Noisy chain / knocking

[sgould]

I've seen your posts on UKS and you seem to be getting somewhere with a couple of guys with some experience.

One odd thought crossed my mind. How do Saab reduce the voltage when needed?  With all the PWM stuff around, I was reminded of the many destroyed TomTom satnavs in the early days.  They ran off 5v.  The in-car charger/power lead came with a standard cigarette lighter plug.  What no one was aware of at the start was that the voltage dropping was done inside the plug that went into the lighter socket. There were 3 wires in the cable to the TomTom.  A 12v feed and earth, and a sensor wire.  The voltage control was a fast on/off switching device that cut the 12v supply and turned it back on again - so effectively a square wave PWM - which once smoothed at the TomTom gave an effective 5v.  The speed of the switching was adjusted by the sensor wire reading the effective 5v at the TomTom end.

The problem arose when people didn't realise that the cable was no a standard 12v twin wire feed, and tried to hard wire the TomTom into the car's 12v system, after cutting off the plug that goes into the cigarette lighter.  It fried the TomTom.   My thought was whether Saab used a similar control which could damage the control module if the sensing wire got overloaded or the circuit failed to switch fast enough to control the voltage.  This may all be complete rubbish, but the TomTom design was almost 20 years ago, around the time that the 5 speed box was introduced.

[carrera]

Yeah, a bit of hive mind going on, building the understanding and gradually most of the pieces of the jigsaw drop into place.

I don't think it's a voltage regulation problem. WIS is fairly explicit that the TCM is fed by power from 30+ which I believe is direct from the battery positive. Reading around I think I saw something that said the TCM was good for 9-16v, so fine for flat battery and an overcharging circuit. Might be a bit of a problem if you are starting the car with a garage charger connected as they can probably put out over 16v when turned up high. (mine can do 40a on max, with start giving even more)

That's the TCM side of things. As I understand the PWM duty cycle side of things if your charging system is running at 14v and the duty cycle is 40%  the DMM will show 5.6 volts. If it was an analogue meter it would be flickering between 0v and 12v 300 times a second, if the needle could move fast enough. In practical terms that too would average around 5.6v

I'm still not sure how I can have a high duty cycle and low amps, when the TCM I am using for the test is (or was) a good one. Time will tell

[carrera]

Time for another update.

The linear solenoids have been dismantled, cleaned and reassembled. Not a lot of crud, but it seems enough to interfere with their operation.

Electrical tests showed that current draw was up to 0.97 amps in park with the engine off, and the duty cycle was 59%

Adaptation complete the car was road tested but after about half an hour, after a stop the P0748 returned with the proper TCM

I have the 2003 TCM and that still seems to be working correctly and the change was noticeably smoother. Only problem is that the gearbox warning light comes on with P0826 as the flappy paddles are not found.

I have a potential solution involving a mini power supply to provide 1.25v to pin 8 of the TCM to extinguish the light, fingers crossed

[carrera]

Well, the power supply arrived today

Pre tested the voltage regulator and set it to 1.248 volts with a spare battery

I picked up 12v from pin 32 (red / blue) and earth from pin 22 (black) on connector A

Bared some wire and soldered the feed wires direct onto the existing TCM wires, then used some heat shrink to protect the exposed areas. Not ideal with it being an unbroken Y joint, but serviceable. I didn't want to cut the existing wires.

Switched the ignition on and the error was still there, but after clearing the stored code there are no warnings after an ignition reset.

So this means a 2003 on TCM can be used with a 2002 car, no other changes, costs about £3

Here's what it looked like before I put the glovebox back. Test drives required