Molly

I am going to battery cables in my 86 951 because their covers are cracking.. I want to test first and see where it is now before I start. So for example to test for resistance on the bell housing ground I guess I can do a resistance check. I have an Innova auto sensing meter. So I assume I will set it for OHMs and contact at each end of the cable.. right? And what should I see if it has good flow and if the ground is good?

Thanks

divil

Resistance checks on battery cables - or anything that carries a lot of current - won't actually tell you anything. Tiny resistances matter a lot for things like that, and ohm meters are just not sensitive enough. As an analogy, it's like trying to check a big sewer pipe for blockage by pouring a glass of water through: it will almost certainly get through without any resistance.

When you crank your starter, the current can reach as high as 600 amps initially. With a 12v battery, using Ohm's law, this means the resistance in the circuit is 0.02 ohms. That is a tiny resistance...your multimeter will have a hard time distinguishing that from say 0.03 or 0.04 ohms. But at 0.04, that's twice the resistance - that would cut your current in half!

The easiest way to check the condition of cables like that is with a voltage drop test - you should google it, or better yet look on youtube, and you'll find plenty of detailed info. In a nutshell, when a lot of current flows through the cable, the voltage will drop significantly, and the voltage drop will be very sensitive to tiny changes in resistance - and it's easy to measure.

All that said, if you actually see cracked insulation, I'd replace the cables anyway. And if they're original, I'd replace them even if you don't see cracks! There is a guy here on rennlist called Robby that makes high quality replacement cables.

EDIT...here you go:



He takes a while to get to the point but it's a good explanation in the end

Cyberpunky

Re-do your grounds anyway. As stated above, a small resistance can cause huge voltage drops which can effect how the cars run and drive you nuts. Tom M’Guinness suggests removing ground bolts, running a tap into the thread and then using new bolts, and I think it's sage advice and well worth doing Click here to see his How to

951kaos

The original battery cables on my 86 looked pristine, no cracks or corrosion build up at the terminals; but I was having charging problems. A check of voltage drops from terminal to cable revealed about a 1 volt drop. Cutting open the crimped lead battery terminal revealed that there was a lot of corrosion/oxidation build up between the cable and the lead. I replaced all the battery cables as a precautionary measure and have had no charging problems since. You can purchase cables from your local auto parts store; Porsche dealer or one of several Rennlist vendors who make replacement cable sets. I purchased a set of IceShark cables from Robby (https://rennlist.com/forums/924-931-...able-kits.html) and found them to be of excellent quality and sells additional ground cable and a power cable to the fuse box.

raleighBahn

The corrosion can hide under the insulation as shown below (when I pulled the strap from my car last year):

mahoney944

So to properly check voltage drop on our cars do you just need to turn the key all the way forward without starting? Or do you need to have the starter cranking? He uses a toggle switch in the video but I'm thinking the starter positive cable is only under load when cranking? Right? I would assume you need to pull the DME relay or a fuse to stop fuel and crank and look for drop. How much drop is allowed to be considered good.

divil

Yes you need to check it when loaded, which means cranking in the case of the battery cable, lights on if you are checking your light wiring etc. I'm not sure what is normal with perfect cables, but I think it would be something low, like less than 1v. If I saw much more than this, I'd be concerned even if the starter was cranking ok - because whatever energy is lost there is being turned into heat. Also, in the case of headlights, bear in mind that the brightness to voltage relationship is not linear, so a small voltage drop will make a big difference to brightness.

divil

BTW, here's one reason why you will still get corrosion even if there insulation doesn't seem to be cracked...

The factory heat shrink doesn't really seal the connections:





Also, this cable didn't look to bad until I took it off the car, only then did the extent of the cracks begin to show in the sleeve:





The IceShark cables that Robby makes have (among other great features) sealed glue-lined heat shrink around the terminal connections, fiberglass protective sheathing, and heavier gauge wire (less resistance). Anyone who wants to learn a lot about this kind of stuff should read IceShark's old posts...absolute gold.

Molly

Thanks everyone. I am definitely fixing this asap, as I now have a new related gremlin..

Sometimes I can hear a loud, very strained, whirring from the right rear while driving. It's an electrical motor for sure. Lasts about 10 seconds. Happens maybe 6 or 7 times a week in daily driving. I've heard it before and I assume it is the fuel pump. So in addition to finding the offending straining part, perhaps most concerning is that it causes a massive drop in the lights and idle when it happens, and the volt gauge shoots right up to above 16 and into the red zone. Usually it reads about 13 during the day, and 12.5 with lights on.

So any thoughts on that 16 volt spike would help.