S6 IGNITION SUPPLY

[SUPRAADD]

Hi when installing a solid state isolator I had a constant 12v supply to the ECU with ignition off. The car now starts OK but quickly develops a bad misfire that gets worse util will hardy run. Have put ignition supply back through switch but sill have misfire. Could having a constant power supply to ECU with ignition off have caused damage to ECU? I use M&W six channel direct fire CDIs on my Supra

[stevieturbo]

The ecu is either off or on....so it wont really care.

Has this setup ever ran correctly ?

[SUPRAADD]

Yes it was fine untill I used the isolator to stop the engine, it cut the power to ecu but ignition was still on so other stuff will sill have had power.

[TimH]

Using an isolator (kill switch) to stop an engine has been known to kill a fair few ECUs.

If it's a solid state one, then the better designs have an ECU shutdown signal which triggers 30-40ms before the isolator disconnects the ground (or power, depending on the type) and this is intended to allow the ECU to start the process of stopping the engine before it is de-powered.

If this isn't done, then when the ECU is de-powered the engine is still running and there's a lot of energy to be dissipated somewhere (coils, any other solenoid-type actuators, dribe by wire, etc.) and it will find the path of least resistance...which is often the ECU.

And if the isolator wasn't removing power to other stuff, then it may not have been wired right anyway since an isolator/kill switch is intended to leave everything unpowered and disconnected from the battery to reduce the risk of fire in an accident.

Even the tried-and-tested "red lever" MSA kill switch has a (should have, if the right one is chosen) a separate pair of contacts to feed the ignition circuits and a bleed resistor to discharge them quickly. But that doesn't do anything to discharge anything else and I have known of 3 instances where S8 drive by wire H-bridges have been cooked as a result of using kill switches to stop a running engine.

YMMV

[MReilly]

What is your opinion of wiring things up the way described in this link Tim?

https://adaptronicecu.com/blogs/article ... age-spikes

[stevieturbo]

What is your opinion of wiring things up the way described in this link Tim?

https://adaptronicecu.com/blogs/article ... age-spikes

I would generally agree with the article. And although I'd probably never use it, I did wire my isolator the way his middle drawing was. It does also create the slight issue that say for a rear mounted battery or even switch, it can require fairly long heavy cable runs, which is also detrimental to how many wire stuff, when the alternator is providing all the power for the devices on the car.

However....there is still a problem with this and with some rules.

So is the switch a "kill switch"...or is it a "battery isolator" ? Therein lies part of the problem, as leaving a line to the alternator, the switch isnt really a battery isolator anymore, which it is supposed to be. It isnt simple a "kill switch", as killing the engine can be achieved many ways without actually isolating the battery.

The fuse on the alternator line does make sense...but bare in mind this will run at high current a lot of time so needs to be rated for this.

[TimH]

What is your opinion of wiring things up the way described in this link Tim?

https://adaptronicecu.com/blogs/article ... age-spikes

Not how I'd want to do it for a track car - there is still a live feed from the battery to the alternator. So if there's a fuel leak, still a risk of fire. MSA would not like it.

The idea is that the battery is completely isolated.

Stopping the engine, and isolation/kill, are separate things.

I would always provide a dedicated and specific means to shut the engine down (remove power to the main relay, for example) that leaves a discharge path for the alternator and solenoid-type actuators. Then you can use the isolator on a stopped engine without issue.

Then you have to consider the "proper" use of the isolator/kill switch by a marshal. You might take the view that if you've had a crash, then stopping the engine and isolating the battery is the most important thing, to reduce the risk of fire and save your life, and that any risk of damage to the engine or ECU is secondary; perhaps irrelevant.

But since the isolator/kill might be used "just in case", after a minor incident, you may want to protect the engine/ecu from the risk of damage. In that case, a solid state type, with an ECU shutdown signal, that allows the ECU to stop the engine before the power is unceremoniously disconnected, is preferred.

This can be taken in as an engine enable signal to the ECU.

There are some very fancy (i.e. expensive) types that signal the ECU via CAN bus and allow a programmed and sequenced shut down, but I've not had first hand experience of these. But I do know someone who has successfully moved to one of these after destroying DBW outputs on an S8 2 or 3 times - they had been using a hybrid type, but with no shutdown signal to the ECU which, with hindsight, would probably have sorted the issue.

[MReilly]

A bit of a mine field really then, for something that on the surface appears simple!?

Thanks for the replies guys