, managed to shock the bejeezus out of myself holding the spark plug and grounding the MSD trigger wire.

So, I'll repeat, minus the shock, to see if I'm getting spark.

Thanks for aiding in the misadventures,
Jake

OMG so sorry Jake. Man I laughed at this, though I do legitimately feel bad. I've edited my post to include a warning for any future searchers, and I truly apologize, I should have made the shock danger more clear.

Reminds me of when I was working on my old pinball machine and shocked the ever loving bejeezus out of myself. 120V 20amp --- Never work on a pinball machine while its plugged in lol.

[75 911s]

Originally Posted by jake28

Update,
Ive got the new MSD 6AL hooked up, powered on, and working, based on the light on the box.

Issue:
- when I put a fresh spark plug in the coil cable, and ground out the trigger cable, I get spark, and blinking light on the MSD confirming it is firing.

- when I put a plug in one of the plug wires, and ground out the trigger wire, I get no spark.

Next up will be to test and replace the rotor.

Anything else I can/ should check out?
Thanks so much, I feel like I’m getting close.

Jake

Jake,

Just making sure, but you have to have the rotor and cap on and the motor turning over to test the one of the plug wires if that isn't clear, use your phone laying on the back of the car over the carrier bolt where the #4 wire will be positioned and grounded using a spare spark plug. Crank the car for a few seconds, view your video and see if there is spark. If this is what you are doing and there is no spark then the rotor or cap is bad. Those are cheap enough that you should have replaced them anyway.

[jake28]

Duane, thanks so much.
The rotor and cap both had what looked like soot on them. I scraped all of the contacts to bare copper and then put it together and got intermittent spark on the number 4 cylinder lead. Which is better than the no spark I was getting previously.

Going to replace the cap and rotor next, which arguably should have been my first (and least expensive) step.

[jake28]

[jake28]

[john walker's workshop]

That cap is junk. So many paths to ground, or spark jumping to a neighboring terminal. That coil is oil filled, so needs to be pointed the other way. I'd be tempted to reinstall the original 6 pin CD box and black coil.