The Good Stuff
The increasing complexity of car electronics requires an ever advancing portfolio of diagnostic methods to pinpoint the root cause of the problem at hand. "The Good Stuff" is a collection of those methods, which we consider critical to our success.
Airbag Testing (full article)
Airbag and seat belt detonator assemblies don't use a separate removable resistance sensors, however they do have an intrinsic resistance value that the airbag ECU uses to "sense" a defective airbag circuit. With that in mind we can substitute a potentiometer (pot') in place of the actual airbag assembly in order to determine if the airbag resistance itself is out of specification or if there is a defect within the harness going to the suspect airbag circuit or the module itself.
I typically use 0 - 25 Ohm range pot to test airbag systems. You only need two of three potentiometer leads for airbag testing. One of the end connections and the adjustable center tap as shown in Figure 1. I have not run across very many airbag systems on which any of the bags circuits measured a normal 2 - 4 Ohms. There are some that are as high as 7 Ohms but again they are rare. Either way you can dial your pot from 1 - 10 Ohms, stepping in 1 Ohm increments to cover your bases if the bag resistance value is not known.
Don't Blow your Fuse Finding Short Circuits (full article)
Tracking down the offending circuit, whether a fuse blows on a constant or intermittent basis, is never easy. With the proliferation of low current automotive electronic devises today, fuses typically feed multiple circuits. Manually separating harness connection points or cutting into harness splice packs one by one in order to install your Ammeter and find the offending leg of a circuit can turn into a game of blind ditch digging in a hurry.
What is needed is a better way to "see" the offending circuit while using a map to enable us to dig a minimum of holes. A low current probe combined with a graphing meter or oscilloscope and a schematic provides just such tools.
First we must understand some facts. Contrary to what is popularly believed fuses do not simply blow "instantaneously" once the current draw of the circuit reaches an amp or so above the rating stamped on the fuse. Fuses are thermal devises meaning they require heat to melt the fuse element. That required amount of heat energy is created by excessive current flowing for a period of time. This means that a fuse will survive current flow, in excess of its rating, for a longer period with a lower amount of excess current. At higher current draws the fuse will last less time before it blows open. Bottom line, the amount of time the fuse takes to blow can vary.
Why is this fact relevant to our excavating adventures? Because many times an excessive current draw or short circuit draw will happen multiple times before the required amount of amperage and time has had the chance to create enough thermal energy to actually melt the fuse or trip the circuit breaker on our favor short finder tool. This means that we can have many chances to catch and track down the offending current draw long before the energy level exceeds the point at which the heat generated opens the fuse. Look at the multiple excessive current draw events traced by current probe and graphing meter shown in Figure 1.. more.
"We come to you so your customer will not go somewhere else!"
Many of the newer vehicles offer airbag module scan date that will read the actual circuit resistance values up to a programmed limit, see Figure 2. Having scan date and multiple airbags on each vehicle can give us the known good airbag resistance values to compare to the airbag circuit that is setting a DTC for a resistance value circuit error. Once this good value is known, we can use our pot in place of the suspect airbag to determine if we have a bad bag assembly or a harness issue... or both!
Test potentiometer installed on equivalent airbag circuit
Figure 1: Equivalent Airbag Circuit
Figure 2: Scan Data
Try graphing the resistance values while wiggling or manipulating the suspect harness.
Figure 1
Caption
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