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EFI Wiring Fundamentals

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Discussion and questions related to the course Motorsport Wiring Fundamentals

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Struggling with hard cranking especially when hot. battery new, starter new (and replaced both to be sure).

I thought for sure I was in over kill land, but perhaps not.. so before I pull the interior out of the car to run new wire I thought I should at least ping the brain trust here.

Car is a 07 Cayman with a LS outback. I am using a Gallardo gearbox/starter. Ironically my Gallardo (I had many moons ago) sometimes didn't like to start hot either. Factory Porsche uses a 1 gauge wire from the battery to the starter. I kept that wire, cut it short and terminated it at a junction block behind the drivers seat. I then ran a second wire from the battery to that same block, 1/0. So 1+1/0 to the back of the driver seat. I then ran a 1/0 from that block to the starter (4ft or less?). I would have expected 1/0 to be plenty front to back, so having the extra 1awg I thought would be more than I need.

The junction block is bolt down style, with the 1/0 from the battery and then to the starter on the same lug, with the 1awg wire tied right next to it. Block is beefy, rated for 300amp+ continuous.

if I hook a jump box up to the starter directly it fires right away

Ground from battery to car is 8", 1/0

Ground from engine to car is 24", 1/0

All the added cable is high strand count "welding" cable, so copper diameter is correct (not cheap china amazon stuff)

Even just typing this its leading me toward just tearing the car back apart and running a 2/0... but what the heck, might as well ask.

edit: didn't think about the solenoid wire. I believe its 18awg. can't imagine it takes more than a couple amps to generate the magnet field required to close the contacts. but maybe im wrong. Rest of the solenoid trigger stuff is just factory Porsche. The solenoid def clicks to be clear, the starter tries to spin the motor over but just can't

Solenoid needs lots of current (especially when hot) to keep the contact engaged. It can be 30 amps, so try a 12 or 14 awg wire.

The thing is that the solenoid is audibly staying engaged and started trying to turn the engine over. I obviously do not know the internal construct of the contacts in the solenoid. I assumed it was binary either it was energized and working or not.. but that is easy thing to test obviously much easier than rerunning the big wiring through the car

First thing I'd do is check the voltage drop between th batter +ve and the starter under cranking, then I'd check between the starter body and the battery -ve terminal. The lower they are the better - been quite some time since I needed to worry about it, so can't recall exactly what the maximum total wiring loss should be, but think it was 1V total - 0.1 might be pushing it.

Second thing I'd do is remove the starter and strip it, while noting how the parts fit together. Polish the commutator (the copper bit with the segments the brushes run against) with some W&D and clean it and the brushes with some electrical cleaner or isopropyl. Clean the bushes and solinoid pivots, etc. with the same and lubricate them with a little grease and a couple of drops of oil. Then re-assemble* and refit - you should notice a significant improvement in cranking speed.

*With some brush spring arrangements it can be tricky, but persevere with, maybe, the assistance of some wire to hold them back initially.

Voltag drops to 8.5v at the dist block under cranking when the starter stalls. Dash is powered off of that so I can see it

Did you read what I wrote above?

One thing I forgot is to also check the cranking voltage across the battery - if it isn't accepting a full charge, or is too small, its voltage may also be dropping excessively low on cranking. I'd like to see it above 11V, if possible, with 10V minimum across the starter.

Electrical resistance increases with temperature, so if the exhaust is run close to the starter and/or wiring, consider a heat shield.

Hi Jordon,

I believe you have misunderstood how to perform a voltage drop test to find resistance problems within a circuit. Measuring the voltage across the battery posts (or distribution block) as you are doing is not a voltage drop test. That is merely the battery's voltage you are observing.

Voltage drop is the voltage lost as it passes through a component or a resistance in a circuit.

When performing a voltage drop test, Your multimeter leads need to be connected in parallel with the tested circuit. In your case, the multimeter leads need to be connected directly to the battery's positive terminal and the starter's positive connection. When voltage is applied, the multimeter should read as close to zero as possible. Anything beyond 0.5v is typically considered excessive on a starter circuit.

The same test will need to be performed on the ground side of the starter circuit. In your case, the multimeter leads will be connected directly to the battery's negative terminal to the engine block's ground strap connection.

Sorry I didnt explain further. I have not had an opportunity to go back and measure. I was only in town for a few hours, and the car was cold so it fired up instantly.

I plan to measure the voltage at the starter, the dist block (which is done by the M1 and dash) and then the battery.

At this point given how fast the engine turns over when cold (cold = 100F, Texas!), my guess is that this starter is just weak and loses too much torque when hot (nature of physics). Sure I could increase wire diameter to reduce voltage drop, but its got to be a band aide at this point. From my research and discussions, 1/0 should be capable of doing what I am asking. The battery and starter have been replaced, and although better when new (clean commutator as noted above, etc) the difference b/w a cold start and a hot start is huge.

Quote= I plan to measure the voltage at the starter, the dist block (which is done by the M1 and dash) and then the battery.

I believe you may be confused about how to perform a voltage drop test. As the name may imply, You do not simply measure the battery's voltage at the different points you described, and your ECU or Dash will not be able to take any parallel voltage measurements on the starter circuit.

ya, maybe I am confused

I had planned to measure the voltage at the battery post (post to post), the distribution block (block to chassis) and then the starter (starter positive to case).

This gives me three voltage during cranking, so I can see the voltage drop.

The M1/dash is tied into the dist block directly so I can use it to measure the voltage there, the other two points will be with a DVM or oscilloscope

Nope, you cannot use the dash to check the voltage there as there is still the ground/earth side to the battery - unless you have it grounded directly at the battery.

It's a very easy process using a volt meter and someone to crank the engine.

From what you have been saying, it still sounds like the starter is getting cooked by the exhaust and you need some heat shielding between them - might be wrong, though, as some pick up a lot of heat from the block. I would also like to see at least 10V across the starter.


Ok, that would not be a voltage drop test...What you had planned to do would measure closed loop battery cranking voltage.

A voltage drop test (drop to zero) is based on Kirchhoff's law. And can be used to identify and locate unwanted resistance due to bad connections, corrosion, faulty cables, poorly crimped connectors, etc.

man, good ole Kirchhoff. its been a few years. I understand the difference. thanks. ill dig into it.


I believe Jordon's problem can be correctly diagnosed before making any changes. I am skeptical about assuming this is a heat-related problem and jumping to conclusions without adequately testing and verifying the problem first.

Paul, he actually said it was worse when the engine was hot/cranked better when cold...

I expect there is a problem with excess voltage drop(s) in the conductors to, and from, the starter and reducing the starter torque, compounded by a heat soak problem, which is not uncommon.

You are correct. That is why I thought I was oversizing the supply conductors.


Yes, I am aware of the temperature conditions that he has mentioned. And that may or may not be a contributing factor to the problem. However, everything is speculation on our behalf without verifying all the data to make a legitimate diagnosis.

I was not implying that you were incorrect. I am just cautious about misleading someone based on inaccurate and subjective information.

No worries, I just assumed you didn't read my first two posts ;-)

It's primarily this bit that has me thinking heat soak is going to be a significant factor - "the difference b/w a cold start and a hot start is huge".

I don't know what power/torque the starter you're using has, but there may be higher torque options, such as https://m.mrfiat.com/lamborghini-gallardo-v10-high-torque-starter-motor.html

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