Hereís my, so far, unsuccessful attempt to repair the hatch release motor on my brotherís 81í S.

The problem with this mechanism is that it does not park the latch release lever, as the motor keeps running until the electrical connector is disconnected. The failed component appears to be a transistor; its plastic shell was broken/cracked. My brother tried to repair the circuit board, but gave up.

I found my brother's mistake and got it working again when powered by a 9 volt battery. But with a 12V car battery, the repaired circuit board failed again, and the replacement transistor cracked as well. The motor generates too much juice for the replacement transistor. I couldn't find a suitable transistor replacement at Radio Shack.

Itís a simple motor mechanism; just a small DC motor and gears. The internal circuit board is just a simple transistor switch to electronically brake the motor to a stop at the park location. If the electronic brake circuit fails, the motor has enough momentum to slide past the park point where it reconnects with the power source (thus, the motor keeps running).

You can see that the transistor is switched off with +12V power, but when power is cut, the motor acts to generate electrical current, switching on the transistor, which causes the the motor to quickly come to rest.

Components:

Resistor, 110 ohms, 2% tolerance, Ĺ watt
Switching Diode, 1N4005, 1A
NPN Transistor, C337

Couldnít find a datasheet for the C337. I did find a datasheet for a BC337, which is think is equivalent. Looks like the original transistor is rated at 800 mA collector current. I replace the transistor with one from Radio Shack, a 2N2222, but these seem to be rated at 300 mA in spite of the packaging being labeled at 800 mA.

Until I can find a suitable transistor, this repair is a work in progress. Any comments appreciated.

Yes, you can stop the motor from running continuously by pulling the 1 Amp fuse, as it provides constant +12V. But pulling the fuse doesn't fix the problem of a defective brake circuit and proper parking of the latch mechanism.

If you remove the 1 amp fuse, you can still momentarily run the motor with the pull switch which gets power from the inside lights power source (7.5A fuse).

All my testing is bench testing.

borland
90' S4
Slate Metalic

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I found a suitable transistor at the local Fry's Electronics store. After installing this transistor, it now works great!

I used a NTE transistor, part number NTE31. It has a higher current rating (1000mA) than the BC337 (800mA) so its slightly larger, but still mounts inside the gear motor case. Here's a link to the NTE31 datasheet:

http://www.datasheetcatalog.org/datasheet/nte/NTE31.pdf

Hope this helps someone in the future.

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borland
90' S4
Slate Metalic

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I don't understand how the circuit is supposed to work exactly...
Alan
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1994 928 GTS Black(741)/Black Manual

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Alan,

The motor brake circuit works something like this.....

When the motor is powered by battery +12V, the transistor is always in the OFF state. This is because the transistor's base is tied to low potential or ground.

When battery power is cut (by the gap in the gear/wiper), the motor, by its rotation acts as a current generator and provides current across the 110 ohm resistor to the transistor's base; enough voltage potential to turn the transistor to the ON state. The diode also helps with this potential by isolating the base from the emitter.

With the transistor in the ON state, the transistor acts as a zero ohm load for the motor (generator). This load causes the motor to stop rotation much faster than if it was allowed to coast to a stop.

Consider if the braking circuit is not functional, the motor will have enough momentum to cause the wipers to glide past the gear/wiper gap and again acquire +12V battery power; thus a continuously running motor with noticeable periodic pauses in motor power.

The down side of this circuit implementation is that the 110 resistor is drawing power (12.5V/110ohm = 114mA, or 114mA*12.5 = 1.4 watts) while the battery is providing +12 power; maybe that resistor is rated higher than 1/2 watt?

borland
90' S4
Slate Metalic

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I second MrMerlins gratitude to all contributors. I have just spent the afternoon fixing the tailgate / hatch open, close and release problems I had.

My problems stemmed mainly from a crumbling cushion stop which made the hatch hard to close and stopped the release from working. I foolishly buggered about with the release mechanism and servo before researching this forum (won't do that again!) and broke the servo in the process. I ordered a new urethane cushion stop and once that was installed I got to work on the servo. The transistor was busted and I replaced it with a 2N5551, which must be fitted in the opposite orientation to the original as the leg configuration is EBC as opposed to the original CBE. It is a 625mA device but as it is only dissipating the motor brake power this isn't an issue. I also had to replace the 1 amp fuse (with a 3 amp at the moment - on to fleabay after this to get the correct rating).

Once I had repaired those three items (cushion, transistor and fuse) the hatch worked perfectly with the servo replace in its original position (no surprise).

So my contribution to all this is to recommend that the cushion is the first point to rectify before anyone starts messing with the hatch adjustment or the servo position.

Thanks again to all contributors.

DaveUK

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