Help with signal leaking to power input

[Daidhi]

I'm trying to fix an intake manifold runner control board from a Ford Focus ST170. The board is set up to take a -5V signal input and a 12V power input from the car's battery. The main components in the circuit are:

• a transistor
• an IC (LM1949n injector driver)
• a darlington transistor
• and a motor

The signal source switches the first transistor, turning on or off the IC, as well as providing a timer to the IC to control the output current. The current from the IC begins at its peak value, switching the darlington to fully open, turning the motor. The current then drops to a hold level, where the motor is braked. The motor is used to pull on a cable which opens a set of valves inside the intake manifold. At the hold current, the motor should be able to keep the valves open, while the current is lowered to prevent the motor burning out.

Here's a circuit diagram taken from the file I attached.

Input 1 is the -5V signal
Input 2 is the 12V power
Input 3 is the earth
Input 5 & 6 are a switch which is opened/closed when the cable is pulled, to tell the ECU that the valves have opened.

I'm using a pair of battery packs to get roughly those input voltages, -4.5V from 3 AA batteries and 11V from 3 AA batteries and a 9V battery. Each battery pack is connected to a common earth at 3. When testing each source individually, the voltages are correct at each point, however when both are on, the voltage at 2 drops to 2.5V, while 1 remains at -4.5V.

I've tested resistance across each of the D1 and D3, and both seem to be working, with high resistance in one direction and low resistance the other, when tested with a multimeter at the 200K setting. After many problems with components failing already, the board isn't looking very healthy, although it doesn't seem to be causing the problem. (I've replaced Tr2 three times and D1, due to faults, as well as IC1 and SMtr1 twice each while troubleshooting). I've checked that there isn't a short between any of the terminals on the transistors, the chip or the inputs, and they all appear to be correct.

Can anyone suggest components that may be at fault to cause this problem? So far I've replaced SMTr1 as I thought there may be a current leak from the base to the emitter, but that hasn't made a difference.
Thanks for any help!

 

[shrtrnd]

Not a part of your question, but after reading your post, thought I'd mention something that might be helpful.
I have not heard of this particular problem, as often as you experience it.
My initial thought, is to take a good look at the connector to this assembly, and make sure your connections are not corroded, or experiencing a conductive short across them. Maybe heat damage or environmental contamination from the weather/road.
I would suspect what you do, some circuit fault. But the fact that this is a repeat problem, makes me think you
should also be checking for external factors which might contribute to this recurrent failure problem.

 

[Daidhi]

Thanks for the reply. I probably should have mentioned it, but this is a new problem. The darlington is run very close to it's current limit which causes it to overheat. That's been my repeat problem, this is the first time I've come across this. I'll look for some damage or a short across the connector just in case though.

 

[poor mystic]

Perhaps if the Darlington fails again you might try replacing it with a higher specification unit, and/or a heatsink.

 

[Daidhi]

Thanks, I already have though. I'm using a TIP102 now rated at 8A continuous, I don't remember the original designation, but it was only rated at 5A continuous, with 8A peak. It supposedly runs at 7.8A for a couple of seconds every time the engine is started, so it is a tight tolerance. Unfortunately I'm not aware of any higher rated options.

Unfortunately, I haven't managed to get any further with the problem. To give some context, before this came up I had replaced both the chip and the darlington, but the motor ended up running really slowly. There was only a small voltage getting to the 1,7 and 8 on the chip. I tested it in the car and it wasn't strong enough to pull the cable. After I desoldered the board from it's connection it stopped working altogether. After that I replaced SMTr1, as I mentioned in my original post. I'm not sure which component failing could cause this behaviour, any suggestions for which to test would be greatly apreciated!

 

[shrtrnd]

Unfortunately, this could be anything if a diode or cap bit the dust.
I don't know if this is your own car, or you're working in a shop.
About now, I'd be trying to find somebody who has one of these boards, borrow it, and find out if it actually IS the
board. You're convinced it is, and it may very well be. But the motor could be weak, the cable that opens the valves
could need adjustment, or one of the valves themselves might have a problem, not to mention head pressure or any leaks in the system.
After spending all this time on the board, I'd want to verify that the board itself is still bad, before I did anything else.
We have a lot of junk-yards or 'wrecking yards' here in the States where we can pull circuit boards or auto parts from 'junked' cars to try to avoid paying full price to some dealership. Any possibility of doing that there if you don't know anybody who has a similar vehicle to borrow this part for test. Or maybe buying a used board on-line from some auto-parts vendor?
Like I said, if your board is bad, and you've got one lone cap or diode causing this failure, it'll take you forever to find it.
Another idea, is to Google your make and model vehicle, and see if somebody else on-line has run into the same problem you're seeing, and find out what they did about it.
I know this is frustrating, but hang in there.

 

[Daidhi]

Yeah, it's my own car, unfortunately they're quite rare here. I've been looking through scrap yard searches and only finding 2 in the country. (If I go across the 30 mile sea to the UK though there are hundreds)

I've got the board on it's own, with 3 wires soldered to where it would usually connect to the car's ECU, so it should be fully isolated from other problems that may be the cause for why it failed in the first place. I've had to do it a few times so far, and the voltages I'm getting aren't what they usually are when it's working properly. I take your point that the lack of power in the motor could have been caused by a worn out motor, but I feel there being a failed component on the board fits the issue I'm seeing.

Thanks for the suggestion about the scrap yard for a comparison though.

On a side note, could anyone suggest a decent free/cheap circuit simulation program? Preferably one that would also have a database of components, particularly integrated circuits

Edit: Nevermind, I spoke too soon. It turns out the motor is dead. I tried putting the 9V battery across it and nothing happened. However I'm not still sure if it was actually the cause of the problem. When I desoldered it the voltage still dropped, but only to 8V, and when I tested the current flow between what was the two motor terminals I got 0.2A, which slowly dropped to 0.

 

[shrtrnd]

Sorry this is hard to do in your location.
I'm betting you're right that it's the board.
Only other thing I can think of is the fact that the ECU gets its information from all of the sensors on the vehicle, and if one of those sensors is supplying a bad signal to the ECU, the ECU may be sending incorrect signals to your problem board.
Did you have a chance to surf the web for information about your symptom with this vehicle? I've found a lot of good information there with problems I've seen with my cars.
Don't know about your simulation program, but maybe someone will have the info you need in a follow-up post.
Hope you can get helpful information Googleing your car problem, or find a second board cheap.

 

[Daidhi]

Ah, sorry I edited the previous post just as you posted that.

Thanks for all the help though.

 

[shrtrnd]

Well, I'd try to replace the motor, and THEN see if you still have the problem.
Good luck with finding an inexpensive replacement.