Engine keeps running a few seconds after the ignition is turned off!?!?

[skidd]

So.. I figured out why.. and this was a funny one.. so I thought I'd share in case anybody else is having the same issue.

In short.. when I turn off the ignition, the engine keeps running for .. oh. about 3-4 more seconds. Any by running, I mean perfectly smoothly running. It's definitely not dieseling or anything like that.

Today after a few hours tinkering on the car, I asked my wife if she wanted to go for a quick GoKart spin around the block. "Hell Yeah" she said. :)
So.. fired it up.. backed out of the garage.. and shut it off to run and grab my phone... you know.. just in case.
And it shut off RIGHT away. No 3-4 seconds of extra running. ?? :confused:
Interesting.. so.. went for a good run around the block.. took a couple of neighbors for a spin.. and back to the garage.
This time.. it did the 3-4 seconds of extra running. :confused:
Ok.. enough of this. So. I grabbed my Voltage tester.. put it on the coil.. flicked on and off the ignition... and it cuts power perfectly.
Then.. fired it up.. and did the same thing.
WTF... The voltage tester showed 12+ volts for that 3-4 seconds.. and then steadily dropped to 0 over another 3+ish seconds.
:confused:
I noticed.. it was perfectly timed with the "Spin Down" of the radiator fan!
Aaaaaha!!! :cool:
And there it is. The inertial rotation of the cooling fan was acting like a dynamo and pushing 12v back into the positive side of the wiring. It was actually keeping the coil powered up! go figure!!
So.. I grabbed one of the extra 10A Diodes I had on hand that I used to wire in my Hazzard Flasher. Put the diode inline to the cooling fan.. and problem solved!! No more continued running.

And there you have it.. if anybody else is noticing that the motor won't die right away when the ignition is cut.. see if it happens to be timed to the cooling fan. If so.. an inline Diode will fix it.

[NAZ]

Good troubleshooting and interesting observation, I'll have to file that one away for future reference. Of course at my age, I better see that one inside a week or I'll have forgotten what causes it.

[Big Blocker]

Shameless attempt at raising post count.

Yup . . . that's the culprit . . . them damn fans not wired thru a relay that cuts both hot AND ground. But, as you've discovered, diodes work well in "clamping" generated voltage from a spinning motor.

Doc

[boat737]

I have a different, but related, issue with the fan windmilling and sending voltage back into the system. It's not the coil (at least not that I have discovered), but a warning light. I too (just today as a matter of fact) tried the 10A diode trick inline on the 12V feed to the fan. Fixed it perfectly, however, with the fan running for any extended time (I tested at 10 minutes) that diode gets damned hot. 250 degrees F as close as I can tell. I tried Three 10A diodes in parallel, thinking it would spread the voltage and amps between them, and it still fixed the problem, but all three still got up to the 250 degree range.

Are these diodes supposed to run that blistering hot? I didn't try sticking the diode on the ground side of the fan motor, but I suspect it would be the same result. Are there diodes out there that have a higher amp rating, say 30 amp for the fan? I'm a bit hesitant to keep that diode in the system knowing how hot it gets.

[Big Blocker]

What is the amperage draw of the fan when running - 30, 40? What fuse do you have protecting it? Diodes need to be at or above that amperage. 30 amp draw on the fan trying to push thru a 10 amp diode is GUNNA GET HOT!
I have a 50a rectifier (diode) on mine, 35 amp draw running. But, I also have mine wired thru a relay that drops both leads to the fan, so nothing can feed back past the relay contacts.


FWIW, If you're running a battery cut-off switch, try turning it off with the engine running . . . if it keeps running anyway, it's your alternator back-feeding thru to the battery, keeping everything going.
Cure for that is cutting the stator lead when the ignition switch is turned off (relay again)

Doc

[skidd]

Woh! Thanks for the heads up. I'll have to check the current draw on my fan. Might in fact be more than that dude can handle.

PS. You generally can't parallel diodes. As they heat up, their resistance goes down, pulling even more current through it away from the others on parallel.

[michael everson]

Just use a relay in the positive wire of the fan with a trigger wire that shuts off with the ignition.
Mike

[CraigS]

My 2200 cfm fan was occasionally popping a 25A circuit breaker so 30A is certainly possible.

[rich grsc]

Just use a relay in the positive wire of the fan with a trigger wire that shuts off with the ignition.
Mike
Yup, just wire up your relay correctly.

[skidd]

Just use a relay in the positive wire of the fan with a trigger wire that shuts off with the ignition.
Mike

This is already how its wired. Ground triggered relay wired to a thermo sensor in the block. The block is still hot, so the thermo sensor is still giving the relay a ground. I'm guessing the relay has its coil positive , and the switch positive wired together, and that then the 12v generated by the fan is enough to keep the relay triggered closed, and as a result pushes that 12v all the way through the system.

[skidd]

Now that I think about it. I might dig into the fuse block (fan relay is built into it) if I decouple the positive join between the coil and the hot side of its switch, and run the fan hot side to a constant 12v source. That way, feedback can't get into the switched circuit when the ignition is cut. Might be easier than the large diode.

[phileas_fogg]

Interesting information on cooling fans. The fan that came with my complete kit is spec'd to draw less than 15A (http://www.maradynehp.com/pdfs/m166k_data.pdf). Good thing, because the Weatherpak connectors I used for it are only rated to 20A. I also changed the fuse to 20A.


Good troubleshooting and interesting observation, I'll have to file that one away for future reference. Of course at my age, I better see that one inside a week or I'll have forgotten what causes it.

Now that's something to which I can relate!


John

[skidd]

I took a peek inside the back of my fuse-block.. and confirmed that the main power feed to the switched input on the relay does infact come from the "switched" side of the main power. Worse yet, there is no easy way to change the source within the block.. so.. I'm just going to get a bigger diode than the 10A I already have. The fan doesn't blow the 20A that is in my fuse block. So.. it's somewhere between 20A.