Secret sauce?

[tschall]

Hey guys, I am a vacuum of desperation. Sal C. has been mentoring me so graciously, but my stupidity has reached i=ts limit me thinks. 1986 Porsche 911 3.2 M491. Car has undergone extensive renovation. Now a no start issue. No problem. Found bad Speed sensor. Replaced. Still no start. Checked methodically with Sal C's help and guidance. No power to pin 30 on DME relay. Ah... Power checked red wire from socket pin at DME to firewall. Excellent voltage. Reversed and checked from battery to last access point at fuel filler inlet at tank. All good. Somewhere in (DME harness Relay) in a 16 inch section of harness (4 wire total, 12v constant from Batt, Fuel pump, coil x 2) 12v constant from battery is not conducting power... seems to be broken wire. Failure occurred while car parked and not driven for a few weeks. Hmmm. This harness is inaccessible to my efforts from cabin firewall floorboard penetration to aforementioned point outboard of fuel filler nozzle at tank. Schematic shows this part of DME relay harness joins an addition 4 group of conduit into a single conduit for the 16" run downward and thru the frunk floor and into cabin firewall. So who has the shortcut, secret sauce, at gaining access to this harness, and how in the world could it be severed, burned, cut, etc... Rodent free environment. No fire damage. Really immaculate low 60K mile car, totally rebuilt except trans, new everything. Struggling here for a method?? HELP??
Thanks!

[John W.]

Timmy2 is the electrical wizard. I’d suggest reposting this with a title that indicates your problem.

[timmy2]

You have voltage at both ends of the circuit according to what I’m deciphering from your description.
It may be corrosion or a bad connection at a terminal that is limiting the load.
(Meter reads voltage but is limiting the power flow when load applied)
Check at fuse panel where the power to the wire connects. Check the fuse terminals and fuse are good.
DME relay sockets have been known to fail with solder failure that can cause the same problem.

[tschall]

Dennis, Thank You for joining in. Maybe I am not being descriptive enough. The # 30 pin 12 V direct from battery terminal provides no power to DME relay pin 30. Battery end of this factory wire has been severed and inspected. No corrosion problems. Temporary connector applied and connected to power and wire tested for voltage and idiot light is bright in addition up tp the fuel filler at tank where the conduit is NOT accessible. Reversed this procedure. Disassembled DME relay and removed female socket pin 30. Disconnected the battery connection at positive terminal for this wire. ran a direct feed from terminal to female socket pin 30 and tested the wire to the cabin firewall. All good 13.25v, idiot light bright.
Before all this ran a continuity test on this line with ohm meter and it is open. No connection from both ends. Sal states he has never seen a harness failure in this location on just the 30 pin wire in his experience. This is my second M491, and I have worked on a few friends cars as well. No clue as to how why this could occur, BUT there is a wire break in evidence somewhere.
Forgive my stupidity, but there is NO fuse involved here. If pin 30 is not powered, DME will not work. Pin 87 functions fine for fuel pump from ignition when powered/tested isolated separately.
You are the wizard and all help appreciated. I am about to run a new direct pin 30 feed direct to DME (with a new female socket installed on test wire and placed in DME relay to feed all) and test the system to see if it starts. Frustrated I can't access the DME harness portion effected as it passes into top floor portion of frunk and then appears to run under that floor nd between the void of the real floor (about 3") until it penetrates the cabin firewall, where it has been tested and verified. Kroon shows this harness with a special additional tube encasing the harness proper where it makes this body penetration, including the formed grommet. From Kroon the three wire battery feed conduit joins another 4 conduit group into one single conduit to make its path rearward to cabin. Please see pix ...
Again thank you VERY much! PP forum is a wonderful place and repository of great knowledge!

[ChrisHoover]

Like Dennis says, a load test would help. When measuring with a volt meter place a load in series. The load could be a light bulb. Measure the voltage drop. Without load 12V with load way less?

[sus911]

Was the fuel tank removed from the car?

[tschall]

Update: Still no joy. The 3 wire harness carrying ignition wire, 87 DME pin wire, and DME 12v constant wire was found to NOT be as advertised. This is a factory correct car and harness. Unmolested. There is one additional wire black coated/red wire 11 gauge that connects to positive terminal. It is NOT a part of the 3 wire harness. It is the DME 30 pin wire. It travels to the main harness just aft of the fuel filler nozzle to tank junction, where it joins the main harness run to the DME. It emerges from harness (exposed) as a RED wire and continues to pin 30. All DME functions good, proper.
Now engine will not spin/crank. New ignition switch a few hundred miles and 9 months ago. Will check main battery cable to starter and then the solenoid jumpers to see if problem is there, or it is ignition switch. All ground points are cleaned and reinstalled. The mystery wire black coated/red had a break about one inch from positive terminal at battery post.
Words of wisdom, please??
Thanks to everyone, especially, Dennis, and Sal, for procedural steps and isolation. Still have hope it will again run!

[3rd_gear_Ted]

Suggest a nearby linked terminal board with a single source fused wire from battery.

[gerakroo]

Hi, we might be able to help you further!

Here's a drawing with the layout of the 3.2 DME harness
I think it's best to do some measurements step by step.
You can contact us by: info@kroonwireharnesses.com.

Regards from the Netherlands,

Gerard Kroon
Kroon Wire Harnesses.

[tschall]

To all: Problem still exists. Localized to Ignition Switch to six pin plug to starter 14 pin relay near engine compartment fuse panel. I have queried Kroon and am awaiting their second reply. I only need the 6 pin connector to the Ignition Switch socket female pin receptacles, as new, Maybe. Problem may be in the harness run from dash to rear engine 14 pin #1. I have 3 specific tests from the one of the most gracious guys on this Forum Sal C.

1 - Unplug the ignition switch and at the harness side just jumper 30 to 50 with a jumper, starter should crank.

If it does not crank do this test next:

2 - remove the yellow crimp connector so you have the yellow wire that's on the 6 pin firewall connector loose and NOT connected to the white wire. Then with the 6 pin connector plugged into the firewall jump pin 30 of the ignition switch harness directly to that loose yellow wire, does the starter crank?

if no crank do test 3
3 - Still leave the yellow wire disconnected from the white and leave the 6 pin plugged into the firewall. Now get a long piece of wire and wire the yellow wire directly to the battery + and see if that cranks the starter? This third test simply hot wires power to the yellow start wire at the 6 pin firewall connector. If still no crank you have a break between the loose yellow wire and the starter solenoid post.
Do those tests in that order

[911pcars]

Put away that 12V test light when troubleshooting digital circuits. Many are low voltage (<12V). Connecting a 12V test device will draw excessive current which can fry the circuit.

Use a digital VOM to be safe. Voltmeters have high internal resistance that won't compromise/damage digital circuits when checking for voltage.

Sherwood

[tschall]

I have a power probe III kit that I use all the time also, it is just a difficult place to reach and hold multiple devies and connections. Idiot lights are appropriately named! Great advice also, and thank you.
Todays status, followed last 3 diagnostic tests to isolate and guess what? There is no joy on any tests, 1, 2, or 3. Test 1 & 3 yield identical results. Fuel pump doesn't kick on, but you can hear a relay clicking to connect. It is not in AC smugglers box, but sounds like underneath it? See Pix. I figure there is a relay near fuel pump that is being obviated by NOT having the ignition socket plugged in and that is not allowing the circuit to be completed. I included a labeled pix of the bck of the ignition switch for anyone interested.
Thoughts??
3.2 1986 Porsche 911. 62,500 original miles. The only original systems in the car are the Transmission, R&P rack, Coil, fuel pump, and starter. All else new/rebuilt/less than 850 miles...

[tschall]

To all: This problem has been identified and resolved. Thank you to everyone who contributed. Our PP forum is populated by helpful folks, so far in my experience all of them have honestly put forth their solutions/problem causes.
Here is the last word on this, and hopefully to all future '86 & '876 owners, as well as '88 and '89.
Our cars have excellent schematics from PET and Bentley. The schematics DO NOT show distances/locations for connectors and the type of connector you are looking for. I have over 100 hours on this '86 Ignition Switch to J50 connector to Connector for Heater block relay/Heater level control levers to starter to 14 pin connector pin 1 (near rear fuse panel) and then to starter solenoid. The following pix represent my problem, and I hope never yours, but if it happens you will have a good guide.
Problem: Clear Lexan rectangular connector has J50 power in/stacked with A/C relay power out in a factory unit. Outbound side has two 1/4" spade connectors leaving. One powers the Heater level control (levers between seats 16 ga.) the other 4 mm starter circuit to Pin 1 of the 14 pin connector, and then on to the starter solenoid (16 ga.).
I need a moment to explain the location of this particular connector i n depth. Reason is to assist in locating it, and prepare yourself for the difficulty in accessing it. In your frunk directly under the fuel filler inlet at the tank, you will find a veritable snakes nest of harness's and three connectors. This place is the nexus of all things electric in our cars. You have main DME harness penetrating into the cabin here, and another harness with Ignition and lights and more. This ruins behind floorboard and behind pedal cluster to enter the tunnel at the right edge top of the accelerator pedal location. From here thru the tunnel until it exits in a 2" harness pax side of transmission then on to the engine bay where it terminates in the 14 pin connector.
Now the difficult area under the fuel filler and frunk floor where all of our power lies. There are cable ties that are placed every 1.5" here - 4 of them in a 6" space very difficult to access. They must be carefully removed. The rectangular fuel plastic cannister must be removed. Now you can see our problem lexan clear connector, but only the power in side. Using two silicone 1/2" by 6" tools you must carefully separate harness from harness (about 9 total, then free up the lexan connector until you can remove it to the center of the car an inch or two. This took hours. The Starter/Pin 1 spade connector was unplugged! How? When all the cables were bundled/tied locked into an unmovable mass? Go figure, it's a 911!
My moist sincere thanks to Sal C. here! He is a passionate dedicated electrical genius. Without his assistance I would have given up!
Simple fix from that point forward plus 12 hours of reassembly, harness patching, floorboard/carpet/Drivers seat and so much more, plus reassembly of the engine compartment.
Car starts, and runs like it did, until it didn't!
I have about 70 pictures if any one needs a reference library.

Best of luck!

[Marc Bixen]

That yellow cable connector is a common place that aftermarket alarms are connected. Is it possible there was previously a hook up there, then on removal of the alarm that wire was not properly connected.
Additionally I once had a 3.2 with an intermittent running issue. The owner of the car was female, and wore high heels most of the time. Down on the left side of the drivers footwell the DME harness is routed at the seam where the floor pan meets the wheel well. She was "grinding" her heel into the harness right at that point, and damaged the harness internally.
What a mess. Very difficult to find as the harness showed no outside damage.