Alternator Backfeed

[darud35]

Hi all,

I've been struggling with some engine run on issues (remove key - engine keeps running). I thought I would post what I've learned and hopefully help someone else with similar issues.

I installed an aftermarket engine management system. This eliminates the CDI box and CIS. After the install I started having issues with engine run on where I would remove the key from the ignition but it wouldn't stop the engine. Of course this is not ideal..

What I believe happens in a normal vehicle:
-Key into ignition: wakeup signal (12V) to engine components driven by the ignition circuit
-Engine starts and alternator begins charging
-Key off - ignition circuit open (slight backfeed from the alternator isn't enough to keep the CDI box awake) so the ignition system stops and engine shuts down

What I believe happens in my vehicle:
-Key into ignition: wakeup signal (12V) to engine components driven by the ignition circuit
-Engine starts and alternator begins charging
-Key off - ignition circuit open -slight backfeed from the alternator IS enough to keep my ECU awake and engine keeps running.

My solution, as many already know, is to place a diode in the alternator excitation wire. This prevents backfeed which keeps the ECU awake.

Does this seem reasonable to everyone? Anyone have a similar issue? I searched and searched but couldn't find this straight answer elsewhere.

[T77911S]

i have not heard of anyone else having this problem with an ECU.

have you checked the alt for bad diode.

is your wiring correct.

[darud35]

It's possible there's a bad diode in my alternator. Not sure how to check, maybe this is the indicator? Wiring is another possibility. I've gone through it a couple times and it's a relatively simple system but not ruling that out.

I see about 7 volts on the 12v wakup circuit when my ignition is off. This is likely close to the threshold for most standalones. That could explain why it's not so common?

[Porboynz]

Is it a real 7V though? Is it enough to light a lamp or only enough to register on a digital voltmeter. Try clipping a test lamp across the circuit then measure for voltage. Then start disconnecting things until the 7V vanishes. Start with the alternator feed at the starter motor, the engine connector, fuses etc.

[1990C4S]

I would have thought that you also killed the ignition with the key, and therefore the car stops running as there is no spark, no?

[rick-l]

Was the alternator light on with the key off before you installed the diode? Is the blue wire what you mean by wakeup signal?

[skybolt]

Sorry, I know this is an older thread but I'm looking for the same info and I couldn't find it anywhere else.

I have a 1972 914-6 conversion with a 2.4/Valeo alternator and MSD CDI box. Wiring is decidedly non-factory so no help with their diagrams. I have the same problem in that the engine continues to run with the ignition switch off. I can kill it by disconnecting an inline spade connector for the blue alternator wire.

My question is, did the diode solution work, and if so what were the diode specs? This is also the solution I arrived at independently, which led me to this post. Any help greatly appreciated!

[theiceman]

you could try PMing the original poster, that may garner you more info. click on his name nd you should get the option.

[Zaz]

I had the periodic/unpredictable engine run-on issue while my alt was failing (74 with aftermarket CDI). Had the alt rebuilt with a new VR and issue appears to have gone away. Not conclusive but fingers crossed.

[rick-l]

a 1N4001 would work (or any 1N400X) but do you have the right wattage bulb in the alternator light? You could try a smaller wattage bulb (higher resistance).

[Jim2]

I never noticed this thread before.

I put an aftermarket ecu in a friends 86 and encountered this exact same run-on issue. Out of frustration I put a switch his dash which interrupts the ecu power. Done. Bandaid I know, but the car hardly gets driven.




.

[darud35]

Sorry for the delay. It did in fact fix my problem, however I've had several people install this same ECU without issue. It makes me question the health of my regulator.

The part I used was a 3A diode, part number at Fry's Electronics: NTE5800

Hope that helps!

[skybolt]

Hi darud35,
Thanks for the reply and sorry for my delay in responding as well. I did fix my back-feeding problem with a diode in the blue alternator wire. For others that might be searching for this potential problem, the diode that I purchased at Radio Shack is an epoxy rectifier diode, 3amp - 400PIV, part number 276-1144. About $1 for 2 of them.

[mikedsilva]

Interesting thread.. I just installed the Rasant efi system and Infinity ecu in my car.. and had the alternator freshly rebuilt.. I am using a Bosch RE55 external regulator.. and yes, my car also continues running.. so where exactly is the diode installed?

[mikedsilva]

I installed a diode in my blue wire before it clips to the regulator, but it did not cure my run on condition. Would like to hear others experiences; perhaps I need to install it somewhere else? It was the diode that came with my kit.

[kuehl]

I had a similar issue after installing an AEM Infinity ECU on my mod'd 3.2.

Sal Carceller found my problem quickly (thanks again Sal!); best MAF systems for your 3.2

Basically .... "If the motor does not shut off, remove the alternator charging light on the dash and see if it then shuts off. It's possible you'll need a diode on the charging light. Try that simple test without that bulb plugged in! "

So I tossed in a diode and all is well again.

[kjchristopher]

I put the diode behind the dash. Works perfectly now.

[mikedsilva]

Interesting.. I am not sure why positioning the diode behind the dash is different to between the regulator and the alternator.. but I will give it a go tonight..

[rattlsnak]

If you step on the brake or raise the handbrake, does the engine stop running at that point? If so, the diode needs to go between the light bulb in the Tach and the Alt. ( the factory location is behind the dash)