911SC only starts with alternator disconnected

anthony · 06-26-2015, 03:11 PM

I'll try to make this brief and understandable.

First, I've been having an ongoing hard starting issue for a while. In trying to track that down I noticed that the alternator light was glowing faintly so I decided to tackle the alternator. My 1981 SC had the external voltage regular and I found that my alternator was NLA so I bought a newer style alternator with the integrated regulator.

After removing the old alternator, I noticed that the hard starting issue completely went away. I could easily start the car and run it for a few minutes in order to move the car. I never ran it for more than a few minutes so it wouldn't overheat.

I finish installing the new alternator and now I can't start the car. It fires and sounds like it wants to start but doesn't. I get lots of popping. I just had the car running earlier today so I'm pretty certain that if I took everything apart and disconnected the alternator I could start the car again.

Please help - I need a clue!!!!

timmy2 · timmy2's Garage

Did you check and clean the ground strap connection at the altrrnator to the top of the engine?
How is the ground connection between the transmission and the body of the car?

anthony · 06-26-2015, 03:59 PM

The ground from the alternator to the engine looked solid. I didn't do any investigation of it other than seeing that it was solidly attached. I didn't see any corrosion on any of the wires behind the alternator.

I can check the transmission ground. Would that really keep the car from starting?

anthony · 06-26-2015, 04:06 PM

BTW, it's not a starter issue. Here is a video of the non-starting:

https://www.youtube.com/watch?v=9BT_B2GpfSc

wayne robson · wayne robson's Garage

Hi Anthony,are you sure while removing your alternator you never
touched or did something to your distributor ,the reason the video you showed
reminded me of the exact problem i had years ago ,not with a Porsche
but 65 Buick Riviera GS ,and it was the pertronix electronic ignition had burnt
out module ,caused by the shipping company had left the key on while shipping to Australia
and had, battery dead flat,,etc,etc, plus the pertronix,and that is exactly what the
Buick would do, fire for approx 5 -8 seconds then cut out.

timmy2 · timmy2's Garage

Trans to body cable is the main ground path for the alternator.
Yes it would have an effect.
Also remove, clean and re-seat the 14 pin engine harness to fuse panel connector.
They tend to get corroded.

anthony · 06-27-2015, 03:55 PM

Wayne, there is no pertronix installed and the car was starting just before I installed the new alternator.

I checked the 14 pin connector and found no corrosion. I still cleaned the alternator related pins with a little emory cloth. No joy. I'll check the transmission ground tomorrow when I get a chance.

Any theories why the car would start with the alternator disconnected?

boyt911sc · boyt911sc's Garage

If the car starts without the alternator connected and won't start with alternator connected, you found the culprit. Either the alternator is defective or the wirings are incorrectly connected. Check if the wires at the back of the alternator are in the right terminals. Keep us posted.

Tony

anthony · 06-27-2015, 11:00 PM

Well the strange thing was that I was having the starting issue with the old alternator but it was intermittent. I took off the old alternator and had zero problems starting the car to move it around. Now, with a new alternator installed starting problems again.

ant7 · ant7's Garage

Usualy, the Alternator' unless its spinning will have no affect on the electrics of the car, as the diodes in the stator of the alternator [if working correctly] will block any current being passed from battery to ground via Alternator, it could be a weak ignition switch i supose, but i suspect there is something going on with the Alternator wiring. Does the car start with the Alternator connected, but with no belt ? Also, is the battery a good one, and fully charged ?
Hope you get it sorted!
A...

boyt911sc · boyt911sc's Garage

Quote:

Originally Posted by ant7

Usualy, the Alternator' unless its spinning will have no affect on the electrics of the car, as the diodes in the stator of the alternator [if working correctly] will block any current being passed from battery to ground via Alternator, it could be a weak ignition switch i supose, but i suspect there is something going on with the Alternator wiring. Does the car start with the Alternator connected, but with no belt ? Also, is the battery a good one, and fully charged ?
Hope you get it sorted!
A...

Anthony,

Try to start the engine using 'ant7's suggestion (alternator installed less belt) and make sure you have a fully charged battery. Interested to see what happens next. Keep us posted.

Tony

T77911S · 06-29-2015, 03:34 AM

i could not see the video.

did you remove the old regulator. i have not converted one so i dont kneo what is envolved as far as wiring.

if it turns over but will not fire here is what i would do:
check for spark
i would check the CSV for power at the TTS. (or, lift the sensor plate with the key on to put some fuel into the cylinders. then start it)
if it starts you have a CSV circuit problem.
with the alt connected, remove the alt wire from the starter and see if it starts.
remove the rear connector on the AFM and turn the key on to run the FP.

the FP relay gets its ground thru the D- on the alt, that could be something to look at. (also the AFM switch, thus remove the connetor)
the speed relay also gets its ground thru the D-. not sure how it works. looks like it might put a ground on the FP relay to kill the FP at redline. i have heard of this thing causing run/starting issues. dont remember who but he said he had to "reset" to fix the issue he was having.

first i would check spark then the CSV. (lift the plate)

anthony · 06-29-2015, 03:53 PM

Thanks for all the help. The car is running perfectly right now. The new alternator is putting out voltage. I get 13.5v at the battery terminals and the alternator light is off.

But I have a feeling that I will be back soon to still figure out the non-starting issue. I think the non-starting issue is not related at all to the alternator. It just looked like cause/effect because I disconnected the old alternator and magically the car started every time without an alternator while I worked on the car and moved it around the driveway. Then, as soon as everything was back together it decided to not start again. ;-( It appeared as if the alternator was the culprit.

Today, thinking about what you guys have written, I've jiggled the fuel pump relay, looked at the connections and fuses and boom the car starts right up and is purring right now as I type. I've even shut it off and restarted it several times just to be sure. I'm going to go ahead and check the ground strap on the transmission and a few other things mentioned.

Still, I think I'll be back. :-( I'm hesitant to even drive the car in case the issue happens out on the road.

anthony · 06-29-2015, 05:05 PM

Sure enough, the starting problem happened again. Are you supposed to hear the fuel pump buzz every time you turn the key? I noticed that I'm not hearing it now but was hearing it before.

More jiggling of the FP relay didn't see to make any difference.

ant7 · ant7's Garage

Could be a faulty fuel pump relay, or a wire broke, to be honest, it could be a lot of things! You need to go through the connections properly, and examine each and every one, including the cabling, if the car seemed to start ok after fiddling with the relays, and cabling, then its probably around that area, also, dont forget to check the fuses, they may look intact, but can break down with age, the fusebox connections are also an issue, my advice would be to go through every fuse and contact, clean them, and check each one for continuity, and if you suspect that they have seen better days, just change them, they are not expensive!
A...

Quote:

Originally Posted by anthony

Sure enough, the starting problem happened again. Are you supposed to hear the fuel pump buzz every time you turn the key? I noticed that I'm not hearing it now but was hearing it before.

More jiggling of the FP relay didn't see to make any difference.

T77911S · 06-30-2015, 04:58 AM

hearing the FP with the key on depends on if you have the connector on the back of the AFM connected or not. with it connected and everything working as it should you will NOT hear the FP with the key on.
while cranking the FP should run.

does the not happen when its hot, cold or anytime.

when it wont crank, pull the connector off the AFM and try it see if the pump runs with the key on.
also check for spark when it wont run. (dont remeber if did that or not)

anthony · 06-30-2015, 12:21 PM

I'm not sure why you couldn't see the video. It's a standard youtube video with permission to anyone who has the link. The car turns over. It just doesn't start. It sounds like it's popping, sometimes. It's not just turning over.

https://youtu.be/9BT_B2GpfSc

I don't have someone to help me check for spark. Is there a way to do that solo?

I turned the key to on and then lifted the plate (under the air filter right?). The car didn't start after that. Should I hear the fuel pump running putting fuel in? Do I have to hold it for a certain period of time.

I also cleaned the connections at the fuel pump fuse. They look very clean and not corroded in general but I sanded them anyway. No luck there. I've swapped relays around and that doesn't make any difference.

anthony · 06-30-2015, 02:32 PM

I've been cleaning various connections with no success. After trying to start it I smell a little fuel at the tail pipe. I think it's spark related. I'm going to work from that angle for the time being.

anthony · 06-30-2015, 03:34 PM

I found my ohm meter and tested the coil. I get .5 to .7 ohms between the terminals. (It usually settles on .7 after moving the leads around a bit.) And I get 698 ohms testing from the center of the coil to one of the terminals. Either terminal actually gives the same reading.

Are these good readings? I've searched a lot and find conflicting values.

Is there a list of the ignition troubleshooting steps I can run through?

47silver · 47silver's Garage

Sounds like fuel. The engine revs then shuts off. It appears to me getting spark.

Test: get a spark plug. Take off a plug wire and plug into the spark plug. Ground the plug. Have some one turn the key. If you see a nice intermittent spark then you need to check for fuel.
Remove filter. Turn on key lift the air flow plate you should hear the injectors and pump working. Do not over do it you do not want cylinders full of raw fuel.
If you don't hear the injectors you have a fuel delivery problem.