Bleeder resistor

[TROOPER88]

Chaps
I have an issue with a cars ECU.

Basically, I have now partially fixed the faulty but the vehicle will not restart for about 30 seconds once the car is turned off. I am guessing the reason for this is that there is still power left in the ECU (long story).

Would installing a resistor on the power lead work to quickly drain any excess voltage?

If so, could someone please post a link to the part required.

Many thanks in advance

[spikeyhead]

What's the car?

[NMNeil]

ECU's always have a keep alive voltage/current, so a resistor will do nothing, except eventually drain the battery.
Using guesswork where sensitive electronics are involved is never a good idea.

[TROOPER88]

spikeyhead said:

What's the car?

Boxster 986

[GreenV8S]

TROOPER88 said:

I have now partially fixed ... I am guessing ... long story

What problem do you think you have fixed and what problem do you think remains?

[TROOPER88]

GreenV8S said:

What problem do you think you have fixed and what problem do you think remains?

Non-starting 986.
Would only start if power cut.
Problem located to DME.
Switched from 12v constant to 12v switched.

Are you an ECU specialist?

[Polly Grigora]

@ 12 volts 1k Ohm will dump to ground @ a max current of 0.012 Amps

What could possibly go wrong?

Try it and time the dump, a lower resistance value will dump quicker

You could begin testing with a 2 or 3K Ohm resistor

[TROOPER88]

Amazing info Polly, thank you

Do you have a link to the type of resistor you suggest?

Am happy to try a couple and time how long they take to dump.

Cheers

Polly Grigora said:

@ 12 volts 1k Ohm will dump to ground @ a max current of 0.012 Amps

What could possibly go wrong?

Try it and time the dump, a lower resistance value will dump quicker

You could begin testing with a 2 or 3K Ohm resistor

[Polly Grigora]

Or

Use a potentiometer instead of the resistor to verify the minimum resistance needed, keep lowering the resistance until obtaining the desired result

1K ohms should be good though

[Polly Grigora]

TROOPER88 said:

Amazing info Polly, thank you

Do you have a link to the type of resistor you suggest?

Am happy to try a couple and time how long they take to dump.

Cheers

Polly Grigora said:

@ 12 volts 1k Ohm will dump to ground @ a max current of 0.012 Amps

What could possibly go wrong?

Try it and time the dump, a lower resistance value will dump quicker

You could begin testing with a 2 or 3K Ohm resistor

You're welcome

Visit a computer, radio or TV repair shop

Ask for a low wattage resistor, it doesn't need to be a heavy wire wound resistor

0.25, 0.5 or a 1 watt rating will be sufficient

Will look similar to this

[Polly Grigora]

Having given the problem some more thought

Are you able to prove that there's still a voltage at the ECU pin when disconnecting the supply from it?

There's a possibility that the circuit that needs dumping to ground is internal, if this is the case you would need to find out what needs dumping, might need an ECU expert to help and if this is the case they might be able to fix the fault for a similar cost

If there's a voltage at the ECU pin when its supply is disconnected, go ahead and try dumping

You won't need to wire in a relay to test if a resistor will dump ok, simply connect one end of the resistor to ground and touch its other end to the ECU pin, if all is good the voltage will no longer be present at the ECU pin after dumping

Good luck

[GreenV8S]

TROOPER88 said:

Would only start if power cut.
Problem located to DME.
Switched from 12v constant to 12v switched.

If that's the extent of your diagnosis then rather than bodging it, get the ECU fixed.

Do you have any reason to think that pulling the positive supply terminal to ground is safe for the ECU? If there is any internal charge then you would be reversing the circuit back to the supply. Electronics often don't like being given a reverse voltage.

[TROOPER88]

Thanks for your help. I’m happy to see how the car behaves and if a new ECU set is required then this is no problem.

I’ll also research an ECU repairer.

What I’ve been doing is more about trying to locate the fault and I believe I’ve done this successfully 😃

Are you an ECU expert?

If so, I’m happy to pay you for your services

GreenV8S said:

If that's the extent of your diagnosis then rather than bodging it, get the ECU fixed.

Do you have any reason to think that pulling the positive supply terminal to ground is safe for the ECU? If there is any internal charge then you would be reversing the circuit back to the supply. Electronics often don't like being given a reverse voltage.

[Polly Grigora]

TROOPER88 said:

Non-starting 986
Would only start if power cut
Problem located to DME
Switched from 12v constant to 12v switched

Something else to check before attempting to dump to ground

Have you used a switched ignition supply instead of the 12 volt permanent supply?

If answer is yes

More often than not, a switched ignition supply taken from the fusebox or ignition switch will be of negative polarity (Ground) when switched off (ignition when off goes negative polarity through relay coils, other control units and ignition supplied circuits)

This means that when you switch the ignition off there will be a direct dumping to earth of that ignition supplied ECU pin

The thing is, I don't know where you have picked up the switched supply from

If the switched ignition supply has been picked up from a relay, the supply might not go negative polarity when ignition is switched off

In summary

There is no point in attempting to dump the ECU pin circuit to ground through a resistor if it's already being dumped directly to ground through the switched ignition supply circuit that has been connected to it

Suggest testing with multi-meter at ECU pin for what polarity is present when ignition is turned off

[TROOPER88]

Polly Grigora said:

Something else to check before attempting to dump to ground

Have you used a switched ignition supply instead of the 12 volt permanent supply?

If answer is yes

More often than not, a switched ignition supply taken from the fusebox or ignition switch will be of negative polarity (Ground) when switched off (ignition when off goes negative polarity through relay coils, other control units and ignition supplied circuits)

This means that when you switch the ignition off there will be a direct dumping to earth of that ignition supplied ECU pin

The thing is, I don't know where you have picked up the switched supply from

If the switched ignition supply has been picked up from a relay, the supply might not go negative polarity when ignition is switched off

In summary

There is no point in attempting to dump the ECU pin circuit to ground through a resistor if it's already being dumped directly to ground through the switched ignition supply circuit that has been connected to it

Suggest testing with multi-meter at ECU pin for what polarity is present when ignition is turned off

Hi Polly
Many, many thanks for the time and effort you have gone to in assisting me with this, it is massively appreciated

Basically what I have done is taken a cheap, high mileage non-running Porsche and have made it useable again. There does not appear to be any damage as the car runs and drives exceptionally well.

I took the 12v switched from an ignition controlled relay.

There is certainly something wrong with the ECU as the car will not re-start immediately after turning the ignition off. It seems to take 30-60 seconds, I can only guess that something is taking this long to de-charge.

I will see how the car behaves and at the same time, I will see if there is a local ECU repairer who can have a look.

Thanks again

[Polly Grigora]

TROOPER88 said:

Hi Polly
Many, many thanks for the time and effort you have gone to in assisting me with this, it is massively appreciated

Glad to possibly been of some help, there's nowt like a good challenge

TROOPER88 said:

Basically what I have done is taken a cheap, high mileage non-running Porsche and have made it useable again. There does not appear to be any damage as the car runs and drives exceptionally well.

Did read somewhere that you know a thing or two about Porsche cars

TROOPER88 said:

I took the 12v switched from an ignition controlled relay

In which case the switched ignition to the ECU very likely won't go negative when ignition is switched off and dumping won't be taking place

TROOPER88 said:

There is certainly something wrong with the ECU as the car will not re-start immediately after turning the ignition off. It seems to take 30-60 seconds, I can only guess that something is taking this long to de-charge.

Yep, understood your earlier posts that came through loud and clear

It may well be worth trying a resistor to dump to ground if the ECU pin is remaining of positive polarity when ignition is turned off

If you do try dumping to ground please do post back about the result, it's not a fault that shows up often and there doesn't seem to be much to lose

[gazza285]

GreenV8S said:

TROOPER88 said:

Would only start if power cut.
Problem located to DME.
Switched from 12v constant to 12v switched.

If that's the extent of your diagnosis then rather than bodging it, get the ECU fixed.

Do you have any reason to think that pulling the positive supply terminal to ground is safe for the ECU? If there is any internal charge then you would be reversing the circuit back to the supply. Electronics often don't like being given a reverse voltage.

There's no reversing of the circuit, all that will happen is any charged capacitors will drain quicker, the issue might even be that a capacitor has a drain resistor that has gone open circuit...