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740 Alternator won?t turn on -

Slimssousa

Slimssousa

New member
Joined
Mar 12, 2017
Location
California
My alternator doesn’t turn on during startup sometimes, so I have to jump it with a wire coming off the battery. Would there be a problem if I just constantly had that wire hooked up to the alternator that’s connected to the battery ? Or would it ruin the alternator by possibly giving it too much power? Help
Picture of wire that connects to alternator https://ibb.co/hHOmXm
Picture of wire that I use to kick on the alternator https://ibb.co/im1tdR
 
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Every automotive alternator that I have seen requires some form of field flashing to start generating voltage. Field flashing consists of applying a small voltage to the field winding to generate a small field current which then starts the alternator generating voltage. Once it is generating voltage it is able to internally supply the required field current through the voltage regulator. When you jump 12v to that connector, you are doing the field flash. Leave that permanently connected and the alternator will try to run at max output which can be over 16 volts - dead battery in short order

My old 1987 745 had a voltmeter and I can't remember the details of the field flashing circuit. If your 1991 has a charge indicator light, check to make sure that the charge indicator light is working. It is very common to use the small amount of current that flows through the indicator light bulb filament to flash the field. A burnt out bulb results in loss of field flashing.
 
You won't overcharge. It's just the exciter wire to make it start charging. You will just lose the dummy light.
 
I do have Burnt out taillights and I do get the bulb indicator, they’ve been out since I’ve had the car and this problems been going on since I’ve had the car I will look further into it and replace the bulbs as soon as I get a chance to, maybe that just might save me.
 
The thing is, sometimes the alternator works and sometimes it doesn’t. Lately it’s been happening a lot more than normal and I need to fix this problem because I want to drive it more.
 
ZVOLV said:
You won't overcharge. It's just the exciter wire to make it start charging. You will just lose the dummy light.
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You are correct if he is jumping to the alternator D+/61 terminal. This will work if he uses switched power. If he uses a continuous un switched connection off the battery the alternator voltage regulator will remain permanently energized and since the battery voltage is below the regulation point that regulator will attempt to boost the voltage on a non running alternator likely draining the battery. Best solution is to fix the flash circuit which may just be a burnt out / loose bulb.

If he is flashing to the alternator Df terminal then he will most certainly cause high voltages with a permanent connection to the battery terminal. On alternators with integrated voltage regulators the Df terminal is sometimes brought out to the external case for testing purposes. It is typically a stud and capped with a plastic insulating cap with no permanent external connection.
 
Slimssousa said:
The thing is, sometimes the alternator works and sometimes it doesn’t. Lately it’s been happening a lot more than normal and I need to fix this problem because I want to drive it more.
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If operation is hit and miss, you likely have a bad connection to the D+/61 (excitation) terminal. Sometime when the car is running / alternator is not working, pull the wire to the D+/61 terminal and measure the voltage on the wire (not the alternator). You should have 12v to ground. If not, you have a bad connection in that circuit. Best solution is to find the wiring diagram for the car and trace that wire back (likely to the instrument cluster) to find where the problem is.
 
ZVOLV said:
Welp....do you have 12v at the exciter wire always?

Does the charge light work???
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No I don?t have the exciter wire connected to the alternator at all times because I?m afraid somethings gonna go wrong. I do connect the exciter wire when I know the alternator is not on, then I disconnect the wire when it turns on, and put the factory wire back on. I know when the alternator is not on because almost all the lights warning lights dim up on the dash.
 
HAVE YOU CHECKED FOR 12V AT THE EXCITER WIRE?

Does you little battery light turn on in the cluster when you turn on the key? Kp2.


12V just needs to be "slapped" to the exciter once per drive. I drove with my cluster out on my 240 and had to remember to slap 12v to the correct pin ONCE, for just a moment, and the engine would noticeably rev up as the alt started charging.
 
ZVOLV said:
HAVE YOU CHECKED FOR 12V AT THE EXCITER WIRE?

Does you little battery light turn on in the cluster when you turn on the key? Kp2.


12V just needs to be "slapped" to the exciter once per drive. I drove with my cluster out on my 240 and had to remember to slap 12v to the correct pin ONCE, for just a moment, and the engine would noticeably rev up as the alt started charging.
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I read your previous thread wrong, 12v is at the exciter wire sometimes, and that?s when the alternator is able to turn on. No the battery light doesnt come on when the alternator is on. But when the alternator decides not to work multiple lights are on including the battery light.
 
Calm down, Zach. A true Master Jedi Tech excercises patience.


OT: Regulator on its way out, probably. Does it self excite when you rev it to cca 4k rpm?
 
ZVOLV said:
Does the battery light turn on when you turn the ignition to the on position???!!!!!!
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When the alternator works, no the battery light doesn?t turn on. When the alternator doesn?t work all dash lights light up, including the battery light.
 
I thought that generation 7/9 series had an instrument cluster flexible circuit board that was commonly bad and caused this problem. I know I had it on one of my 940s. I had to check when I turned the key that the charge warning light came on. If it didn't, I had to smack the dashboard until it did, otherwise I'd drive with no alternator and run the battery down. Eventually I replaced the circuit board and never had another problem.

Watch for that dash light before you start the car.
 
LloydDobler said:
I thought that generation 7/9 series had an instrument cluster flexible circuit board that was commonly bad and caused this problem. I know I had it on one of my 940s. I had to check when I turned the key that the charge warning light came on. If it didn't, I had to smack the dashboard until it did, otherwise I'd drive with no alternator and run the battery down. Eventually I replaced the circuit board and never had another problem.

Watch for that dash light before you start the car.
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I’m almost sure that this problem is coming from the cluster. Possibly a bad soulder. It’s very finicky. The alternator only works half the time.
 
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