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Voltage drop switch how to?

Voltage drop switch how to?

Voltage drop switch how to?

Hi Guy's I am new hear and electronics is not my favourite subject hence I have searched out this forum to gain some knowledge and hopefully help me solve a problem.
So here goes. I have a idle control issue with my car which is due to it running on an ECU that is a bit basic in some respects and does not have some quite key functions. So I want to try and plug something in to create a work around for this. The main issue I currently have is poor idle due to voltage drop when any electrical loads are applied such as fans, lights etc..
The work around I want to try and achieve is this. I want to wire something in off the back off the alternator or another suitable 12V source which under normal running should of course read around 14.0-14.2 volts with a good battery. If the voltage temporarily drops down to say 13.8V when a load is switched on I then want this to flip a switch which will allow a 12v supply to be sent to an input on the ECU. Then when the voltage picks back up to 14.0 the switch will flip back and close off the 12v source to the ECU. I hope that makes sense. I think this should also only happen when at idle so I guess that means wiring a microswitch onto the throttle pedal or throttle body that is on/off when the throttle is closed. or maybe the 12v switch and microswitch need to run to a relay to control that?
Can anyone advise if there is any off the shelf item that will do this or coul dbe adapted. Maybe I need to make something from scratch and if so what components am I looking at?
One other thing is ideally I want to be able to adjust the voltage that the switch flips at in order to fine tune the point at which this needs to happen. i.e I might need it to trigger at 13.9 volts or only at 13.5 volts.

Thanks for any help.


RE: Voltage drop switch how to?

The off the shelf item you need is a specialized wire brush called a battery terminal cleaner. It is different for side terminal batteries or for stud terminal batteries. In either case, disconnect the battery, and use the appropriate tool to clean the battery terminal and all of the connectors that attach to it.

For good measure, use a more conventional wire brush to clean the alternator terminals, the mating connectors, and both ends of all the ground straps. Also replace the ground straps that you or corrosion have removed.

DO NOT mess with the ECU.

Mike Halloran
Pembroke Pines, FL, USA

RE: Voltage drop switch how to?

I'd also observe that the minimum drive voltage for an OEM ECU is 8V or so, otherwise the engine would never start.

I suspect the problem is not low voltage.

However, a simple method would be to fit a diode between your second battery and the ecu. This will drop 0.6V, but if the ecu supply voltage ever drops below second battery voltage -0.6V, then it will take over.


Greg Locock

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RE: Voltage drop switch how to?

That's got to be a very piss-poor ECU to require such a convoluted work around. What will this 12V input to the ECU do exactly?

RE: Voltage drop switch how to?

I think maybe the point didn't come across properly. The car starts fine and reads correct voltage of 14.2 but when you apply an electrical load this drops momentarily while the engine stabilises. The car is a aftermarket ECU and engine swap with no idle control unit on it currently. the ECU has some scope to run an idle control unit but most people don't both and do other work around. The ECU also has an input specifically for connecting to an electrical load such as cooling fans in order to create and idle up when the fans kick in. I want to take this one step further and use a detection in voltage drop when any load is applied to trigger this ECU input.

Thank you

RE: Voltage drop switch how to?

Sounds like you really need a governor function. Not being told a 'new load' has appeared but instead something to keep the engine at the same idle speed. That way the idle speed doesn't change regardless of how many or which loads are added.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Voltage drop switch how to?

Ya, it needs the IAC circuit.

But, if you insist on running without it then why can't you just use an analog input to detect the battery voltage?

RE: Voltage drop switch how to?

If the connectiomns are good and the voltage is low, where are you going to get the 12 Volts?
Give us some idea whet you have.-
Engine size,
Electrical loads Watts of lights, Amps of fans.
Did you do all the wiring yourself?
If you are turning on a monster light setup and your engine is small with a low idle, you may have idl problems, but they may not be related to voltage drop.
Maybe you just need to set your idle up a hundred RPM or so.
Maybe you used undersized wires when you wired everything up.
You can waste a lot of time and money by "Guessing" that you have a low voltage problem when the real cause of the issue is something else.

"Why not the best?"
Jimmy Carter

RE: Voltage drop switch how to?


aftermarket ECU and engine swap with no idle control unit on it currently

All OEM throttle bodies of which I am aware include an Idle Air Control Circuit, which is basically a ~1/4" bore carburetor, with a pintle valve or other throttling device driven by a stepper motor or maybe a pulse modulated solenoid, driven in turn by the ECU. The IAC circuit is responsible for feeding air/fuel mixture into the engine when the throttle is closed. When that little carburetor is clogged, or the IAC electrical feed is disconnected, the engine will not start with a closed throttle, but otherwise behaves normally provided you keep your foot on the gas pedal.

I can't imagine that an aftermarket ECU would lack the circuitry to drive an IAC, and connecting it requires fewer than half a dozen wires in the harness.

So, am I correct that you didn't bother to connect the IAC, but instead adjusted the idle throttle stop to keep the big butterfly open a little? If so, the ECU has no means to control the idle air flow in response to load. It might be able to compensate a little with more fuel feed, but that doesn't solve the basic problem, and I have a hard time believing that adjusting the ECU supply voltage would change its behavior in any useful way.

Mike Halloran
Pembroke Pines, FL, USA

RE: Voltage drop switch how to?

At least in the US, IAC are an endangered species. They have been almost completely replaced with all electronic throttles (drive by wire).


The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

RE: Voltage drop switch how to?

I guess I have to go new car shopping, just to examine throttle bodies.

Right now, I am failing to see how moving the main butterfly with a motor somehow eliminates the need for an IAC built into the throttle body, which was/is there in the first place because the airflow vs. butterfly angle gain is extremely high when the butterfly is nearly closed, which would require the throttle motor and/or position transducer to have extremely fine resolution in order to allow the ECU to effectively control idle airflow.

Mike Halloran
Pembroke Pines, FL, USA

RE: Voltage drop switch how to?

Mike - you may fail to see how, but cars with an electronic throttle do not use an IAC valve.

I'm still curious how the OP is going to increase the idle rpm without any form of IAC valve or idle control. Retard timing normally then advance it to increase idle rpm?

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