924Board.org

[Patrick]

Hello all! I had asked this question a few yrs back when the author`s name(me) was pat. I have a 1979 U.S. version NA 924 w/ the Bosch ignition control box #0227-100-010. At the time i noticed the problem(2004), the box was only about 4 yrs old.

I was checking something in my car one morning and turned the ignition switch ON for several minuets, and then touched my ignition box and wow was it REALLY HOT! I had not started the car at all this particular morning. Its apparently been like this for the past 4 yrs. The car starts and runs fine though. I`m just concerned that this might eventually destroy my box! The box is located in the original position on the left handside. The only reply i got (from Lizard) 4 yrs ago was that i should make sure i was getting a good ground to the box. Would this be along the same lines as when your ground connector to the (-) battery terminal is loose and you develop some heat, or SOMETHING ELSE? Anyone else experience this? Anybody have any ideas? THANKS!

[Smoothie]

I don't know - how hot is "hot"?
To really know, you'd have to get a reading with an infrared non-contact thermometer, then have a few more more people with the same model year do the same and compare results.
-Or touch the same part of the ignition box for the same amount of time after it's been on for the same amount of time in the same ambient temperature, then measure how big and how red a mark it leaves.
_________________
"..it's made in Germany. You know the Germans always make good stuff."


'82 924T, US version, dark green metallic, 5 speed Audi 016G gearbox

[RC]

Since this is a first generation transistor ignition it switches ON at power up, and OFF when triggered, to generate a spark. Some were even designed like that, with an extended dwell to fully saturate the coil. It wont receive a trigger signal without the engine (dizzy) turning. The transistor acts as a switch but will have voltage drop between collector and emitter even when fully on. At say 5A coil current this will generate heat. Not only in the unit but the coil too.

The ignition is not designed to be on when not running. Eventually you will likely overheat the igniter and/or coil.

Roger

[Patrick]

SMOOTHIE- no thanks for the great idea, i`m definitely not into big red marks!(ha ha). But yes,i plan to use an infra red thermometer. But just to give you guys an idea, i did try to keep my hand on it for several seconds, but let go for obvious reasons!(thats how HOT it was!)

RC- i vaguely remember how the electronic ignition box works (from the haynes manual). But if i understand you right, the HOT ignition box is caused by the engine not running so that the transistor switch doesnt interrupt the current going to the coil`s primary winding which eventually heats it up! Thats good enough for me. Though i will probably start the engine up for several minutes and check the box temp w/ an infrared thermometer just to see what its like.

THANKS A LOT GUYS!

[RC]

The unit will get hot anyway as it is still dissipating some power when running. Bosch engineers will have designed the heat sinking capacity of the case to acceptable level at a fairly high ambient temperature considering it is mounted under the hood. With a nominal dwell of say 45*, or 50% duty cycle with the engine running, the temp rise will be within design limitations of the heatsink. Power transistor maximum junction temperature can get up to 150C or higher in some cases. It is imperative that this is conducted via the case to a heatsink of appropriate thermal conductivity to dissipate this heat.

With the ignition on but not triggering (not running) the duty cycle is 100%. The transistor will get hotter and so will the case which is the heatsink. Although there is a degree of design tolerance the temperature will increase so much above ambient that it is a gamble as to when the critical temp is exceeded. Should be OK for a brief period if necessary but is just a matter of time if the ignition is left on.