[1965_SSChevelle]

Took the car out for a drive. First time out at night. At hard acceleration above 5 grand I noticed my headlights got real dim. I have a chevy serpentine system GM 10si 105 amp alternator. They work great and a bright under 5000 but this has me a little concerned. Should I be?

 

[Steve340]

belt slippage?

 

[1971 Chevelle]

Spin her up to 7 or 8 K and see if it comes back out of it.:hurray:

 

[Olle]

You might just be approaching the speed of light... Seriously, it sounds like it could be belt slippage. Have you tried belt dressing?

 

[1965_SSChevelle]

With it being a serpentine system I didnt think the belt could/would slip. Ill check that tonight.

 

[Steve340]

With it being a serpentine system I didnt think the belt could/would slip. Ill check that tonight.

Yea most serpentine belts have some sort of spring loaded tensioner, if its weak, it could slip as the engine rpm rises, unless your alternator is failing at highg rpm. which I have personally never seen.

Might even consider a volt meter in the car as you rise rpm, and see what the voltage is doing

 

[1965_SSChevelle]

Setup is new so I dont think its the spring. I may run a temp volt meter to watch it.

 

[Steve340]

how does it energize the field, is it a new internally regulated alternator?

Also, when hard on it, the engine is going to move, any chance somethings grounding, maybe a cable on a header pipe, or pulling or pinching a ground wire?> Rubbing the belt or pully against something?
try revving in neutral and see if it does the same thing, because without a load from the drivetrain, the engine shouldent move nearly as much

Also see if you have a good engine to frame ground

 

[1965_SSChevelle]

New Internal alternator. Ill take a closer look at the grounds today.

 

[1965_SSChevelle]

Went out to the garage tonight and moved the negative wire from the battery to the head for a better ground. Turned the lights off in the garage and fired her up. Watched the tach pretty close this time and at 5300 everyting starts to dim, not just the headlights. Dash lights, dome light. Im more confused now than when I started.

PS. This is what I used to switch to the Internal Regulated Alternator.

External Regulated Alternator to Internal Regulated Alternator
Conversion Kit for 1963 to 1972 GM Cars and Trucks
If the harnesses in your car are in good shape, but you would like to get rid of your external regulated alternator and install an internal regulated alternator this kits for you. Easy to install: 1) Disconnect your external voltage regulator leads and plug in our "Special Regulator Adapter" to the regulator lead wires. 2) Disconnect your alternator lead wires. 3) Install new GM internal regulated alternator. 4) Plug in our "Special Alternator Adapter" to the alternator lead wires. That's really how simple it is. Part # 27555. Price $19.00. Includes both adapters and instruction sheet. We also offer free technical support if you can't figure it out.

 

[bochnak]

It seems like you are bypassing/not taking full advantage of your voltage sensing wire from the 10si. The sensing wire should be wired as described here:

http://www.madelectrical.com/electricaltech/remotevoltagesensing.shtml

Your alternator doesn't know there is a voltage drop in the system?

 

[Steve340]

So your like 14-14.6 until 5K, then it starts to slide back? obviously to below 12 or so right?

Just doesnt make sense, im beginning to think your alternator is defective if everythings wired right, because if its putting out 14 volts at 3,000 whats the difference to the alternator at 5,000? None, Either the Alternator is slowing down, or its bad, did you call the manufacturer?

And I didnt ask this, but did you put a voltmeter on the output wire of the alternator at 5300, see if its putting it out or not?

If your throwing 12 volts to it, the internal regulator is setting field voltage, which sets the output

 

[1965_SSChevelle]

Have not put a voltmeter on it yet. Im going to do that this weekend when I a have some help. Im beginning to think the alternator is bad also. I bought a complte GM serpintine kit from Summit last year so I doubt it would be under warranty.

UPDATE: I just went and checked with a volt meter. At idle I have 14.56 coming out of the alternator. When I get some help Ill check it at 5300. I really appreciate everyones help so far

 

[bochnak]

I don't think it matters what the back of the alternator is putting out. However, I would compare this voltage (back of alternator) to other areas of the electrical system, headlights, ignition, and any distribution blocks you may have. These old electrical systems have huge voltage drops due to long length wires. I would read through all of MAD Electrical tech articles and possibly re-wire your system per their kits.

 

[Steve340]

Well it does matter, you need to know if you have an alternator issue, or a car wiring issue
But think of this
the body of the car, doesnt know that the engine is turning 5000 RPM
see my point?
Your fusebox feels happy at 1000 RPM and no changes occur of any kind when you increase the engine RPM

So if the alternator is putting out 14.6 at 5000 RPM, you have to think of what changes does the car go through at 5000 RPM, this will help find the problem

 

[1Fast7D]

PS. This is what I used to switch to the Internal Regulated Alternator.

External Regulated Alternator to Internal Regulated Alternator
Conversion Kit for 1963 to 1972 GM Cars and Trucks

It seems like you are bypassing/not taking full advantage of your voltage sensing wire from the 10si. The sensing wire should be wired as described here:

That conversion puts the sense wire in the same spot as the original did, at the main splice, so the conversion kit is not the problem.

1965_SSChevelle, most likely your problem is belt slippage, a wire loose or being pinched and short circuiting when the engine reved. (soft short) To check for belt slippage, try a little belt dressing on the belt. If you have any chrome pulleys, they are known to allow belt slippage at higher RPMs. Also check belt length, the belt tensioner will not keep the belt tight with to long of a belt. The tensioner needs a good 3+ in from the at rest position to keep good tension on belt.