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Engine dies when the alternator is connected

I have a 1989 Nissan D21 pickup. About 4 months the engine stalled, it would refire for just a second or two when you tired to start it. Assumed fuel issue, checked pressure, all good. Checked spark, all good. Then discovered if I disconnected the alternator, the engine would run fairly normal. Checked relays in the engine compartment, fuses, grounds on the engine. Replaced the fusible links at the battery. Also discovered I could duplicate the stalling if I left the alternator disconnected and connected a battery charger to the truck. Anytime you turned the battery charger up and the voltage would get around 12.8 to 13 volts, the engine would die. Installed a used ECM, same issue. After several weekends I let the truck sit. Then I decided to take another stab at it. Purchased a new battery, the old one was 7 years old and the discharging and recharging took a toll on it. When I installed the new battery, the negative cable would not quite reach, when I bent the inline mounting/ground lug up and installed the cable, started it and after connecting the alternator wire, it stayed running. Drove it for several weeks, put around 800 miles on it. Went to leave work the other day, started and then immediately it died. Disconnected the alternator wire and was able to make it home with it.

II sounds like an alternator issue to me.How old is it?

Did you get your alternator checked?

Not the alternator, it was replaced earlier, also I can duplicate it by using a battery charger with the alternator disconnected. Anytime the system has 12.8 to 13 volts, the engine dies, below that, it will continue to run.

With the battery charger connected and the ignition on, place your positive volt meter lead on the alternator positive post, connect the negative lead to the battery post, then to the engine, next to the body ground. Compare the voltage readings at these different ground points.

will do and repost, might be this weekend before I have a chance

checked the voltages, every combination, with alternator wire connected, not connected, battery by itself 12.4 volts, with charger on with 13.5 volts at the battery, verses, no charger. No matter where I checked it, the voltage was the same or only .01 difference.

I have to say, this is a very, very strange problem. My best guess about the cause is there is a bad connection to power somewhere and when there is more voltage added to the circuit there is more voltage drop across the bad connection, which then causes the load to have trouble. You might be able to find out what circuit area is causing this issue by applying a separate power source to the suspected area via the fuse panel under the hood. Like the ignition system, fuel system, or power supply to the PCM.

I agree, I’ve turned wrenches for about 40 years mainly on heavy duty trucks. Was hoping someone had heard of this. I was thinking the same line as you were, have you ever heard of a relay coil shortening out, maybe at low voltage its ok, then high voltage it arcs and opens back up? I know I’m reaching, but trying to figure out how and what components would fail under those conditions. There are several diodes in different circuits I checked them, but of course, that is just with a ohm meter, ever hear of one shorting out under load?

Will the truck start if you just wait a while?
I am looking at the ignition components for your model that are listed on RockAuto. You have a couple ignition control modules listed there. Apparently, these are known for flaky symptoms, including No Start. If they are intermittent, you might need to look immediately after you stall. Check to see if they are hot to the touch. Also, checkout some YouTube videos for your truck problem.

It will start and run for about a second, when the positive cable is connected to the alternator. If you remove the cable from the alternator, it will start and run. It will have a tendency to skip every now and then when the cable is disconnected. A friend of mine mentioned the distributor, so I ordered the new one on Rockauto, along with both coils. It has two spark plugs per cylinder, with the two coils and modules. From what I have read, they will sometime both be used and sometime only one will be used. Not sure what determines how the ECM decides which one or both to use. Thought about biting the bullet and buying the two modules when I ordered the distributor, might end up doing it yet.

I don’t think a relay is causing the issue but I could agree with your idea about a diode possibly causing this issue. Especially if it is a reversed diode or possibly a Zener diode used in the ECU. The slightly higher voltage may be causing the diode to break down. This is a wild guess on my part but this a strange issue.

The problem is either within the ignition or the fuel delivery system. You might try to keep the engine running by spraying some starter fluid into the intake to see if that keeps the engine going. If that doesn’t work then it seems the ignition system is causing the problem and seems the more likely culprit to me due to the electronics involved with it. If I was working on this I would use a separate battery source to power the ECU via the fuse position to see if separating that area makes a difference. If not, then check the ignition and fuel circuits one at a time to help pin down the area of trouble.

I wish I had thought about the starting fluid before ordering the dist. I was able to confirm fuel pressure, but with all the electronics on the throttle body, I couldn’t figure out how to trouble shoot them, but that will work. Hope I remember to disconnect the intake heating element first, that could open up a lot of new repairs. Anything to help narrow down between fuel delivery or ignition is a great help. Both great suggestions, thank you

With the starting fluid, it continued to run, looked closer and I could see the injectors (throttle body) were no longer dumping fuel. Also found a diode pack (5), ohmed them, one in the pack showed continuity the wrong way for a split second. Found another one in a junk yard, swapped it, no luck. When I tried the second battery, did it at the fusible links (5 of them) coming off of the battery, the result wasn’t consistent. Did some reading in the manual, the ECM relies on the distributor for spark timing, and for injector timing. Received the new distributor, going to go ahead and install it and the coils. Next will be to back probe the voltage inputs (forgot to bring them home this weekend) to the ECM ( 5 positive and 4 grounds) going into it.

I wonder if the ECM has input over voltage protection that is failing. They may have something like a zener diode to limit voltage. It could be failing. The zener will not conduct in reverse direction until it’s designed voltage is exceeded. Sometimes when they fail, their inverse voltage starts to degrade. Isolating the ECM power while allowing rest of car to go higher might help narrow it down. Not the easiest diag to do for diyer…

Just thought of something to try- do you lose OBD comm when engine dies under high voltage conditions?

There is a “safety relay” in the harness that is designed to cut voltage to the ECM if the voltage is reversed, I bypassed it last time and it didn’t have an affect. Also have a second ECM, bought online from a junkyard, responded the same way. There is a chance it is also bad, so not totally able to rule out the ECMs.

Have not connected a scanner to it, since the check engine light has never come on. Not sure if my small OBD 2 scanner will even read it, but can try.

Well hopefully the new disty will solve this very strange issue. If there is a cam sensor that may be another thing to look at if the new disty doesn’t solve the problem.