Won't charge until after going over 4000 rpm

[E30SPDFRK]

Left my car outside warming up one night while I cleaned up at work. Went outside and it wasn't running, so I went to start it and my battery was dead.

Checked some stuff and noticed the alternator wasn't charging until I exceeded about 4000 rpm, then it would be fine. It was the original alternator from 1988 and has coolant peeing on it at one point in time, so I put a new (remanufactured) alternator in. It still does the same thing.

I took a video to explain what I mean because I usually suck at typing lol




Ideas?

[PeaveyBassist]

Check all the connections for corrosion and check the engine and battery grounds first. The red wire that comes off the back of the alternator, follow that back to the starter and the connection on the passenger side firwall above the battery tray.

[E30SPDFRK]

No corrosion on anything. I should mention that this is on my m30 swap car. I took everything apart and cleaned it before reassembly into the car.

The 2 wires on the back of the alternator (small says D+, large says B+) go into a metal tube that runs along the side of the engine block. Only the large wire emerges from the other side.

[jlevie]

The first thing to check would be to see if exciter voltage is present on D+. If it is check resistance from the end of the B+ wire to the battery positive (disconnect B+ from the alternator first. If that is okay, check resistance from the alternator body to chassis ground. If you don't have problems with any of those, the alternator is bad.

[Danny]

I love when Jlevie posts.

Its like a wise old friend.

[E30SPDFRK]

The first thing to check would be to see if exciter voltage is present on D+. If it is check resistance from the end of the B+ wire to the battery positive (disconnect B+ from the alternator first. If that is okay, check resistance from the alternator body to chassis ground. If you don't have problems with any of those, the alternator is bad.

I get no voltage at all at the exciter until I rev it over 4k, then I get 14.7 on both B+ and D+. Only battery voltage at B+ until it is revved too.

Resistance from the case to ground wont stay at a certain number, but jumps around from 50-.5 ohms.

How would I go about checking the resistance from B+ to Battery Positive?

I love when Jlevie posts.

Its like a wise old friend.

+1 I always hope I get a response from him whenever I post a technical question.

[DJB]

Does the alternator lamp (the one with the battery symbol) light when you first turn the key?

If not, replace that lamp. The voltage regulator is powered through the lamp. Later clusters have a resistor in parallel in case the lamp burns out.

[jlevie]

I get no voltage at all at the exciter until I rev it over 4k, then I get 14.7 on both B+ and D+. Only battery voltage at B+ until it is revved too.

Resistance from the case to ground wont stay at a certain number, but jumps around from 50-.5 ohms.

How would I go about checking the resistance from B+ to Battery Positive?

You have two problems. The ground problem could be dirty connections between the starter body and its mount, but is more likely to be a bad connection in the ground strap from the engine to the frame.Remove the alternator and its mount and clean the contact points. Pull the ground cable and clean its connections. If the ground cable has evidence of corrosion damage, replace it. A generic ground cable, available from any auto parts hose, is an acceptable substitute.

The bigest problem is the lack of exciter voltage. That is supplied via the warning light in the cluster. That signal reaches the alternator via C101. So a problem in the cluster or corrosion in C101 are the most likely causes of your problem. Ohm out the exciter path from the alternator back through the cluster.

The first check I'd do would be to check the resistance from the D+ wire to the cluster connector. If more than about an ohm, there's a problem in the body or engine harness or C101. A check at C101 will tell which side has the problem. If that's okay the problem has to be in the cluster.

[E30SPDFRK]

The light is out in the cluster, but I believe the late models have something that skips over that. The alternator will charge too so I ruled that out pretty quickly when I was trying to diagnose it. Could it be the problem?

Alternator mounting points are mint as they were just cleaned and the ground strap is new.

[jlevie]

Late model clusters do have a resistor in parallel to the bulb to allow the alternator to charge even if the bulb is burned out. Replace the bulb and lets see if that has any effect.

[xwill112x]

voltage regulator?

[DJB]

The light is out in the cluster, but I believe the late models have something that skips over that. The alternator will charge too so I ruled that out pretty quickly when I was trying to diagnose it. Could it be the problem?

Alternator mounting points are mint as they were just cleaned and the ground strap is new.

In the later clusters there is a spot on the circuit board for a resistor in parallel with the lamp. But I have seen cluster circuit boards where that resistor where that resistor was not installed. I just checked one on my workbench, and it was a 4/1990 VDO board with no resistor in place.

If you replace the lamp with a known good one and it still does not light, you have narrowed down the problem.

In normal operation the voltage regulator gets its power from a small set of secondary coils in the alternator. It uses this power to power an electromagnet in the rotor. But when starting up there is no power generated in the secondary coil either, so the regulator is initially powered by the battery, through the alternator light.

What is happening here is that the rotor has just a little residual magnetism. If you get the alternator spinning fast enough, this induces enough current in the secondary coils to power the voltage regulator. Once the voltage regulator is powered, it turns on the rotor current and the alternator works normally until the engine is shut down.

This could have been broken for a long time without you noticing. (Or perhaps you never hooked it up correctly, and ignored that the warning light wasn't working.) Your car just runs off of battery power until the first time you really rev the engine. Then the battery start recharging. No problem, besides a shorter battery life, until something goes wrong. Perhaps you don't rev it to start charging. Or the residual magnetism (which is randomly perturbed each time the alternator shuts down) isn't strong enough to start the alternator up the next time. Or the cold causes the rotor to need a higher current (thus high RPMs) to start up.

[bsbrilesm3]

Late model clusters do have a resistor in parallel to the bulb to allow the alternator to charge even if the bulb is burned out. Replace the bulb and lets see if that has any effect.

Not trying to thread jack but this saved me a bunch of money thanks. My bulb was burnt out and couldn't figure out what was going on. Thanks