Y Cable Testing?

Just been checking over my engine wiring loom this evening to make sure it all looks OK before I put it back on my engine after its rebuild, and looking at the Y cable, which I had intended to replace while the engine was out, I can't see any issues with it. No corrosion on the connections at each end, and I pulled down the rubber boot at the centre connection, and there's no corrosion there either, the copper is even still shiny. Is the reason these cables fail usually visible or do they corrode somewhere in the middle under the sheathing? Would I be able to maybe test resistance across the cable with a multimeter to check? I could do with saving the money if it doesn't need to be replaced but I'd like to be sure.

I changed mine recently as a precaution whilst I was replacing a broken oil tiller tube. Like you, I found the old one to have zero evidence of corrosion. I had a new one handy that I'd bought new unused at a good price so I went ahead and fitted it. Unless you were getting low voltage readings from the alternator or slow cranking, yours may indeed be okay. A resistance test sounds sensible, someone here will hopefully provide the value ranges....

The biggest thing I noticed comparing the new one to the old is the old one was stiff but the new one was soft and bendy. So I'd say play around with your old one and if it is stuck in a certain shape and doesn't feel like it wants to bend then it's had it.

Mine was the same (clean and no corrosion) so I put it back in. What was needing replaced due to many cores broken was the earth strap. I made a new one and the very slow cranking I had was gone. I also cleaned up all the junctions really well.

Mine had a bit of corrosion where it bolts on the alternator. I just cleaned it up and wacked some Vaseline on it.

Thanks all for the feedback. Still thinking about my idea of checking the resistance of the cable. Assuming that no-one else has a brand new one lying around to check the resistance of, I've found this thread: https://rennlist.com/forums/996-forum/764630-updated-alternator-starter-cable-5.html Which has measurements of the length and width of the cables. I then used an online calculator for wire resistance to get these values: So I'm thinking that if I measure the resistance of my cables, measuring each cable individually, ignoring the join and connectors at each end, if I get values similar to the above I should be ok? I will fully admit that electrics and wiring are really not a strong point of mine so if anyone would like to point out something I'm forgetting please do.

I suspect given the fractions of an amp and voltage which a multimeter may use to provide a resistance reading, seem likely to provide little confidence in the test procedure of a high current carrying circuit such as that of a starter motor. As an approximation I think at the point of initiation, the solenoid contacts will be exposed to circa one thousand amps, all be it for a fraction of a second, till the starter motor begins to rotate. If one adds heat as being related to slow starter operation.... I assume the cable will be cool when testing with the multimeter.. As has been suggested clean the points of contact and more so on the earth/ground cable points of connection as those seem more exposed to corrosion build up and flexing causing cable damage.. A replacement y cable may not resolve any issue but can be good insurance . As for the cost saving, I guess one measures that against the task of replacing it with the engine in place.....and against that... The Y cable is very unlikely to totally fail, leaving you at the side of the road. I suspect most breakdowns are related to the cumulative effects gained over time, old battery, alternator output perhaps reduced, dirty or loose connections, the solenoid and starter motor as a unit... An intermittent starter circuit fault can leave you at the side of the road, but unlikely to be caused by the Y cable only. Unless anyone knows of that ever happening, and proved positive... ? Just thinking in type,

I do have a new unused one lying around actually as I bought before I fixed my issue…

If you happen to also own a multimeter that would be extremely helpful

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TL;DR no, even disregarding the effects of temperature, you want an ohmometer to get a usable reading, the resistance functionality of a DIY multimeter is to measure resistors and do some basic open/short troubleshooting not to measure wire resistance

I'm not sure those cables are that big in mm2 , they've included the very thick insulation. The main starter cable is what I've always called 35mm2 (the one they measured OD of 11.7mm) , the alternator much smaller probably 20mm2. The flex I used for the earth strap was almost definitely 35mm2 but let me check...let's get this right it's a really interesting thing to do. Edit: just checked and yes the cable I use for big loads (inverters etc) including the Earth strap is 35mm2 and has an OD inc. insulation of 11.8mm. Maybe ATM could measure the small alternator cable from his old one if he still has it. Remember if you take the diameter of the copper it doesn't mean much because it's multi core so not solid copper. 8mm diameter solid cable would be roughly 3.14 x (4x4) = 50mm2 Multi is less as it's the diameter of each strand x the amount of strands

Quotes source post: 2818764

Even a professional quality ohm meter or megger kicking out 500volts, may not be enough to detect a connection in a cable joint that fails intermittently when carrying far more current than a megger or any other similar instrument may be able to generate... Having read of these hot start issues, I can not remember anyone definitely able to resolve the situation by only replacing one item. ie when replacing a suspect starter motor, you will likely have disconnected the battery, as well as the terminals at the starter motor solenoid, either or both of which could have had crud at the points of contact which may well have caused the issue.... But cleaning the contacts while in the process of putting things back together, may equate to the starter being defined as the cause, thus binning a working starter.... when the fault was actually cruddy connections..? Don`t ask me how I know...

I just spotted this warning on an extension lead and thought it might help some people out. This extension cable feels a lot thinner than the chunkier side of the y cable - possibly both sides of it. You can see its rated at 10 amps. 10 amps in your home is a lot because at 240v that's 2400 Watts. I'd guess the starter motor drags or demands over 50 amps when you crank it - 600 Watts. Our batteries have over 80 CCA - Cold Cranking Amps. And that's the difference here - the amps. More amps demands more cable or less resistance. Any tiny resistance in the cable or joints or fastenings or anything will be amplified by more Amps. And this resistance will get worse with heat. Heat increases resistance. That's how you get electrical fires started in cars. So even though cranking is quick and only lasts a couple of seconds it demands a lot of Amps. I think age and heat cycles damages the copper - by creating oxidisation and it definitely damages the insulation because its rubber based so like an old tyre that starts cracking. The insulation is there to protect a bit against the heat from the engine and oxidisation of the copper as well as prevent short circuits.

On the Rennlist thread there's a pic of the crimp on the end of the thinner cable, it says 8-25 which means the hole in the lug is 8mm and the wire cross section is 25mm! I think Uncle Monty is spot on with his size estimation for the larger cable too of 35mm2. In your calculator the estimated sizes are too large. Also the starter on these engines is around 2.5 kw which means it will pull around 300 amps when cranking and maybe as much as 800 initially! 800 amps requires a big surface area, if the y cable has corrosion on the inner surfaces of the crimp where you can't see, then the surface area is not great enough meaning the resistance is too high and you get rapid heating which increases the resistance further. This lowers the voltage available to drive the starter motor, hence the slow starting. If you measure the resistance of good and bad cables with a multimeter they both will show a low resistance as the multimeter only tests using a couple of milliamps, a tiny fraction of the current required to drive the starter. Therefore you get no heating and can't measure the increase in resistance that occurs during cranking. There maybe a very small difference eg a good cable may show 0.0 - 0.1 ohms, and a bad one maybe 0.1- 0.2 for example, but the average multimeter doesn't have the resolution to use these readings as definite proof of good or bad. A better way of checking is looking for signs of corrosion or heat at the point the cable enters the crimps! ATM ATM your currents and wattages are a bit out there, but your point is still a good one!

By the way the resistance of 0.36m of 25mm2 copper cable is 0.26 m ohms, or 0.00026 ohms. The average multimeter only has a resolution of 0.1 ohms !! I suppose if people are measuring their cable at more than say 0.2 ohms then that would suggest there is something wrong with it. A new cable should be virtually zero which means it should be the same as if you just touch the two multimeter leads together! Still not sure if this is a valid way to diagnose a faulty y cable. Probably just easier to replace if you are having starting problems!