want a Fixed 5% duty cycle, 12v input 50hz

[Gloyale]

Hi, I am new here and not very knowledgable about electronics.
My Question is this. I want to basically overide the computer that controls the All Wheel Drive in my Subaru. There is a Duty Solenoid that controls the hydraulic pressure to the AWD system. Recieves a PWM from computer at 50Hz. Duty Cycle Ratio varies between 5-95%. At 5% maximum torque is sent to system, at 95% minimum torque.
I want to make or buy a controller that would let me send that 5% duty cycle signal to the solenoid. Bypassing the computer control which tends to wait too long to apply torque and does not provide enough(low enough duty cycle).
Can someone tell me my options? Again, it is a 12v system(auto) and the targeted value is a fixed 5% duty ratio. with a 50hz Pulse Wave Modulation

 

[jrz126]

I highly doubt this is a good idea. The computer is a pretty sophisticated control system. If you force one of its outputs and it doesnt know that it has been forced, it could cause some serious problems.

 

[Gloyale]

I highly doubt this is a good idea. The computer is a pretty sophisticated control system. If you force one of its outputs and it doesnt know that it has been forced, it could cause some serious problems.

I want to set up the controller for the solenoid standalone. With a DPDT On-On switch that interupts the signal going to the Solenoid from computer, and closes the signal from new controller. no modification to the computer at all. The only effect to the computer would be that it would percieve the solenoid as having failed or become diconnected. This will generate a "fault" code. But doesn't effect the rest of the operation of computer at all. I have reviewed the factory transmission manual. the computer seeing an open solenoid circuit will not change the opreration of the rest of the system. Some "fault" codes will change other operating values but not this particular one. It is only in control of the 4WD portion of the transmission and will not effect other things like shift points and lock-up.

The only thing that would be connected to the controller would be one wire to the solenoid and a power supply. I only want to use it on the occasion that I need full 4wd to all wheels like deep snow. the rest of the time the computer would be controling the solenoid as normal.

 

[Sceadwian]

You need to more fully flush out the EXACT specifications of the signal coming from the car computer before anyone can help you.

I'm going to hazzard a guess that the signal is actually 12 volts directly to the solenoid that controls the hydraulics and the duty cycle is positive as the 50hz signal drives the solenoid directly? But what is the current expected of the signal? These are all asumptions and you need to exaclty specify how the system works.

 

[Gloyale]

Hmmm...not sure excactly what you mean.

But what is the current expected of the signal?[QUOTE/]

There is only one wire to the solenoid. it is the only one that powers the solenoid. The computer varies the on-off cycle between 5% and 95%, depending on what info it gets from other sensors in the car. Factory Service manual for the car says it operates at 50Hz with a pulse wave modulation. The 50hz frequency is constant(.02 second cycle). The Pulse Width Modulation controls the ratio of on to off. I don't need a variable control. I really would just like to get an output of 5%.

Will a voltmeter tell me what voltage amount is being pulsed? or will the voltage reading show lower because of the on-off action repeating every .02 seconds? There is a mode to lock it fully on(95% duty ratio) If I put it in that mode, and then measure the voltage going to the solenoid, will that tell me what voltage it's sending the signal at? would it then be
(signal voltage X .95= measured voltage)?
I am really good with cars and straight forward type electronics. It is just this PWM and duty cycle stuff I am confused about

 

[Analog]

This is a relatively straightforward circuit to build, two op amps and a mosfet should do it.

I am reluctant to endorse the idea, messing with the controls of the car like this may not be a good thing, unless you are ready to experience any unforseen consequenses.

 

[Gloyale]

This is a relatively straightforward circuit to build, two op amps and a mosfet should do it.

I am reluctant to endorse the idea, messing with the controls of the car like this may not be a good thing, unless you are ready to experience any unforseen consequenses.

I am very sure of what it will do for the car. In fact many many people have done a similar mod on subaru's. Most people including myself simply put a switch in to interupt the signal. This sets the duty ratio of the solenoid to 0% obviously. this provides the locked 4WD pressure we want. however with the solenoid not opening at all, it puts alot of stess in the hydraulic system. None the less many people have done it and tested it for over 5 years. When the computer selects lock it sends a 5% duty ratio signal so the solenoid opens just slightly, which keeps hydraulic pressure from building.

Please could you give me a way to make a device to send the 5% ratio signal? It would make an already effective modification even better. It would allow the manual selection of 4WD and still allow the solenoid to operate in it's normal range. Granted the far end of the range but that is OK.

BTW, the car is almost 20 years old, I bought it for $400 dolars. Ready for anything

 

[mramos1]

The car is 20 years old and has a computer in it? Do you have details on the computer and what is does?

So you want to override the PWM the current computer is handling to make it faster?

Can you post a schematic or link to what the others have done and you want to do on your own or make better?

 

[Gloyale]

Hmm. well I don't know how to draw it up on the computer. but as I said, there is only one wire going to the solenoid. What we have done for the mod in the past is to simply put a switch inline in this wire. leave it closed and it functions normally. open it and the signal to the solenoid is cut. I'd like to use a double pole double throw switch so that when circuit from the computer is open, the circuit from what ever type of controller I add will be closed.
I just need something to generate a 5% duty ratio signal at 50hz from a 12v system. The one thing I don't know is whether the signal is on a 12v scale or stepped down to 5v or something like that. I am workin on making the measurement. I just have to borrow the right multimeter to measure with.

 

[Sceadwian]

But you have no idea what the exact voltage and drive current is, you can't build a driver without knowing what you're going to drive. You need at least a basic view of the signal on a scope, and perhaps through a small shunt so you can sense the current as well or there's no way to suggest what kind of driver would work for it. The solenoid could draw a few hundred ma's or a few dozen amps during each pulse. I wouldn't recommend using anything less than a mosfet capable of 20 amps to cover your bases. If you use 12 volts for your switching voltage and the solenoids are using 5 volts you could fry them. It's difficult to check this on a meter as it's a pulse. But you should get an idea of the line voltage when you're in the 90+% duty cycle range.

 

[Gloyale]

Thanks, I wil try to measure it at the 95% duty cycle that can be manual selected. Is there a way to measure how many amps the solenoid will draw? The manual mentions it is a low amp circuit but does not specify how low. could be a few ma's or could be 5 or 10 amps. The multi meter I was going to use has an RPM function that can be used to measure low frequency pulse signals. You have to divide the reading by 30 or something, gotta read the instructions. Thanks for advice. still workin on it.

 

[Sceadwian]

Your meter should have a DC current measuring mode, basically the meter is inserted inbetween one of the circuits lines and set to measure DC current, most meters have a 10 or 20 amp measuring limit. An internal shunt resistor provides a small differential voltage which is what the meter actually registers. It usually only drops 200mv's at full scale (10 or 20 amps) so it shouldn't effect the circuit. At 95% duty cycle the meter isn't really going to notice it's not a solid DC signal, expecting perhaps for intermitant blips when the sampling rate and duty cycle sync.

 

[Gloyale]

Okay. So I measured voltage at 95% duty cycle. Registered at about 10.9V
so it seems like the signal to the solenoid is on a 12v scale.

 

[Sceadwian]

How much current though? That's the last thing you need to know before you rate a FET or transistor for your circuit.

 

[Gloyale]

Well, my digital meter does not have a current setting, only Volts and Ohms. I have a cheapo analog meter that has a DC mA function. It has a 250 mA limit though, so I don't know if that will cut it.

 

[Sceadwian]

That will blow the meters fuse easily, or fry the meter if it doesn't have one and the current is in the ampere range. Buy a new meter with a current measuring setting. Even a cheap Radio Shack model will give you 10 or 20 amps of curent sensing.

 

[Sig239]

Can't you just go to whomever sells the solenoid(autoparts dealer) and find the specs for it. A certified mechanic may even know. I'm sure the information is out there.

 

[mramos1]

Gloyale, can you post a schematic or links to what others have done? Or a part number. I really think that will get you a solution a lost faster.

 

[Gloyale]

Can't you just go to whomever sells the solenoid(autoparts dealer) and find the specs for it. A certified mechanic may even know. I'm sure the information is out there.

No, not really. the only place that sells it is the subaru dealer. They just know what it does and where it goes, not really what it IS Maybe I can get an actual manufacturer and part specs.

 

[Gloyale]

Gloyale, can you post a schematic or links to what others have done? Or a part number. I really think that will get you a solution a lost faster.

No one else has done what I am trying for. I don't know of anyone who has made a controller for the solenoid. The only thing others have done is to simply put a switch inline on the one wire that goes to the solenoid so all power to it can be cut. I want to be able to energize it 5%.