# Short life failure explanation

• posted on October 7, 2005, 12:11 pm
Hyundaitech said he did not understand what I was talking about in another note so here goes. I think it's something that should be understood. ;-)
When one designs a circuit or mechanical device part of the design process is to take into account the power to be transmitted. In a mechanical part it would be the material choice, cross section area, shape, etc. In electrical components it is mostly rated heat dissipation. Electrical waste is generally produced as heat. To keep it simple if I use a 12 ohm load in the control module sensor connected to it will cause at most a 1 amp current at a nominal 12 volt system. Amps=volts/ohms. That is with the sensor resistance at 0 ohms. As the sensor increases reistance the current drops and the power decreases. The power also divides between the sensor and the control module. Max power is 12x1 watts.
There are 3 failure models. 1. Open circuit - no current flow. 2. Short circuit - currnet limited by source resistance - classic arc and spark with charred remains. 3. Lowered impedance. Result is more current, more power dissipation. If it's low enough you approach failure 2 but what if it drops something less? To pick a number sya a 30% drop. My 12 ohm resistor becomes 8 ohms. Max current goes to 12/8=1.5 amps. Max power becomes 12x1.5 watts.
OK - what's it mean? Not enough power for arc and spark but the extra power becomes extra heat that slowly simmers the parts instead of flash burning them. It is hard to predict what will happen as there are many failure modes as well as conditions. For example if the sensor is the gas gauge and you never let it get below half full you probably won't get to max power so nothing happens. On the other hand if you consistently run low the gauge fails prematurely when either the sensor or the load in the meter circuit cook off. The failure is as likely to be a type 1- open circuit as a type 2 short. Either way it's broken but a type 1 will not damage the rest of the circuitry even thogh the part that fails may be the "good" part that is being operated out of it's design range of power dissipation. Most sensor circuits work at lower power levels than this but the principle remains the same.
nothermark

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• posted on October 7, 2005, 5:20 pm
What I don't know is whether the TCM provides any significant resistance in the pulse generator circuit. They are analog A/C voltage generating sensors. If I remember correctly the sensors provide about 220 ohms resistance. I have no idea what order of magnitude the TCM might be.
Furthermore, I definitely agree that if the TCM provides a significant amount of resistance to the circuit that it's *possible* it caused the sensor to fail. I've only ever seen a failure like this once-- a Kia ECM was causing an ignition coil to misfire. It would literally blow up the coil after the car was driven awhile. And it's probably exactly the problem you're referring to. The ECM's resistance was too low, causing the coil to overheat and fail.

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• posted on October 7, 2005, 9:26 pm
I should have mentioned this in the above post, but sorry to say I forgot.
You've given an excellent explanation and I appreciate your efforts very much. I hope nothing in the above post led you to believe otherwise. In fact, you've made me think about some things I hadn't thought about before, and that might help me fix some more cars right the first time. Thanks.