Starter solenoid stud accessory

[jordan]

Would any one have any idea if it would be ok to hook up a cable from the starter solenoid stud to power an expansion block for accessorys which will include power to the the efi system, I can just imagine there being enough of an amperage draw at that point from the engine cranking to cause the ecu to malfunction due to a momentary lack of voltage while the engine is cranking. Thoughts?

[GTJIM]

That connection is straight to the battery '+' post with no fuse box or damping impedance due to the long wiring looms from the fuse box.
You will get quite large voltage 'spikes' at that connection which would be even worse if you ever jump-started the car .... exit ECU - stage left!

[jordan]

This is what was going to go on the starter solenoid stud, it has a 10 awg wire and it feeds 6 fuses capable of handling 65 amps between all of them so since it is fused it would seem that the ecu would be safe...still a no go?

[GTJIM]

The fuses just protect from too many amps - voltage spikes don't blow fuses.... but they do nasty things to electronics.
I have always understood that the miles of wiring in late model cars acts as a sort of 'back-pressure' to damp out voltage spikes.
Some ECUs do have voltage stabilization circuitry to help protect them. The whole circuitry is a bit like the old distributor points system. When you stop cranking the car the voltage rises quickly and since the current is no longer flowing through the starter as 'surge' of electrical potential goes into the circuit of least resistance which acts a bit like the capacitor in the distributor.
ECUs don't like that sort of thing and your circuit is so directly connected there is not much to stop a voltage spike across its insides.
Maybe some electronics whizz can elaborate ... ??

[simplemind7]

most electronics won't be friends with the ideas, but some will be ok, ones that use transistors, resistors, capacitors, simple things like that, but if it uses a mosfet chip or pretty much anything that is static sensitive, then it will probably die from power surges. things like amplifiers for stereos are fine when they are connected directly to the battery, but an ecu would be risky.

[guyopel]

Most new style ECM's & PCM's are protected by Quad drivers to operate output systems . HTH

[OrangPeEL]

Hey Jordan- doubt I'm familiar with your system, but with constant 12v will ECU always be powered up? Guess not, or you're also planning a key-on relay to switch that ECU source. NOT trying to make this a poll, but TO ME a nearly direct connection to battery seems a GOOD spot to tag some stable voltage.

I have some OEM application and failure diagnosis experience with adaptive Bosch (similar to late GM) fuel injection on small frame industrial engines. Crossover may apply regarding prevention of spike-fried ECUs, so I offer the following: Some voltage spikes can occur as alternator reacts (charge/no-charge) to required battery voltage. The battery acts as a shock absorber for spikes...

• A healthy battery will absorb the excess amps and disperse them slowly/evenly. A weak battery (not meaning just low on volts, but low on life and ability to retain amps) is poor at this job.

• Introduce the "jump start" vehicle charging system to the "weak" battery... and at first successful ignition/RPM of recipient, there's potentially a cooked ECU.

• Spinning/coast-down of cooling fans have backfed voltage into system and contributed to failed ECU... corrected with diodes.

Later
-Dan

[bq97]

Originally Posted by GTJIM The fuses just protect from too many amps - voltage spikes don't blow fuses.... but they do nasty things to electronics. I have always understood that the miles of wiring in late model cars acts as a sort of 'back-pressure' to damp out voltage spikes. Some ECUs do have voltage stabilization circuitry to help protect them. The whole circuitry is a bit like the old distributor points system. When you stop cranking the car the voltage rises quickly and since the current is no longer flowing through the starter as 'surge' of electrical potential goes into the circuit of least resistance which acts a bit like the capacitor in the distributor. ECUs don't like that sort of thing and your circuit is so directly connected there is not much to stop a voltage spike across its insides. Maybe some electronics whizz can elaborate ... ??

What actually blows fuses is an increase in wattage thru the fuse. The fact that fuses are generally labeled in amperage only indicates their current carrying capability at a specified voltage. So a voltage spike can and will blow a fuse if the overall power rating is exceeded for a long enough duration. A 1 amp rated 12VDC fuse will blow after 12 watts is exceeded regardless of the current flow or voltage, the same fuse will only carry 1/2 amp at 24VDC (12 watts). Of course every fuse will carry a % of over current for a specified time.

The length of the wire is immaterial to a voltage spike and does nothing to reduce a spike's effect on the circuit. The overall resistance factor of the wiring will have a minimal dissipating affect on the voltage of the spike. Power spikes on electric transmission lines can travel hundreds of miles blowing transformers and tripping breakers along the way. The only way to eliminate damage by voltage spikes is to capture them and dissipate it slowly or to sacrifice a component.

The power generated by the alternator is AC that is smoothed thru the rectifier circuit to produce DC. In older cars the DC output is actually very dirty; meaning a good bit of the AC component is still present on the DC. The battery eliminates most of the AC component and helps to shunt any voltage spikes that are developed. In cars that are electronic intensive the raw DC power is further smoothed and conditioned before it enters any of the cars electronic components. This is not the case with older cars and especially not in Opel GTs. The GT's alternator and voltage regulator do a poor but adequate job at providing stable clean DC.

Now back to the original question from jordan. The starter stud where he wants to draw power from for his accessory expansion block is the exact same place where the battery and fuse block input is connected. So any fluctuation or spike experienced at that location is also present at the fuse block and beyond. While it's not a great location to draw power from due to it being un-switched it's just as good as any other place in the GT's circuitry. You can draw power from the starter stud and route it thru a relay to create a switched power source that is easily expanded.

hth
Brian