using resistors to reduce voltage

I have just noticed an oddity with the central locking on my car. The system is an OE retro fit kit. I have noticed that when the engine is running or just after running the engine, it is not possible to push the pin down on one of the rear doors. As it seems to happen when one would expect the voltage to be higher, my uneducated guess is the higher voltage puts more effort into the motor which somehow overcomes the physical mechanism. The central locking itself works fine though and can be locked with front pins.

I just thought it is probably a good idea to measure the voltage of the system with the engine running to rule out an issue with the regulator (although I suspect it would generate an error code in the OBDII computer thing, as voltage too low does).

If all else fails, my idea was to try reduce the voltage supplied to that motor on unlocking using resistors, as when voltage is lower it works perfect. I was thinking perhaps a drop of 0.5 or 1 volt. I have read https://www.ehow.com/how_6937424_reduce-voltage-12_volt-circuit.html and based on that I was thinking I could use a 1k ohm resistor and a 47 ohm resistor. This would make 14v 13.4v, and 11v 10.5v (assuming the maths there is correct). I assume I would just splice these in the positive unlock line.

Are any unforeseen consequences likely? Would doing so affect locking, given that the positive wire for unlocking then becomes the ground?

Many thanks in advance

The link is only good for (near) constant current loads.
If current varies, so does the voltage drop.

The side effect of using resistors to drop voltage is that resistors restrict current. This may leave your actuator with not enough current to work properly.

I don't 100% understand what you have on your car, but it sounds like you have 5 wire actuators that lock/unlock all doors when you toggle one. If that is correct, the actuators don't rest with voltage on them, meaning that if you are not pushing anything there is no voltage, and therefore voltage would be irrelevant.

If you really want to try reducing the voltage I would look for a 7805 voltage regulator. It will provide constant voltage and current won't be limited. You may need to parallel several of them together to get the proper amount of current though.

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Kevin Pierson

What I mean is that I think the motor pushes it harder with higher voltage and it pushes the rod in some physical position that makes it extremely difficult to push down. What seems to happen on the other doors is that the rods are pushed up with the central locking, and then come down a fraction. When the other is difficult to push it does not drop down.

It definitely seems to be correlated with voltage as when I go to car after leaving it a while and unlock it works fine, but with engine running and just after turning off it still happens.

The system is 4 wire on two front doors, with mux negative switching, 180ohm resistor unlock.

If it is going to be more complicated than some resistors I think I will leave it alone. :)

It sounds like maybe it is binding up? Try reposition the actuator a little and see if that fixes the problem. Reducing the power going to the actuator will only mask the problem, it won't "fix" anything.

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Kevin Pierson

Geez KP - it sounds like you are saying 5V thru a regulator provide more current than 5V thru a resistor!!(?)

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Amateurs assume, don't test and have problems; pros test first. I am not a free install service.
Read the installation manual, do a search here or online for your vehicle wiring before posting.

My thoughts are above but I was going to use Idiot's crystal ball and guess this is a VW. If you correctly wired it it should have ignition lock because the factory wiring over rides it. Other wise check your mechanical install of that back door motor, it needs re-sitting and freeing off.
Nothing at all to do with resistors the original used that 180ohm as a multiplex trigger for the locks.
If you put resistors on the motor wires, wait for the smell.

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Amateurs assume, don't test and have problems; pros test first. I am not a free install service.
Read the installation manual, do a search here or online for your vehicle wiring before posting.

howie ll wrote:

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If you put resistors on the motor wires, wait for the smell.

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Thanks for the warning.

Going to try reposition/refit the actuator. The holes and rods were all already there and designed to just bolt in, but there might just be enough give in the drilled holes etc there to bring about the fault. I guess it's not even outside the realms of possibility that the MFR drilled the holes slightly off. Seems my aim should be to try site the motor slightly south of its current position.

In the end I found 2 washers under the top mounting point of the actuator fixed it. These were not in the original design, but at least it works.

So score 1 each for KP and me! As soon as you said retro fit, I thought positioning, been there, done that got the (Clifford/Viper) T. shirts, all those years with Mk/l & ll Golfs, Polos etc.
1st rule of fitting a door actuator, test it physically first, nice easy run, in and out.
Second rule, make sure neither it or its supply cables foul window mechs etc.

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Amateurs assume, don't test and have problems; pros test first. I am not a free install service.
Read the installation manual, do a search here or online for your vehicle wiring before posting.