CFM928

So after cleaning the 'rodent pantry corn' out of my alternator, and adding a screen to the cooling air tube, my alternator light is on again.

(Previous rodent cleanup thread:
https://rennlist.com/forums/928-foru...-question.html)

I removed it and had my local auto electric guy bench test it. Works fine, triggers at a low RPM, delivers 90 amps. (He offered that if the small blue wire was shorted the alternator light could be on but no excitation voltage at the alternator to energize it.)

Reinstalled, wiring looked fine. Started the car, light goes off, turned on H4s and driving lights, they dimmed slightly at idle, got brighter at 2,000 RPMs. All good.

At idle, with H4s and driving lights on, the alternator light came on, voltage dropped. Restarting the car it was OK again. Engine off / On a few times, all OK.

This morning, tempted to take it across VT, of course the alternator light is on, low voltage.

Could it be anything other than bad grounds?

It acts like a relay failure. I'm wondering about the integrity of the connection between the alternator body and the engine block. May add a jumper cable to test that.

Any other ideas?

Charlie 80S ROW 5 spd

dr bob

The issue is related to heat, which is related to time and load.

Clean the connections for the excitation wiring of course. For practical purposes this starts at the 14-pin connector in the engine bay, and continues through the front-of-engine harness to the alternator. Since your rodents had full access to the back of the alternator via the vent hose, so it's quite possible that they were munching on the insulation there. Did they eat enough to short the excitation lead to ground? Worth inspecting.

But back to the heat and time issue. The alternator gets initial excitation through the wiring mentioned above. Once the alternator is spinning, that initial excitation voltage is exceeded by the alternator's own current production, and that's what causes the dash indicator to go out. After that initial bit of current flow, the light shouldn't come on again so long as the alternator is making power. You 'voltage drops at idle under high load" isn't abnormal, but the light shouldn't come on unless the alternator has stopped making current. As soon as you raise engine speed, voltage should come up without having to restart. The symptom of the light coming back on may be as simple as worn brushes or a failing voltage regulator. Easy enough to install a new regulator as it includes new brushes. But you'll also want to look at the "slip rings" that the brushes ride on to make sure they aren't too worn.

It's possible that the engine-to-chassis ground cable needs attention, especially since the car lives in a salt-intensive environment. New ground cable there is inexpensive. If you haven't already changed the battery ground cable, do that at the same time. No need for a separate alternator-to-chassis cable if the engine ground is in good condition. Just restore the original pieces and you'll restore original function, which was plenty adequate.

Were it mine, and the alternator original, I'd be looking hard at a rebuilt unit unless you have really low miles on it. You have bearings with 40+ year old grease in them for instance. By the time I buy a genuine regulator, teardown and bearing replace, consider trying to replace worn slip rings, and do a full clean dip and bake on the windings myself, a rebuilt unit is pretty attractive. My memory is that you have a Paris-Rhone alternator based on production year. If you decide to upgrade to later-year Bosch for the added current capacity, you'll need to make a small modification to the excitation circuit in the cluster. There are also some good threads that discuss swapping in a more common Delco (GM) alternator, saving a bunch of dollars but sacrificing the external cooling hose and rear cover.

Mrmerlin

NOTE dont buy a non factory regulator for either Bosch or PR alternators or you will do the job twice.
An alternator thats on off on is probably the regulator or brushes.
Higher mileage alternators should be sent out for rebuilt or replaced.
Make sure that the the first note is followed by your rebuilder
good alternator output at 1500 RPM is 13.5v or higher
charging at 13.4v or lower indicates a bad Diode, you have 6 of them.
This could happen after a big booster unit was applied to the charging system to jump start the car.

NOTE with a flat battery disconnect it from the system then charge it with a 6 amp charger for atleast 4 hours

Hey_Allen

I will also suggest checking that there is no point where the blue excitation wire has been rubbed or pinched and shorting intermittently.

My cooling shroud on the back of the alternator was installed by the prior owner or mechanic with the wire caught under the edge.
Once I found and replaced the damaged wire(rotted insulation and corroded copper as far as I could chase into the harness) it started working.

CFM928

Wow! Thanks to all who responded, the alternator was rebuilt in 2017 and has very, very few miles on it. (Not sure what brand regulator / brushes the rebuilder used, I'll ask...) (The cap says Paris Rhone, so it likely is, I believe I replaced the original Marshall (?) in 1986ish...) I rebuilt the front of engine harness ~10ish years ago (Before I was aware of them being available...)

I measured 20 Ohms from the blue excitation wire to ground - before installation - with the alternator disconnected - Is this typical?

(Certainly not shorted, what I can see looks to be intact insulation, and the shroud is plastic so less chance of shorting there. Will also re-clean and check the 14 pin and revisit grounds.

dr bob

With the 14-pin connector disconnected and the connection at the alternator lifted, the excitation wiring should read infinite Ohms to ground. If you are reading 20 Ohms with the wiring disconnected, there's a path to ground (a "partial short", whatever that may be...) somewhere in your harness.

Billu

The regulator is grounded through every mounting bolt, so no need for extra wires to ground the alternator to the block.
Bad /weak grounds usually won’t light the charge light, especially if this happens at above idle engine speed.
Checking continuity with the wire disconnected only at the alternator, you are reading resistance back through the indicator bulb, so there should be a reading.
Checking continuity with both the wire off the alternator and the 14 pin connector disconnected you should get nothing.
If you can rig a jumper loop between the alternator and the exciter wire with a switch, start the car and the switch off the connection, if the light stays on you have a shorted wire. If the light goes out, you have alternator problems.
The fact that the indicated voltage drops when the light comes on usually means bed alternator/regulator.
Your guy doing the test might not have run it long enough for failure.