Basic electrical

[PatricksDad]

Let me preface this by stating my semi-ignorance about automotive electrical stuff up front, but I have more than a passing understanding of basic electrical and electronics concepts.

I'm helping a buddy with a '70 Chevelle project, and one of the tasks is a trunk mount battery w/ Ford solenoid conversion.

Without the budget to go with a new harness, we've got to piggyback on the existing wiring loom. It's in remarkably good shape after 44 years, but not really up to snuff for this kind of think not to mention elec. fans, fuel pump, etc.

He's been told that the battery should be grounded all the way back to the block, which I think is wrong-headed. The reason I'm mounting the solenoid in the trunk w/ the battery is to eliminate another long run of 0 gauge cable, and provide another solid ground path in addition to existing the block-to-frame ground.

Am I overlooking something here?

[milner351]

I'm no expert.
Most trunk battery conversions run the ground off the battery to the frame or unibody - then run another ground from the frame to the engine block up front, as well as at least one good large body ground from the engine to the firewall.

I see no reason (especially at today's prices) to run two large diameter cables the length of the car.

[DanStokes]

I tried to ground the rear mount battery in 'Stang to the unibody and had issues with charging. I had lots of engine to frame grounds, and body to frame grounds and I checked all of them to be sure that they were clean and were making contact. One of my Michigan buddies told me that you have to run a separate ground and sure enough that fixed it. No idea if this is Fox-body specific but I worked for me. I can't begin to explain why - seems like the ground should have worked thru the unibody.

Dan

[squirrel]

The chevelle has a frame. It will conduct electricity pretty well. But only if all your connections are clean, tight, etc. It's easy to miss something. I guess there's a reason to run a Ford solenoid, but I don't know what that would be. If you have starting issues, it's likely due to the wire from the ignitin switch to the neutral safety switch to the solenoid having too much voltage drop, and you can fix that with a small Bosch type relay near the starter, on that wire. There should be no need to add another relay in the main battery cable.

He should run new wires and relays to power high current stuff like fans, fuel pumps, etc. This really doesn't have anything to do with the original wiring, though.

[MR P-BODY]

A lot of guys that run a unibody car might have a issue with a chassis ground...
the body is all spot welded and after time do develop corrosion ... I run the ground
to the chassis (a full tube chassis car all solid welded) and never had any issues...
I have about a 12" lead from the battery to a bolt welded to the chassis right near
the battery... then up forward I have one on the knee loop which grounds the items
inside the car... then another welded to the chassis near the back of the engine for the
engine to be grounded to... you can always do a voltage drop test once you set up your
ground and see what you have.... then run a lead from the engine to the battery and see
if its better.... ALL CONNECTIONS MUST BE CLEAN

[TC]

The reason I'm mounting the solenoid in the trunk w/ the battery is to eliminate another long run of 0 gauge cable,

I don't get this, are you saying by having a second starter solenoid in the trunk near the battery that you'll be able to run a smaller gauge wire to the starter??

I would think that upgrading the wire from the ignition switch to the starter relay on the starter would be a better way to go, if anything it would be a shorter run than running it to the trunk....

And here is a wire size chart, to help you size the wires for your fans and fuel pump......

[anotheridiot]

you run the extra solenoid for a couple reasons, first to avoid positive power running thru those long cables under the frame that are prone to getting shaved. the ford solenoid is more to throw the current to the front of the car when the ignition switch is on, a good measure for theft deterrence. Think of it more like that fat positive wire on your fuse box on your late model car getting power when the ignition switch is on thru that solenoid. It does absolutely nothing to engage the starter gear into the flywheel.

As far as that separate ground, its always a good idea. TC pulled that wire gauge deal off the internet, but dealing with a frame you are really not conducting thru 1/0 copper wire. Stranded wire is better than solid wire because the voltage runs on the outside of each strand. So solid wire gets hotter because all the voltage is at one outside diameter. Stranded wire allows voltage to travel on the outside diameter of each strand. So look at your box frame and you might have about the same square inches of all the stranded wire added together around the steel frame, but you are conducting on a steel frame, not a good conductor like copper or aluminum.

Long run, you are better off running the extra wire. Best to get a length for both cables and some tape to designate which is positive and which is ground. At least you can get one piece that way, but you will be happier because you wont start getting grounding issues. 44 year old chevelles use the frame or body for alot of grounds at lamps anyway, best to just let the frame handle those. Also run a ground to the frame in back as well if you are using any kind of poly mounts for the motor.

[milner351]

As I recall - welding wire is one of the best to use for the trunk mount conversion- I'm sure that stuff is just slightly less than solid gold with the cost of copper these days.

[MR P-BODY]

As I recall - welding wire is one of the best to use for the trunk mount conversion- I'm sure that stuff is just slightly less than solid gold with the cost of copper these days.

Welding cable is better... its finer stranded and more of them... makes it more
flexible and will carry plenty of amps

[Bob Holmes]

I've done this a couple of times. But, all my cars have computer controls and sensors. I always run a ground to the engine block, as well as a ground to the unibody. I use welding cable.

I also always use mega circuit breakers on the long + runs to make sure that the car doesn't burn down if the cable goes to ground.

I can tell you that just relying on a ground to the body, on the racecar, caused the starter to run slow, and eventually the + found a way to ground through the braded steel line oil pressure line welding itself to some stainless hard brake lines.

[TC]

I know a lot of cars that have the battery in the trunk(BMW) just run the battery negative to the body of the car......

[MR P-BODY]

I know a lot of cars that have the battery in the trunk(BMW) just run the battery negative to the body of the car......

This is true... Chrysler does it also and with all the computer junk
on todays cars they dont have a problem... but we will see what
happens after 15 years or so

[Bob Holmes]

I'm not denying that, just giving my experience.

I think that if you are very careful about the installation of the grounds, and you have enough of them (I think we counted on the SVO something like 7 from the engine to the unibody, as well as a direct hook to the battery) you can probably get away with it. OEMs also have electrical engineers on board that can spec the entire system.

[dieselgeek]

Bob Holmes does it the way Motec and other high end race ECU vendors tell you to wire their cars. I like it that way, but grounding to the frame with carefully cleaned (and maintained!) grounds can work fine too. If you run a lot of Digital euqipment in the car, it doesn't like the frame grounds as much IMO.

Second, I love welding cable for the reasons mentioned. Electricity travels on the surface of wires so finer strands are better at distributing heat and lowers resistance which means a lot on long power runs.

Finally, whatever you do - put a circuit breaker or fuse at the head end of ANY power run that you make. I don't care if it's a hassle or expensive... I've seen too many cars burn down with trunk mounted batteries and no protection, that long run WILL eventually chafe.

[dieselgeek]

JeffMcKC rewired his Malibu recently, he might have some input on this too. He did a sort of hybrid between what Bob did, and what you guys are thinking.