Hi all.
Thats the problem. The alternator is new and it works fine but, when I use D, 3, 2 or 1...volts fall down until 11,5 volts with car running or stopped. If I use P, N or R, volts are right (13,5 - 14).
This Vette is an 86.

Any idea of this?
Regards from Spain

 

You are refering to idle only?
The volts drop when idling in D 3n 2 or 1...

does the voltage go up as the engine rpm goes up ?

dropping to 11.5 at low idle is common. as long as the volts jump up instantly with the engine speed its ok. These cars are plastic and have great problems with good steady grounding of the electrical system. There are ground wires everywhere and each portion of the system has to have its own ground. As the car ages the quality of all these ground connection goes down. Thats part of the reason that idling drops the voltage to 11 witht hings running. The other reason is that the voltage regulator is not set properly in most of these reman alternators because they have no idea of what the demand is since they do not have it on your car, so they go LOW on the guess at where to set the voltage, so it does not over-charge and fry the system. An alt thats got the regulator set too high can easily burn up the delicate electrical system of a computer controlled car.

There is a way to turn UP the regulator output, but you risk doing some damage and ruining the alt...so if its getting more than 12V above idle (600 rpm) then its not worth messing with.

The idle speed is the common denominator in the gears that you mentioned. park, neut or Rev are either low drag or no drag and no pulling the idle down as far as OD or D or the other forward gears.

Like I was saying, as long as the volts go up over 12 as the engine speed increases it should be fine. It should go to 13 <plus/minus> .5v when cruising steady at 55-60 mph (1800-2000 rpm). Over 14v briefly just after start-up then settle down as the battery is full again.

Now IF you are seeing 11.5V anywhere over about 800+ rpm...thats a problem and the regulator needs to be adjusted or replaced. Always clean and tighten any and all battery connections, plugs on the alt, AND the small jumping post that has several power wires from the battery to supply things inside the cabin.

 

Ok, may be I didn´t find the corrects words to explain it, because english isn´t my best language

It happens when idle is at 700 rpm or at 3000. When you use 1,2,3 and D, alternator doesn´t charge the battery, but it works if you use R,N or P. Increassing rpms it continues at 11,5 V and falling down...11,4...11,3...11,2...
It´s like if something deactivates alternator when you use 1 - D, only in that case.
Thanks for your time and help

 

leesvet is likely on track.

Looking at my '87 FSM electrical diagnostic supplement there is no obvious direct connection between the transmission position switch and the charging system. So, when in gear, the only effect should be load on the engine or current in a shared ground connection. You might use a volt meter to double check the dash voltage display is accurate. Since you are getting a low voltage value with high engine rpms a gound problem seems likely.

 

The ONLY other thing that I can think of would be a VERY VERY slick, glazed belt that was slipping a LOT on the alt pulley....did somebody armour-all the belt?

I don;pt know what else could cause that besides bad grounds/connections somewhere.

 

Can you verify the voltage drop with a hand held voltmeter.

 

just found out on mine the plug that plugs into the alt was sending the turn on signal to the wrong pin for my alt. resulting in low volts, loss of charge didnt even know alt doesnt always run.. get a good alt shop to make sure your plug is in sync wuth your alt...

 

There is no connection between the gearshift and the alternator. The alternator should put out 14.3 volts (dash voltmeter) when the engine is cold and idle even as low as 700 rpm. In any forward gear, the engine will idle lower than in N and if too low (say 500 rpm) , the alternator cannot generate its full output. When you drive the car and engine rpm is way above idle rpm, the alternator should easily be able to maintain 14.3 volts. Removing the alternator isn't hard and it would be worth it to take it to a parts store that can test the alternator at low rpm. There is no adjustment for alternator output voltage as the voltage regulator module inside the alternator is a sealed unit. A previous suggestion that you measure the battery terminal voltage with a voltmeter and compare it to the dash voltmeter is good advice.

 

I was annoyed by low alternator voltage output at idle in drive when at a stop or going through a drive though fast food place, or stuck in Chicago traffic during rush hour when I still commuted daily to my old job up there.

What your are experiencing is 100 % normal.
11.5 to 12.2 volts in drive, foot on the brake, and sitting still longer than 20 seconds.
Drive pulley ratio on C4's is 3 to 1.
If the crankshaft is turning 600 RPM's like in Drive trans range & foot on the brake, alternator rotor speed is 1,800 RPM's exact.
C4 CS 130 series alternators don't start charging till the alternator rotor speed is at least 2,000 RPM's...12.6 vdc generated output under 0 to 100 amp load.

At 700 RPM's crankshaft speed,
alternator rotor speed is 2,100 RPM's.

I installed a POWERMASTER XS series high output 140 amp alternator with an external adjustable voltage regulator.
& installed 1 of there very small chrome 28 mm drive pulleys.
I set the voltage output to 15.1 volts DC at 2,000 crankshaft RPM's.
Checked at the alternator charging output lug & at the + positive battery terminal.
There is a tiny potentiometer on the back of these XS series Powermaster alternators for DC voltage setpoint output.
Can deliver over 18 volts DC under 140 amp continuous loads.
Cost for the alternator in 2008 was about $165.00 my cost from my local speed shop.
Small chrome drive pulley was about $22.00.

My 87 in drive & foot on the brake & engine fully warmed up,
625 RPM's crankshaft speed.
I see 12.6 vdc on the LCD dash voltmeter now.

Having the voltage setpoint at 15.1 volts,
car is more responsive than before,
Fuel pump(s) spin faster ( using 1990 ZR-1 dual fuel pump assembly),
Hotter ignition spark present, I did visually check with an external spark tester- Bright Hot Blue at all times.
No problems with overcharging the vehicle storage battery after 3 years.
ECM & ignition module A- OK after 35,000 driving miles since & installed the Powermaster Alternator and tiny driven pulley.

Worked out OK for me.

Power Master XS series alternators were designed for Race car use with up to 18 volt DC storage batteries.
Works A- OK on the street with 12 storage car batteries I can tell all here on c4 too.

Brian

 

Hola. ¿Qué tal?

Is it just the display on the dashboard that reads incorrectly or have you measured the output from the alternator with a handheld voltmeter?

 

Yes. Check the voltage at the battery itself. You'll probably need someone in the car to tell you what the dash is reading.

 

Ok, thanks to all.
I have no time this days, but i will try to do everything you told me.
You'll know all the details when I've done it.

Thanks!

 

This sounds like something a Shop Teacher sets up for his/her class here in the States. Unusual that it doesn't occur in Reverse, though because it doesn't, I'd assume the P/N Switch is ok. If not, it would stall or the RPM's would be extremely low with any transmission load. That leaves the Torque Converter Circuit which also uses a gear position switch (usually closed D, 1 & 2 to prevent lockup in those gear positions) in the console. I'd disconnect it and if the juice comes back, something in the circuit is sucking the life out of it.

 

Hi!!

Ok, solved problem, something stupid
The red alternator wire has a poor connection. When I move it at any gear, it doesn´t works. I suppouse that ¨D¨ gear moves a little bit the car and wire when is geared.
So...no problem. A joker wire simply.

Thanks to all, take a beer!!


P.D: Excuse my poor english :P