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I read in another thread that a bad diode in the alternator can cause a car to continue running after the key is shut off and would like to confirm the information, plus share the issue for any other feedback.
This is a 75 vette with MSD ignition and the voltage regulator is integrated in the alternator. The issue started after I changed out the steering column with a Ididit column.
Some notes:
I don't suspect the column wiring because the key switch continuity tests correct and the car will run on even after the IGN/SOL connector is unplugged from the column. With the engine running, IGN/SOL connector open and the key off there is 12V on the PNK wire to the IGN connection in the fuse box.
I disconnected the alternator and the car runs fine and will shut off using the key.
Many articles reference when using a MSD ignition a diode is needed on the BRN wire to the alternator. The BRN wire exists in the bundle but is not terminated. The alternator connector has a RED and WHT wire. Also, a few years back I replaced the amp meter with a volt meter. I suspect the WHT wire will be the same as the BRN but did not want to try a diode there without confirmation.
I tried putting a diode on the RED ignition feed to the MSD box but it did not help. This feed stays at 12V with the engine running and the key off. I do not know where that feed wire comes from yet but I did verify it doesn't have power when the engine is off.
Once the alternator starts charging, it feeds the brown (white ) wire. If you hook it to the acc circuit instead of the ign circuit, it will solve the problem.
This is my understanding of the line diagram of a 75,
I own a 77 so...
At the 2 wire plug alternator yes, #10 Red and a #20 Brown.
I believe your 2 wire plug at some point was replaced.
The replacement pigtails I have seen the wire colors are Red & White.
The Brown wire (white wire) is 12vdc switched and at the bulkhead connector changes from Brown to Brown with White tracer inside cabin and lands at IGN-3 of ignition switch.
Gauges fuse powers the Pink wire at the CELL cavity marked IGN.
This circuit is switched at IGN-1 of the ignition switch.
Do you have power to the gauges fuses with ignition switch off?
The Diode trio in the alternator only allows voltage to pass in one direction, if NOT faulty.
Currently the white wire from the alternator connects to the ACC circuit in the fuse block and the pink wire goes to IGN. When I disconnect the white wire on the ACC connection the car starts and shuts off properly. With both the pink and white wire removed from the fuse block there is not any significant resistance between the IGN and ACC contacts on the fuse block. Seems like they may be shorted together somewhere.
There is no power to the gauges fuse with the ignition switched off.
Your red wire should be 12 v unswitched on the #2 terminal. This is the sensing wire for the regulator, can be tapped right into the batt terminal on the alternator. The wire on term #1 should be switched to either acc or an ign source that is separate from the distributor or coil, otherwise it will back feed when it's charging and keep it running.
First off what ever wiring changes you made put them back to original. No, a bad alternator won't keep the ignition coil hot. The reason that you need to have the MSD recommended diode in the brown wire (1 on the alternator) is because ignition 3 buss powers both the ignition coil and the gauges and subsequently the alternator brown wire. When the alternator starts working it sends a small amount of current back through the brown wire. This small amount of current is not enough to keep a stock coil powered. However it's enough to keep the relay energized in the MSD box and the MSD box powered up. Remember MSD gets its power from a separate power source. The coil wire now only serves to trigger the MSD on.
Originally Posted by '75
Once the alternator starts charging, it feeds the brown (white ) wire. If you hook it to the acc circuit instead of the ign circuit, it will solve the problem.
Yes, that would work but not the best solution. The alternator will be hot with the key in the accessory position. That's another 2 amp draw while listening to the radio with the engine not running.
Originally Posted by bmotojoe
Again, this is a 77 ignition switch...
IGN-3 is HOT when ignition switch is in the ON position.
IGN-1 is HOT at both Start & ON position.
It's the other way around. Ignition 3 is for the ignition coil. Ignition 1 is for the blower motor, etc.
First off what ever wiring changes you made put them back to original. No, a bad alternator won't keep the ignition coil hot. The reason that you need to have the MSD recommended diode in the brown wire (1 on the alternator) is because ignition 3 buss powers both the ignition coil and the gauges and subsequently the alternator brown wire. When the alternator starts working it sends a small amount of current back through the brown wire. This small amount of current is not enough to keep a stock coil powered. However it's enough to keep the relay energized in the MSD box and the MSD box powered up. Remember MSD gets its power from a separate power source. The coil wire now only serves to trigger the MSD on.
Yes, that would work but not the best solution. The alternator will be hot with the key in the accessory position. That's another 2 amp draw while listening to the radio with the engine not running.
It's the other way around. Ignition 3 is for the ignition coil. Ignition 1 is for the blower motor, etc.
Good catch, at least on the OP 75 and for the 76 that is correct.
In 77 the printed circuit board was installed and the switching also changed as I provided above.
Thanks for all the feedback. Based on what I am learning I am going to revisit my wiring to the new steering column.
Below are the instructions from the manufacturer I followed. The one difference in my wiring is the orange wire has a brown/white wire (alternator??) sharing the same connector. I am thinking that may be the issue or the fact they tie both the accessory wires together.
That brown wire in the picture is not the same circuit as the brown wire to the alternator. I don't like that ignition 1 and the accessory circuit are tied together. However it shouldn't cause the run on problem because pink/ignition 3 is separate. But definitely try disconnecting ether of those two wires and see what happens. If the run on stops, I'm going to guess that the MSD trigger wire is wired incorrectly into either the accessory or ignition 1 circuit and not the pink/ignition 3.
To clarify, the instruction picture shows 6 wires on my dash harness, however, there is actually 7 wires on my car. The 7th wire that is not shown in the illustration is a brown/white wire that is crimped into the connector on the orange wire.
Here is a picture I took before moving the wires to the new connectors.
Unfortunately I'm working off of a 82 diagram and what I can remember from back in the day. There is a tan/white wire that goes to the ignition switch on a 82. That tan/white is ground for the brake light in the instrument cluster. The ignition switch grounds it when in the crank position as a bulb check. I don't know if your year it different. All I know is the bulb check circuit was used in all GM cars.
Fixed the engine run-on issue by putting a diode on the white wire (originally brown/white) going from the alternator to the ACC terminal on the fuse box.
In hindsight, after everyone helping me understand the circuitry, it makes sense. This is what i was thinking was needed in my first post but the different color wire coming off the alternator sent me down the rabbit hole. Now I think the alternator was back feeding the IGN-3 circuit so opening the key switch was not shutting it down. Not sure why the issue surfaced after changing out the steering column.
Anyways, hope this fix holds up and a big thank you to everyone who helped me understand the circuit flow.
Didn't read all the replies But after putting a MSD 6a box on my 77, it would run for a bit after the key was shut off. Read the same bout a diode on alt But my alt was a brand new powermaster. Fix was toggle switch for the MSD box.
I recently ran a thread on this very topic. Just a few days ago actually.
backfeeding is a very common problem when converting a 1 wire alternator to 3 wire operation. (A very good thing to do if done properly)
such a common issue a number of companies offer a plug and play solution.
Like this one being sold under the gen. DELCO name.
But many others like it.
A simple diode installed in the line between the gen light and the alternator white or brown wire. (Depending on year) eliminates this backfeed and the subsequent run on it causes. Not to mention pushing power down wiring not designed for it.
the Voltage regulators in one wire alternators will work just fine with downstream voltage sensing. But are not designed with the same resistor inline to prevent this backfeed. A simple 2 dollar diode cures this issue while allowing one to use a high output 1 wire alternator with all the benefits of a 3 wire alternator.
02-02-2022, 02:12 PM
Bad alternator causing run-on?
