P38A Alternator cable repair advice needed

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JamesBB

JamesBB

Active Member
Posts
761
Location
Sandwich. Kent.
Hi All

GEMS 4.6 with original Magneti Marelli alternator but new rectifier/regulator. Been running fine for a very long time.

My car conked out while driving but first threw a load of errors like it was a flat battery.

It seems the main red B+ power cable on the alternator was slightly burnt out. A bad/corroded connection directly at the spade crimp had clearly gone high resistance and heated up before it all just gave up.

A temporary roadside repair was done by cutting back about an inch or so of the red plastic sheathing that had been damaged and cleaning the copper underneath then putting on a new crimp connector. It loses half a volt between the alternator connector and the battery.

So now I need to repair the cable properly and more permanently. I can see another inch of the cable needs stripping back (hence voltage loss) and then an extension made to replace the cut-out part.

How would you guys do this? New heavy gauge battery cable and splice it in? If so what is the best connector to use to attach to the existing cable?

Also, I have seen guys run new cables direct from the alternator to new terminals on the battery post. Is that a better fix? I guess the original cable still needs to be attached to the alternator to complete the circuit to the main loom.

Advice is most welcome as always.
Thanks
 
Run a cable from the alt to the battery is a good idea as long as the starter has a good feed front the battery it doesn't matter. Any crimped connection will do better if soldered as well;)
 
JamesBB said:
Hi All

GEMS 4.6 with original Magneti Marelli alternator but new rectifier/regulator. Been running fine for a very long time.

My car conked out while driving but first threw a load of errors like it was a flat battery.

It seems the main red B+ power cable on the alternator was slightly burnt out. A bad/corroded connection directly at the spade crimp had clearly gone high resistance and heated up before it all just gave up.

A temporary roadside repair was done by cutting back about an inch or so of the red plastic sheathing that had been damaged and cleaning the copper underneath then putting on a new crimp connector. It loses half a volt between the alternator connector and the battery.

So now I need to repair the cable properly and more permanently. I can see another inch of the cable needs stripping back (hence voltage loss) and then an extension made to replace the cut-out part.

How would you guys do this? New heavy gauge battery cable and splice it in? If so what is the best connector to use to attach to the existing cable?

Also, I have seen guys run new cables direct from the alternator to new terminals on the battery post. Is that a better fix? I guess the original cable still needs to be attached to the alternator to complete the circuit to the main loom.

Advice is most welcome as always.
Thanks
Click to expand...
Never splice high current cables. Run a new cable of the correct size following the route of the old cable.
 
JamesBB said:
Thanks, but the problem is surely that means replacing the engine wiring loom as it is all 1 item.
Click to expand...
I don't know the V8, but no need to replace the rest of the engine loom. Just cut the ends off the damaged wire ensuring that it is no longer connected to anything and run a new wire. It cannot be difficult, there are normally 2 very heavy gauge wires from the battery, one to the starter the other to the alternator. On some cars the the alternator output goes to the starter and from there to the battery.
 
There are 2 heavy gauge cables from the battery terminal. 1 to the fusebox, the other seems to go down to the starter/solenoid.
The cable from the alternator is a smaller gauge and disappears into trunking towards the fusebox.
I will grab a couple of metres of cable to replace that, should be more than enough, then finish tracing exactly where it goes.

upload_2021-12-4_15-40-58.png

upload_2021-12-4_15-41-43.png

Sadly the power dist diagram does not give much away.
 
JamesBB said:
There are 2 heavy gauge cables from the battery terminal. 1 to the fusebox, the other seems to go down to the starter/solenoid.
The cable from the alternator is a smaller gauge and disappears into trunking towards the fusebox.
I will grab a couple of metres of cable to replace that, should be more than enough, then finish tracing exactly where it goes.

View attachment 253835
View attachment 253836
Sadly the power dist diagram does not give much away.
Click to expand...
According to the schematic the alternator output goes to the battery via the starter solenoid. Is the smaller wire from the alternator not the D+ wire?
 
Datatek said:
According to the schematic the alternator output goes to the battery via the starter solenoid. Is the smaller wire from the alternator not the D+ wire?
Click to expand...
Thanks for your help so far. Appreciated.
It may be D+ but if you look at the other diagram below it mentions B+. Both refer to connector Z106. I guess the smaller wire is between the starter solenoid and the alternator, but it's not clear, almost contradictory.

upload_2021-12-4_18-26-49.png
 
Wait, in terms of D+, you are not referring to the tiny thin brown wire running off of the alternator into the same part of the loom are you?
There are 2 wires on mine. 1 x Thick red (slightly charred) but thinner (guess 30% of the diameter) than the main battery B+ cable; and 1 x thin brown wire.
 
JamesBB said:
Wait, in terms of D+, you are not referring to the tiny thin brown wire running off of the alternator into the same part of the loom are you?
There are 2 wires on mine. 1 x Thick red (slightly charred) but thinner (guess 30% of the diameter) than the main battery B+ cable; and 1 x thin brown wire.
Click to expand...
Looking at the wires on it, the thin one is D+, which I guess is AKA the lamp wire. So I am not referring to that one. The main B+ then, which is actually a smaller gauge than the main battery cables. It's original, so I wonder why it's smaller..... Sounds like I need to get under there and see if it does go down to the starter solenoid then.
 
Thin wire with small ring terminal should go to D+ for the warning lamp / sensing the voltage for regulation, fed via BECM and dash lamp.

Heavy cable on Otto runs Alternator output (B+) to starter solenoid, then up to battery terminal. Battery connection then has another heavy cable to the fusebox.

Sounds to me as if yours has been wired with a heavy cable to the starter, and a slightly lighter (too light?) cable to the battery to supplement or replace the original starter to battery section?
 
OK, you are making this way more complicated than it needs to be.

Remove the cable from the alternator to the starter and throw it in the bin. It is bad practice and later P38s run straight to the battery.

Get 50cm or so of 150A cable from your local electrical hobby store.

Put an appropriate loop fitting on each end. Connect one end to the B+ and the other to the bolt that secures the +ve terminal on the battery.

Replace the battery cover, close the bonnet, drive the car.
 
Marshall8hp said:
OK, you are making this way more complicated than it needs to be.

Remove the cable from the alternator to the starter and throw it in the bin. It is bad practice and later P38s run straight to the battery.

Get 50cm or so of 150A cable from your local electrical hobby store.

Put an appropriate loop fitting on each end. Connect one end to the B+ and the other to the bolt that secures the +ve terminal on the battery.

Replace the battery cover, close the bonnet, drive the car.
Click to expand...

This is an early one from1996, so it makes sense to run direct from the alternator to the battery. I could not work out at all why they ran it down to the starter, then back up to the battery.
Cheers
 
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