Posted in section for vans, 74 views, no response. A tough one I guess.
I am concerned it may be an indication of something amiss.

1990 E-150 XL 5.8L. I recently, (6 months) replaced my alternator, and the voltage regulator. I purchased them from NAPA. Everything seems fine until I turn the ignition off. The brake indicator light, and other lights normally on when ignition is on without engine started will briefly brighten up and then slowly fade out. It appears something is allowing voltage to seep through briefly before the circuit is completely dead. I am curious if anyone has experienced this problem before?

 

Is your idle speed a little fast and the engine takes a bit to finally stop? What happens with the A/C off or with the A/C on? The A/C on will make the engine idle a little faster usually.

 

Thanks Dave! The idle speed is a little faster once it kicks down from initial start. This is usually around 20-30 seconds. Also, the engine seems to have a slight hunting issue. The engine stops promptly with A/C on, or off. Once the key is in the off position, the dash lights and buzzer seems to fade away slowly. Only a few seconds. I am assuming the buzzer is from the seat belt warning.

Recent work I have done,

1. New ECT, O2 Sensor.
2. New Radiator, along with thermostat.
3. New injectors, which I absolutely love the performance! Purchased on Amazon.
4. Complete tune-up, including air filters, and fuel filter.
5. Engine, and transmission fluid, and filter change.
6. New Alternator, and Voltage Regulator.
7. New Serpentine Belt.
8. New A/C Compressor, Accumulator Dryer, replaced all O-Rings on spring lock connectors. Orriface Tube, system flush.

I have been trying to narrow it down to something recently done, but still baffled.

 

I would try putting the old regulator back on and see what happens.

 

I found the problem after pulling my hair out. LOL! My controller went out on my Hayden electric fan. This was the second time. The Controller has a lifetime warranty, but I decided to get a refund myself. I tapped into a wire in the ignition harness and have the fan connected to a toggle switch, indicator light, and relay to monitor the fan plus have better control since I have lost faith in the thermostat controller and relay. I changed the set up and have it connected to a spare fuse slot on the fuse panel. Problem solved. I really appreciate your input. In the future this could help someone with troubleshooting.

 

So I guess when power was removed the fan was acting like a windmill and generating a little power in the ignition circuit till it came to a stop?