Replacing Battery Cable from Front Trunk to Starter Motor - What cable to use?

[polizei]

Guys - I know, I know... replacing the trunk-to-starter battery cable is a last resort. Please read on.

I've been encountering sporadic no start issues over the past several months and have had to jump start the car about 7-8 times when in a pinch. My car has visited the mechanic on 3 occasions because of the no start issue. New battery, 2 new start motors, cleaned batter connectors and eliminated all corrosion, and we're still getting no start issues. My mechanic has checked and rechecked that the battery holds a charge, the starter motor is good and the battery is properly grounded.

My mechanic narrowed it down to the trunk-to-starter battery cable and suspected that it was shorting out in a particular section. He tried tugging on the cable from the front trunk as well as from the starter motor, under the no-start condition, and still nothing. Then he removed the petal cluster, tugged the cable and it fired right up! He's concluded that a section of the cable that runs from the back of the gas tank, through the petal cluster and into the tunnel must be shorting out. He's recommended replacing the entire cable to fix the issue.

After reading several old threads in this forum, I specifically asked if trimming the cable near the front and replacing the connectors would be a good first step. He explained that because he's isolated the short to being near the petal cluster that trimming the front will likely do nothing.

He's a very honest and fair mechanic, and I'm inclined to move forward. He said that while my engine is being rebuilt over the next few weeks that it would be a good do-it-yourself project since it's fairly time-consuming. Since the price of a new factory cable is about $160, he mentioned that I could make my own cable and that it should be the appropriate gauge and be both insulated and intended for automotive use. Can anyone recommend what type of cable to use? Where can I source new connectors that work with a factory starter motor?

Thank you!

[Rot 911]

His theory is possible, but I would think improbable. That is one big cable running to the starter. I would think if it was shorting out it would create enough sparks for you to see. If you didn't see the sparks, and the short continued, that cable is big enough to create enough heat to melt right through sheet metal.

[nineball]

i have more than enough 4g cable for your needs. all you would need would be terminals for the ends and they are only a couple cents each at lowes/home depot. shoot me a pm if interested.

[Grady Clay]

I agree. The B+ cable is an improbable candidate.

Have you cleaned and tightened both ends of the ground strap from the front of the transmission to the chassis?
If it is corroded, replace it.

Best,
Grady

[polizei]

Guys - My mechanic has been servicing Porsches for 30 years, and he's done just about everything you can do to address the issue: clean connections, tighten connections, replace the battery, check/recheck ground, replace starter, etc. I don't have the technical knowledge to troubleshoot the problem further, so I'm going to trust my mechanic that the next step is to replace the cable. I'll report back regarding whether this fixes the problem and the actual time involved.

My question is, if you were going to make your own trunk-to-starter cable, what's the right way to do it? Does anyone have experience doing this?

[Quinlan]

4 ga wire from Home Depot, with the proper ring connectors crimped on. I might avoid trying to fish it through the tunnel, and perhaps run it under the sill on the driver's side (inside a 1/2 or 3/4 inch plastic conduit pipe for protection). Not much money out the door that way, and you will find out soon enough if that was the problem.

[euro911sc]

I was helping a friend with a similar problem on a different classic car. Turned out to be that the cable had worn through the shielding where it passed through a firewall hole. There were some spark, but they were very minor and certainly no welding or melting was going on. It is not as easy to see as you might think. I believe your mechanic has isolated the issue well as we did all that other stuff 1st too. Except we just patched his with electrical tape and a grommet because we had easy access to the offending area.

Another possibility is that the actual strands in the cable have broken. Last weekend I was helping another pal put an engine and tranny into an E30. He grabbed the wiring harness and one of the thick battery cables basically broke in his hand. The insulation was hard and brittle and the cable was corroded almost through. Still worked fine when the engine was in the car prior to removal... but needs replacement now.

If your engine is out it might be a good DIY project. You may also be able to fish it out enough to see where the issue is on the cable, fix it, and fish it back without having to do the whole thing. I would try this 1st.

On a side note, I believe that the cables of today are a lot more flexible and may be easier to thread through the car... though it is still a big PITA.

[onboost]

You're car is and 87 and unless its been well beaten and weather worn, or has had serious accident damage, I just don't see this as being the problem. I've owned too many 911's and have rarely run into an issue like this.. especially with a "later" car. Hey, but there's a first time for everything right?

However, I see know mention of you checking the ignition switch.. though it my be a problem to duplicate the issue now that the car is apart.

[Andy911sc]

The 924's are notorious for having this issue. One time the car would start and the next time it wouldn't. Found the battery to starter cable had the insulation burned off near the exhaust manifolds and was shorting itself out on the frame. I replaced the cable and put extra heavy duty insulation on the entire starter wire harness and never had a problem with starting since.

Originally Posted by polizei

Guys - I know, I know... replacing the trunk-to-starter battery cable is a last resort. Please read on.

I've been encountering sporadic no start issues over the past several months and have had to jump start the car about 7-8 times when in a pinch. My car has visited the mechanic on 3 occasions because of the no start issue. New battery, 2 new start motors, cleaned batter connectors and eliminated all corrosion, and we're still getting no start issues. My mechanic has checked and rechecked that the battery holds a charge, the starter motor is good and the battery is properly grounded.

My mechanic narrowed it down to the trunk-to-starter battery cable and suspected that it was shorting out in a particular section. He tried tugging on the cable from the front trunk as well as from the starter motor, under the no-start condition, and still nothing. Then he removed the petal cluster, tugged the cable and it fired right up! He's concluded that a section of the cable that runs from the back of the gas tank, through the petal cluster and into the tunnel must be shorting out. He's recommended replacing the entire cable to fix the issue.

After reading several old threads in this forum, I specifically asked if trimming the cable near the front and replacing the connectors would be a good first step. He explained that because he's isolated the short to being near the petal cluster that trimming the front will likely do nothing.

He's a very honest and fair mechanic, and I'm inclined to move forward. He said that while my engine is being rebuilt over the next few weeks that it would be a good do-it-yourself project since it's fairly time-consuming. Since the price of a new factory cable is about $160, he mentioned that I could make my own cable and that it should be the appropriate gauge and be both insulated and intended for automotive use. Can anyone recommend what type of cable to use? Where can I source new connectors that work with a factory starter motor?

Thank you!

[nineball]

Originally Posted by Quinlan

4 ga wire from Home Depot

i would not suggest this but rather some quality 4g wire made for the car audio people. the jackets are much thicker, the strand count is much higher (easier to bend) and generally they are oversized. this of course only applies to the quality brands, not the cheap stuff. you get what you pay for.

i have about 20-30' left over on a 200' roll from previous installs OP if you want some.

[HawgRyder]

The old electricians trick is to use the old cable to pull the new cable through all the holes.
If the old cable will move (end to end) you might consider this method.
Bob

[timmy2]

Use #4 welding cable, it has the flexibility and protection you need. Take it from a 49 yr old electrician (me), best bang for your buck.

[Grady Clay]

Andy (polizei),

I would not go to all this effort only to find that the repair is otherwise.
More diagnosis is in order.


I don’t see any discussion specifically confirming that the transmission ground strap has been serviced or replaced – just “He did everything.”
See my Replacing Battery Cable from Front Trunk to Starter Motor - What cable to use? post #4 above.



Next, I would contest the diagnosis that the cable is “shorting out”.
That would drain the battery and cause massive arcing in the part of the tunnel that holds the cable.
This would most likely occur while driving and your nose would know.
Your ‘no-crank’ problem is an intermittent lack of continuity somewhere in a circuit – not a ‘short circuit’.

If you want to correctly diagnose the possibility of lack of continuity between the battery B+ terminal and the starter motor terminal, install a pair of wires and a 12 V bulb or voltmeter.
One wire goes to the B+ terminal at the battery and the other wire to the starter motor B+ cable terminal.
When you have a failed attempt to crank and if there is an ‘open-circuit’ in the cable or connections, there will be 12 V between these two wires (the bulb will light).
If the bulb lights, something is ‘open-circuit’ between these two points.
If this test shows an ‘open-circuit’, the connection between the terminal at the starter to the cable is most likely.
I would then consider re-soldering and crimping the lug terminal at the starter motor connection and re-clean the battery B+ connections.

Another worthwhile diagnostic test is to install a wire between the terminal 50 (yellow wire) at the starter and a light bulb or voltmeter in the cockpit.
The other side of the bulb should be chassis ground.
If you attempt to crank, this bulb should light every time.
If it fails to light, the intermittent ‘open-circuit’ is in the starter circuit through the ignition switch.

A clue that the B+ cable is not the culprit is this ‘intermittent open-circuit’ would not be limited to only when you attempt to start the engine.
It would most likely also become intermittently open while driving.
This would disconnect the battery from the alternator as the alternator feeds current to the battery at the starter B+ cable connection.
At the least, the alternator light would come on (intermittently).
At worst, having the alternator disconnected from the battery usually causes an ‘overvoltage’ situation, damaging the alternator.


Summarily replacing the cable without performing these diagnostic tests sets you up for having done a lot of work and still having the problem.

Best,
Grady

[Justin@Athens]

+1000 what Grady said. My mechanic spent a ton of time fishing a new positive lead from the battery to the starter only to find out later that it was a faulty ground from the negative terminal to the frame.

[Nine17]

I've had nearly the exact problem with my '88 Carrera coupe and my mechanic has completely solved it. It's one of those trial-and-error things that can't be completely explained...

First off, let me introduce my wrench-twirler: he's Tim Benson of Fastlane Porsche Repair in Santa Cruz, CA. He started out as an IMSA GTU mechanic before being hired to work at Vasek Polak Porsche-Audi 30-odd years ago, migrating north to Anderson-Behel and My Porsche (formerly Wester, in Seaside). He spent over 20 years in the Porsche dealership trenches, with the training certificates to prove it. Eventually he needed more time for his kids and opened up a little werkstatt of his own here in Santa Cruz. Great guy.

My car, another customer's '87 M491 Targa, and an '86 930 were all exhibiting the same behavior. We all had new batteries, and I also replaced that crappy Denso "high-torque" starter with a Bosch unit. Tim finally cut off and replaced the connectors on the cables to the positive battery terminal and installed a new ground strap to the negative terminals. That seems to have solved the problem on all three cars. I haven't had a non-start since we did the job in January -- even after sitting a week in a cold garage.

We have NO IDEA why this fix worked so well. Tim suspects that there's something about the metallurgy of the connector that has changed its conductivity over time. All I care about is that my car lights right up with a twist of the key every time that I ask it to! Give this a try before you start trying to pull cable.

[Canada Kev]

The battery to starter cable may possibly be the issue regardless of whether it's arcing or not. If the insulation is worn enough that it is making good solid contact with some grounded metal on its path, then it likely won't arc. Arcing usually comes from a not firm contact. What arcing is, is electricity jumping across an air gap between a positive and negative. No gap = no arc, just current flow.

HOWEVER, if that was the case (good of contact with a ground), your battery should be getting REALLY warm because the positive terminal was making such good contact with essentially the negative post (a ground).

It shouldn't be all that difficult to determine if it was making contact with a ground. When you're having this no start issue, unplug cables from battery and check for continuity between your starter cable and ground.

Another [more likely] possibility may be a spot where for some reason the starter cable has become damaged enough (though not grounding) that some of the strands are severed and the cable resistance is too high to engage the starter. In this case, I would suspect some arcing, but perhaps not, or maybe it's in a spot you can't see.

[E Sully]

As usual Grady has the best advice. I have spent 32 years as an electrician. I would have to disagree with a grounding positive cable also. If the cable intermitantly grounded, as stated by others, you would definitely know it. I would suspect a fault in the yellow wire from the ignition switch to the starter, a bad ground, or loss of current flow in the cable from bad connectors.

[timmy2]

Originally Posted by E Sully

As usual Grady has the best advice. I have spent 32 years as an electrician. I would have to disagree with a grounding positive cable also. If the cable intermitantly grounded, as stated by others, you would definitely know it. I would suspect a fault in the yellow wire from the ignition switch to the starter, a bad ground, or loss of current flow in the cable from bad connectors.

+2 for Grady's response. I thought you had already proved an open and wanted a type of cable to use. Electrically prove it needs repair/replacement first.

[T77911S]

some of this reasoning is just crazy!

you have a resistance problem, not a shorting problem

there is no such thing as a "little short" when it comes to the battery cable "slightly" shorting or arcing to ground. dont believe me. if you are brave enough, and you have good insurance, go try to "barely" arc a wrench across the pos and neg terminals of a battery. one little arc and most likey that wire is welded to body and you have much bigger problems.
the ONLY way the battery cable can be shorted to ground is if the battery is not making a good connection to ground.

i also noticed that you said that the battery connections where cleaned.
the grounds have to be cleaned too. the one from the battery to the body, and the body to the tranny.
you also mentioned concern of the cable terminals themselves. that is a real possibility. corrosion inside the crimp at both ends creates resistance. resistance drops the current available to the starter.

now to the yellow wire.
the solenoid requires a lot of current to pull in. resistance is the killer here too.
bad ignition switch, the connector on the relay panel, car alarms are problems for the yellow wire. after market car alarms that have been hacked into the yellow wire are a very bad idea for the 911 since that is such a long run to the starter.

another thing to try is to make a remote starter switch. mine is a simple puch button switch with 2 wires and aligator clips on the ends. i can clip the switch to a starter for testing.
when the car will not start, clip it to the battery cable and the yellow wire and give it a try. if it starts, the yellow wire is the problem.

gradys idea was good too. both may be worth a try before pulling a new battery wire.

here is an example of what a little resistance can do.

Automobile Battery Example

[spuggy]

Originally Posted by T77911S

now to the yellow wire.
the solenoid requires a lot of current to pull in. resistance is the killer here too.
bad ignition switch, the connector on the relay panel, car alarms are problems for the yellow wire. after market car alarms that have been hacked into the yellow wire are a very bad idea for the 911 since that is such a long run to the starter.

+10. Check, clean and spread the connectors end-to-end (e.g. from the ignition switch harness/bulkhead plugs through the engine bay 14-pin connector) would be my advice.

You cannot rely on it "buzzing out"; it has to carry the current to operate the solenoid. Voltage drop is the red flag.