Car died at intersection
10-02-2014, 01:42 AM
#17
Burning Brakes
If the Motronic DME is bad and reflow of the solder joints (especially the 3 joints on the Bosch 1012 ignition trigger transistor) did not fix it, perhaps the ignition trigger transistor itself is toast, after a mere 3 decades of use.
I replaced the Bosch 1012 ignition transistor is my Motronic with a BU941 automotive ignition transistor from DigiKey online in Minnesota, for about $3.50.
While your'e at it, get some heat conductive grease and make sure there is good thermal conductivity (thermal grease) between the ignition transistor aluminum heat sink and the 4 little square aluminum cubes, and the aluminum chassis, which is intended as one big heat sink.
If there is not good thermal conductivity because the connections are loose, the transistor will not be able to shed enough heat. (It's rated to 175 degrees Celsius, damn hot, so don't touch it if the box is worm, or you'll wish you hadn't.) A good way to cause transistors to fail is to overheat them. This may explain why the car may start and run for awhile, then die, especially on a hot day.
I've gone through the Motronic on my Turbo, reflowed solder joints, new ignition transistor, thermal grease, locktite on the screws that secure the heat sinks, etc. I may add an extra finned heat sink, or even move the ignition transistor to the outside of the box, for better heat dissipation. This is what Bosch did on later models of Motronic.
My theory on at least one cause of Motronic failure or intermittent running is that over many years of operation, and many thermal cycles with constant thermal expansion and contraction, the solder joints develop hairline cracks, the aluminum heat sink parts get loose, the unit overheats, and the process is exacerbated until failure.
Moral of the story: Before you spend big bucks on a new or refurbished Motronic, at least consider reflowing the solder joints and securing the heat sinks by yourself. I used a cheap soldering pen from Harbor Freight, about $3. Since then I upgraded to HF hobby heat pen with 6 or 8 interchangeable tips, about $8.
By the way, the Motronic uses a Bosch 1012 ignition trigger which completes the grounding circuit to the coil--so the spark plugs only work when the coil fires, and the coil only fires when that ignition trigger's innards complete a grounding circuit. If that sucker fails, you're dead in the water.
I replaced the Bosch 1012 ignition transistor is my Motronic with a BU941 automotive ignition transistor from DigiKey online in Minnesota, for about $3.50.
While your'e at it, get some heat conductive grease and make sure there is good thermal conductivity (thermal grease) between the ignition transistor aluminum heat sink and the 4 little square aluminum cubes, and the aluminum chassis, which is intended as one big heat sink.
If there is not good thermal conductivity because the connections are loose, the transistor will not be able to shed enough heat. (It's rated to 175 degrees Celsius, damn hot, so don't touch it if the box is worm, or you'll wish you hadn't.) A good way to cause transistors to fail is to overheat them. This may explain why the car may start and run for awhile, then die, especially on a hot day.
I've gone through the Motronic on my Turbo, reflowed solder joints, new ignition transistor, thermal grease, locktite on the screws that secure the heat sinks, etc. I may add an extra finned heat sink, or even move the ignition transistor to the outside of the box, for better heat dissipation. This is what Bosch did on later models of Motronic.
My theory on at least one cause of Motronic failure or intermittent running is that over many years of operation, and many thermal cycles with constant thermal expansion and contraction, the solder joints develop hairline cracks, the aluminum heat sink parts get loose, the unit overheats, and the process is exacerbated until failure.
Moral of the story: Before you spend big bucks on a new or refurbished Motronic, at least consider reflowing the solder joints and securing the heat sinks by yourself. I used a cheap soldering pen from Harbor Freight, about $3. Since then I upgraded to HF hobby heat pen with 6 or 8 interchangeable tips, about $8.
By the way, the Motronic uses a Bosch 1012 ignition trigger which completes the grounding circuit to the coil--so the spark plugs only work when the coil fires, and the coil only fires when that ignition trigger's innards complete a grounding circuit. If that sucker fails, you're dead in the water.
10-02-2014, 01:48 AM
#18
Burning Brakes
If the Motronic DME is bad and reflow of the solder joints (especially the 3 joints on the Bosch 1012 ignition trigger transistor) did not fix it, perhaps the ignition trigger transistor itself is toast, after a mere 3 decades of use.
I replaced the Bosch 1012 ignition transistor is my Motronic with a BU941 automotive ignition transistor from DigiKey online in Minnesota, for about $3.50.
While your'e at it, get some heat conductive grease and make sure there is good thermal conductivity (thermal grease) between the ignition transistor aluminum heat sink and the 4 little square aluminum cubes, and the aluminum chassis, which is intended as one big heat sink.
If there is not good thermal conductivity because the connections are loose, the transistor will not be able to shed enough heat. (It's rated to 175 degrees Celsius, damn hot, so don't touch it if the box is worm, or you'll wish you hadn't.) A good way to cause transistors to fail is to overheat them. This may explain why the car may start and run for awhile, then die, especially on a hot day.
I've gone through the Motronic on my Turbo, reflowed solder joints, new ignition transistor, thermal grease, locktite on the screws that secure the heat sinks, etc. I may add an extra finned heat sink, or even move the ignition transistor to the outside of the box, for better heat dissipation. This is what Bosch did on later models of Motronic.
My theory on at least one cause of Motronic failure or intermittent running is that over many years of operation, and many thermal cycles with constant thermal expansion and contraction, the solder joints develop hairline cracks, the aluminum heat sink parts get loose, the unit overheats, and the process is exacerbated until failure.
Moral of the story: Before you spend big bucks on a new or refurbished Motronic, at least consider reflowing the solder joints and securing the heat sinks by yourself. I used a cheap soldering pen from Harbor Freight, about $3. Since then I upgraded to HF hobby heat pen with 6 or 8 interchangeable tips, about $8.
I replaced the Bosch 1012 ignition transistor is my Motronic with a BU941 automotive ignition transistor from DigiKey online in Minnesota, for about $3.50.
While your'e at it, get some heat conductive grease and make sure there is good thermal conductivity (thermal grease) between the ignition transistor aluminum heat sink and the 4 little square aluminum cubes, and the aluminum chassis, which is intended as one big heat sink.
If there is not good thermal conductivity because the connections are loose, the transistor will not be able to shed enough heat. (It's rated to 175 degrees Celsius, damn hot, so don't touch it if the box is worm, or you'll wish you hadn't.) A good way to cause transistors to fail is to overheat them. This may explain why the car may start and run for awhile, then die, especially on a hot day.
I've gone through the Motronic on my Turbo, reflowed solder joints, new ignition transistor, thermal grease, locktite on the screws that secure the heat sinks, etc. I may add an extra finned heat sink, or even move the ignition transistor to the outside of the box, for better heat dissipation. This is what Bosch did on later models of Motronic.
My theory on at least one cause of Motronic failure or intermittent running is that over many years of operation, and many thermal cycles with constant thermal expansion and contraction, the solder joints develop hairline cracks, the aluminum heat sink parts get loose, the unit overheats, and the process is exacerbated until failure.
Moral of the story: Before you spend big bucks on a new or refurbished Motronic, at least consider reflowing the solder joints and securing the heat sinks by yourself. I used a cheap soldering pen from Harbor Freight, about $3. Since then I upgraded to HF hobby heat pen with 6 or 8 interchangeable tips, about $8.