No charge light comes on at low RPMs

[manoa matt]

Hello All,

I've got a 1978 Fiat 1800cc The car originaly came with a Bosch 55 amp alternator with the internal regulator. I had a 65amp alternator of the same design and decided to upgrade. I took the 65 amp unit to a local shop that is dedicated to rebuilding starters and alternators. The guy took it apart, cleaned everything, and checked it out. I installed the 65 amp unit in place of the 55 amp unit. I the only difference I could see was that the 65 amp unit had a slightly smaller pully than the 55 amp. Which means that the 65 amp unit spins faster than the 55 amp unit.

Everything was fine for a couple weeks of driving. Now when I'm stopped at a light or just idleing the no charge battery indicator light will come on. Sometimes it will just flicker a bit. As soon as I apply the gas pedal the light will go off.

The light goes off right at start up. After the car is warmed up and driven for a bit the light will come on when I come to a stop. Regardless of wether the car is idleing at 500, 700, or 900 rpms the light will come on, but as soon as I nudge the gas pedal the light will go off.

I've done all the charging system checks as outlined in the Fiat factory service manual Page 55-31 and everything checks out good and strong. There are no blown fuses and all connections and mounting surfaces are bright metal clean. I even took it to Checker and had them check the battery, alternator, and voltage regulator with and without load and everything checked out good and strong.

While looking for alternators I came across a tech tip for a 65 amp alternator for a fiat. Thats not the alternator that I have but I think the test should be applicable. Here is the link: http://www.partsamerica.com/product_ima ... RW/518.pdf

In case the link dosen't work Here is the crux of it: (The B+ wire should have battery voltage at all times. The D+ wire must have AT LEAST 2 VOLTS when the ignition switch is "ON" Lower voltage readings will prevent the alternator from charging. Check D+ wiring for breaks, indicator lamp(burned out bulb) and all fuses.)

According to that test my D+ voltage reading is about 1.4 volts.

No blown fuses, but I have not traced the wire all the way back yet. Could that wire be rubbing on the gas pedal some where? Or is it the voltage regulator?

Matt

[mdrburchette]

Could it be a loose alt belt, Matt? I'm assuming the D+ wire is your field wire activating your alternator light? Is it reading 1.4v when the light is or while it off too?

[So Cal Mark]

I'd also suggest checking belt tension. How in the world do you get the engine to idle at 500rpm? As for electronic regulators, once they turn on they usually stay on no matter what the rpm goes to. Most of those internal regulators need 1000-1200 rpm to turn on initially. Have you checked output from the alternator at idle?

[manoa matt]

As I drove around yesterday the RPMs at which the light would go off increased. At the start of the day it would go off around 1000 rpms. By the time I was driving home it would not go off untill 2500 rpms. On the way home I stoped and bought a new voltage regulator and installed it. No change the light still came on. After I got home I removed the 65 amp unit and re-installed the 55amp unit and put the new regulator on it. Drove around for about 15-20 minutes and the light came on again.

Given that the light comes on with a new regulator and does it with both alternators I can only assume that the fault is in the wiring from the alternator to the gauge. Possibly one of the connections in the dash or by the pedals. Tonight I'll trace the wire back to the dash and inspect all the connections and test for continuity. Hopefully its just a bad connection or ground.

[So Cal Mark]

while many cars have power to one side of the light and the other side leading directly to the alt for ground, some of the Spiders use a relay to operate the light

[mbouse]

i believe Fiat tune up spec calls for a 750 rpm idle speed on a warm engine. if you are idling at 500 rpm you are too low, and should be near stalling. add to that any inaccuracies in the tach reading and you might even be lower than that.

what does your VOM say at various times checking the battery itself? are you recording these readings with all other uses of electricity in the off mode (no lights, no radio, etc)?

a squeamish idiot light is annoying, but only an indication of an issue. it does not guaruntee a problem exists.

send me a ticket. i will fly out and fix it for free. GAWD, i am tired of Michigan Winters!!

[manoa matt]

I have sufficient belt tension. No relay in my circuit. No blown fuses.

Volt readings at battery are:
1). With key "ON" 13.25v
2). With coil disconnected crank test 11.3v
3). With car idling at 2500 rpm and no electrical on 14.44v
4). With car idling at 2500rpm with low beams on and heater blower fan on high 14.18v

I checked voltage output at the alternator but didn't write it down and can't remember right now.

Went to the starter/alternator rebuild guy yesterday at lunch. Light was on while driving there. Got there and he hooked it up to the "machine" and the light would not come on. Everything tested fine. Drove away and a few miles down the road the light came on. I knew that would happen. Same thing happened at Checker on the way to work.

As for Idle, It is set at 800 to 850 rpms. Durring the warm up period it idles a little high sometimes like 950 rpm. After warm up it settles back down to around 800 rpms. But every now and then when I come to a stop the rpms will drop to mid way between 0 and 1000 because the brake booster is sucking air. (its a new booster from IAP so its not a bad diaphram, just "sucks" more than the original.)

matt

[mbouse]

i'd be surprised if your idiot light was not on at 500 rpm.

readings from the alternator output are gonna be critical to determine where the issue is. faulty reading from the idiot llight, or are you having alternator issues at 2500 rpm?

and, when presented to the mechanic, the intermittant problem will ALWAYS be in remission. Murphy's law #608

[manoa matt]

My tests were at 2500 rpms as per the testing procedure in the Fiat factory service manual page 55-31. However the tests performed by Checker auto parts and my local alternator rebuild guy were at regular idle speed.

Idiot light is on from 0 rpms up to 2500 then it goes off. I think Murphy was the previous owner of my fiat

Mike, About flying out to fix the problem: You got a free place to stay and I'll show you around the island, but you got to buy your own plane ticket.

[So Cal Mark]

with over 14v at the battery while running, the alt is obviously charging. It's important to test it under various conditions though. I recently had a Dodge truck in that charged normally until 3500rpm. Then it would overcharge at 18v, melted the instrument cluster and gauge! Turned out the rebuilt unit that the customer had installed had a loose wire in the stator and at 3500 the centrifigul force would send it to ground, maxing the output.

So, when the light is on, you're measuring 14 v at the battery? The way the light circuit works when there isn't a relay is that there is power to one side of the light with the key on, the other side grounds in the alternator. Once the alt is turning and charging, there is power to both sides of the light. With no ground the light is out. So you could disconnect the light circuit from the alt and see if the light is illuminated. If so, then the circuit from the light to the alt is shorting somewhere, providing a ground

[mbouse]

My wife is packing now; give us a couple of hours. it was MINUS 3 farenheight here this morning....and that does not include the wind chill factor. S'plain that, algore.

Hook up a portable VOM to the alternator, and drive around the island while watching gauge and not the bikini clad beauties on the beach. Get the engine to 500 rpm, 1500, 2500, and if possible 3500 rpm while grabbing periodic voltage checks at each major change in speed. Betcha see that same 14 volts when the engine is at 1000 or greater rpms; proving a solid alternator.

Process of elimination then says it is a defective idiot light circuit.

[manoa matt]

Bought a volt meter last night. Its real simple to hook up, but with a friend in town I did'nt have the time to hook it up or to chase the wiring and test it. Weekend project time.

[manoa matt]

And stop calling it the idiot light, you are starting to make me feel bad

[So Cal Mark]

no idiots here, it's a charge indicator light

[mbouse]

i beg your pardon. did not mean to infer.

in michigan, they are called idiot lights, and i resemble that remark!