Thread: 90 Classic charging circuit woes

The charge light and its connection to the alternator will play no part in the alternator's faults. The main power cable may be faulty though. Check that you can drag a decent current through it without there being any big difference between the terminal at the alternator and the battery. Normally though all such faults are internal to the alternators. My choice of alternators for these used to be something like a Magna alternator off a wreck, or any Australian made Bosch alternator that you could easily adapt to the bracketry. Modern aftermarket alternators all seen to be unadulterated crud.

Originally Posted by bee utey

The charge light and its connection to the alternator will play no part in the alternator's faults. The main power cable may be faulty though. Check that you can drag a decent current through it without there being any big difference between the terminal at the alternator and the battery. Normally though all such faults are internal to the alternators. My choice of alternators for these used to be something like a Magna alternator off a wreck, or any Australian made Bosch alternator that you could easily adapt to the bracketry. Modern aftermarket alternators all seen to be unadulterated crud.

Many thanks BeeUtey, that relieves me from delving into the ignition light circuit. Will check over the main power cable carefully. Have also got an old Bosch Ford alternator I can get rebuilt. I agree that the too cheap to be true new Baxters USA alternators seem light-on (excuse the pun).

The plot thickens...checking the power cable connections from alternator to battery for resistance etc, discovered a short at the terminal post that the alternator and all the starter and battery cables anchor to at the firewall. Now I understand why the alternator was getting so hot. Removed all the cables on the engine bay side - post still showing a short. Got in under the dash and removed all brown power cables - no more short from the terminal post, its nylon insulator being in good condition.
Then faced with finding the source of the short when all the numerous brown cables (battery feed circuit and ignition circuits etc) originate from this terminal post. First separated the battery fed circuit from the post and short still there so not in the (always live) battery fed circuits.
Then tried to check each circuit left identifying the components powered and removing the fuses one at a time to see if I could find the shorted one (though surely a short in a component would blow the fuse). Nothing found.
Ignition switch checked out ok.
Then tried removing relays controlling components identified from the wiring diagram one at a time - nothing found.
This seems to leave the so called "connecting points" for the aircon condenser fans, for the electric seat motors and the EFI harness plug which are unfused prior to the "connecting points".
Am I on the right @!#$% track?
The short is somewhere in the bunch of six brown power cables joined together into a single eyelet attached to the firewall terminal post. These all head off to the left behind the heater/centre dash assembly - presumably to the main fuse board.
Anyone happen to know any more about this bunch of power cables?
Would it make sense to reconnect the battery and try all the electrical equipment and hope not to fry anything?
I'm dreading what it would cost to get an auto electrician to track this short down.

How are you determining that there is a short? If the car starts and runs I'd be suspicious of this "short" as it would smoke out the loom. Is the "short" appearing only when the alternator is connected? Fat plain brown wires are always unfused battery live so a real short would stuff your battery within minutes.

Originally Posted by bee utey

How are you determining that there is a short? If the car starts and runs I'd be suspicious of this "short" as it would smoke out the loom. Is the "short" appearing only when the alternator is connected? Fat plain brown wires are always unfused battery live so a real short would stuff your battery within minutes.

Thanks heaps BeeUtey for your ongoing interest - I'm not that flash on auto electrics and appreciate your advice.
I've been testing for short with 20kohm and/or 200ohm resistance between lead and earth (bolt head on steering column mount) with a $20 multimeter.
The apparent short is there (i.e 0 ohms between power cable and earth) without any battery or alternator connected.
Yep I'm confused by the whole pattern of how a power cable can seem to have connectivity to earth (and it's a bunch of 6 cables) without fires etc. The other power cables that connect to the temrinal post show infinite resistance.
Am I concluding wrongly about the short?
What else would would've cooked up two alternators plus ignition light on with engine running. I haven't yet tried to run the engine with alternator disconnected.
All accessories/ electrical equipment and the engine itself working perfectly prior to what started as the pattern in the previous sentence, last week. Inmiitally I changede the alternator due to a luldly knocking and grinding bearing and replaced with a brand new cheapie from TRS - within 48 hours the "pattern" started. Swapped in the old Magneti Marelli which had been fine when last used as a back up. Same immediate pattern of light on and BBQued alternator with no load except engine running at fast idle. This why I think it's not alternator. Am currently having a revuild done on a Ford Bosch converted for RRC for more permanent alternator reliability, but convinced something else is going on.

OK so a simple multi meter will show a low resistance that is not strictly a dead short. For example a 1 ohm resistance would draw 12 amps at 12 volts so only 144W, not a dead short. The meter may not be very accurate that low down. In your case I would start by not using the main alternator cable in the RRC harness but instead constructing a new one and running it directly to the battery positive terminal. Insulation on the original may have broken down and the short is intermittent with movement. I'm assuming your '90 is very similar to the '89 wreck I have down the back. On that the original alternator wire goes to the starter motor, behind the RH exhaust manifold. Undo the old cable at the starter motor terminal.

As for the rest of the brown wires, they would go to various fuses and relays. On the '89 there is a rack of relays under a cover behind the washer bottle and at first glance many have plain brown unfused inputs. The ECU under your seat also has unfused power near it and you should look for wires on relays and fuse blocks touching their mounting brackets. Pull out any relays one by one and see if the short goes away.

Thanks again BeeUtey, I'll follow up on your recommendations tonight. Looks like I'm chasing shadows with my ohmeter! I'll be mighty grateful if it does turn out to be the alternator to battery cable.
This cable is as you describe - running from alternator to terminal post on firewall behind the R/H exhaust manifold. It isn't in contact with anything and is still encase in its porteectivepolytube but I guess may have broken down or be fried inside.

I had a longer look at the '89 wreck just now, a good time to clear the blackberries invading it from the other side of the fence...

There are 3 lots of brown wires at the bolt through the firewall. The bunch of 6 includes one single wire to the ignition switch and 5 going across the back of the heater box to the fuse box. Circuits with 15, 25 and 30 amp fuses hang off them. Loads of other plain brown wires from the other 2 bunches of 7 go to the relay block behind the washer bottle, the ECU, an ignition relay up high to the right of the steering column and one plug that I assume was for the air conditioning. So many live wires, so little protection. Your multi meter will come in handy in determining which bunch contains your short.