1G PTU relocation/wiring question

[Drewpy Dawg]

I made a complete new engine harness for my car and while I was at it decided to relocate the PTU to inside the console. This solved a mounting issue and I figured it certainly won’t hurt to keep the PTU away from engine heat. Now I’m at the stage of fitting the harness to the car and making connections inside and I realized that when I extended the wires between the PTU and the coil I used 20 gauge wire just like the original harness used. That was fine when the PTU and coil were only inches from each other but now they’re several feet apart and I’m concerned that I should have used heavier gauge wire. How much current does the PTU switch? Looking at a wire gauge chart it appears that 20 gauge wire in a 12 volt automotive system can handle 6 amps over a ten foot length. I can replace those wires in the harness with a heavier gauge if I really need to, but it will be kind of a PITA so I’d rather not unless it’s necessary. Thanks.

 

[danl]

You would need to know the inductance of the stock coil to figure out how to size the wiring correctly. Even then some assumptions would need to be made such as how long the ecu keeps the coil energized after it has been fully charged. The second bad thing happening is that the longer wire will have more resistance which will affect the charging time (Time Constant = L/R). The longer wire will increase the resistance (R in the formula) increasing the time it takes to charge the coil. If it were me I would not move the PTU. If you must move the PTU then you will want a wire with similar resistance as the factory short run. 1 foot of 20AWG has approximately 0.01 ohms of resistance. 10 feet of 10AWG wire also has 0.01 ohms of resistance.

Wire Resistance Calculator - Cirris Inc

cirris.com

You may get lucky and it all runs ok. If the PTU has a lot of internal resistance then the resistance of the wire drop may be insignificant. The same is true of the power running to the PTU. Or it may run ok but end up with weak spot at high rpm (when coil charging time is reduced) and/or under heavy load.

Whatever you do, mount the PTU to something metal to conduct the heat away.

 

[We're on Boost]

I did relocate my PTU to the cabin in 2022 and it works great, I love it. I used 14 gauge wire for my long wire runs though.
If you haven't seen my thread for it, it's here.

My thought about what gauge to use was like danl said about not wanting to increase the resistance of the wires, or at least not by much.
I have no idea how much current is in those wires but let's just say for example that it reaches 5 amps.
20 gauge wire has a resistance of 10 ohms per 1000 feet. So, 0.1 ohm for 10 feet.
If you had 10 feet of wire with 5 amps going through it, the voltage drop would be:
V=IR = 5 x 0.1 = 0.5 volts. You'd be losing half a volt in the wire.
If there's only 12 volts to begin with, that seems like kind of a lot.

Anyway I have a lot of beautiful new 14 gauge GXL wire lying around, so I used it.
The pigtails coming out of my connectors are 16 gauge, to work with the pins and seals that we get for new connectors from Sheridan. But the long runs were all 14 gauge.

I left my new PTU mounted to the steel plate that it comes on like stock, and mounted that steel plate to an aluminum hanger that I made to go under the center stack.

I watched temps for a while by checking at the end of a drive with a Milwaukee IR temp gun. There was nothing to get excited about there. Temperature rise was as I stated in the 2022 thread.

I'm not saying 20 gauge is gonna be bad. It might be ok. I just don't know.

BTW Ignition coils do a thing called "flyback" which is something I don't know much about and I don't know if it might affect what we are talking about here.

 

[Drewpy Dawg]

Thanks for the input, guys. I did some more searching and found info in an old thread about wiring up COP ignition. The resistance of the coil primary and PTU is about 2.2 ohms, which means there will be 5-6 amps going to the PTU (12 volts / 2.2 ohms = 5.45 amps). Looking at more charts on wire ampacity and taking into account things like engine bay temp and a preferred safety margin I have decided to upgrade the wire to at least 16 gauge. I'd rather not dig back into this harness now that I'm nearly done, but the consequences of pulling too much current through that small wire could be much worse.

The main reason I'm moving the PTU is because there is a bolt broken off in one of the holes where the PTU mounts to the manifold. It's a Cyclone manifold so it only has two bolts holding the PTU bracket, and I can't remove the broken bolt without removing the manifold. At this point that's a lot more effort than making the change to my harness and putting the PTU inside. It will be mounted to a metal bracket near the MPI relay, similar to what We're On Boost did, so it will have some heat sinking.

Thanks again.