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.
The 30 amp fuse that fed this main relay circuit was exceeded by 23 amps nine times in just 30 seconds. Yet the fuse never blew! That is nine more chances I had to locate the short compared to someone who was simply waiting for a fuse to blow or a circuit breaker to trip.
Using the low amps current probe and scope or graphing meter graph the circuit output over while operating circuits or doing a wiggle test. Start by pulling the schematic, or map, of all the circuits that the offending fuse protects. Then clamp your current probe around the feed circuit to the offending fuse. Then one by one operate each devise on that circuit until you see the current draw exceed the fuse rating
If the current draw exceeds the fuse rating as seen on the graph it is time to chase down the circuit regardless if the fuse blew or not. How exactly do we chase it down or what if the excessive current draw is not relative to a devise being operated and the draw occurs simply by turning on the key? No problem. Find a splice pack(s) and simply wrap your current probe around half the wires branching off of the splice, then retest the circuit. If the first bunch of wires displays the excessive draw then further divide the bunch of wires by half until you have narrowed down the offending wire. If the first bunch does not display the excessive draw move your current probe to the second bunch and again keep dividing till you find the highest current draw wire. Once you find the offending wire recheck your schematic to see if that splice pack feeds yet another splice pack. The interior lights circuit on my plain-Jane 1998 GMC Safari van has no less than 3 splice packs totaling 16 connections! Once you have singled out the offending wire cut it from the splice pack and retest your remaining circuits for excessive draws.
I say cut the wire of the offending circuit because many splice packs typically do not label the branches of the splice by circuit number. Cutting the feed to the circuit is a way of identifying the circuit in question. If the feed to the offending circuit is dead the devise it feeds will also be dead, thereby identifying the defective circuit. Next we need to see if the devise or the harness to it is causing the excessive current draw. To determine this disconnect the power feed harness wire from the devise connector. Then run your own fused jumper wire from battery power to the devise and recheck the current draw again using your current probe. If the draw is excessive you have a defective devise. If the draw is not defective you must now test the devise feed harness for a short. Do this by removing the battery feed fused jumper wire from the devise and attach it to the power feed wire previously removed from the devise connector and recheck your current draw. Now that you have located the offending wire it is time to dig just that one hole in order to find that harness short.
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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.
Figure 1: Equivalent Airbag Circuit
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!
If possible, graph the resistance value data while doing things like turning the steering wheel and adjusting the tilt column. Move seats back and forth and up and down if you are testing a seat mounted side airbag assembly circuit. Perform a wiggle test on all suspect harnesses.
Figure 2: Scan Data
Try graphing the resistance values while wiggling or manipulating the suspect harness.
As you watch for incorrect resistance values on the scanner don’t expect absolute true readings. I have never seen scan data read 100 ohms or “Open”. Most of the time the scanner will read around 25 ohms when in fact the circuit is open. This is simply due to the Airbag modules software programmed limits.
If resistance values are not available in scan data you can always use your potentiometer and an Ohmmeter set to min/max mode to do your dynamic testing. A graphing meter is even better if you have one. Install the meter in place of the airbag module to test the entire harness.
When installing your pot’ in place of a removed airbag or seat belt detonator you must either consult the vehicles wiring diagram or visually inspect the connector you are to install your pot for the presence of connector “Shorting Bars”. See Figure 3. Shorting bars are a safety devise that connects or “shorts” the airbag terminals together when the connector is seperated. This is a safety devise employed to prevent an open harness connection from simultaneously being shorted to voltage and ground accidentally deploying the airbag. If the connector half of which you are connecting your pot’ has a shorting bar assembly then you must first carefully disengage the shorting bars from the connector terminals before making your measurements. I use flat plastic toothpicks for this purpose. The old cut off ends of plastic zip ties also work well. If the shorting bars are left in their default “open connection” shorted position then the circuit resistance will read almost zero ohms due to the mechanical short. This is obviously not a true resistance value of the circuit.
Figure 3: Shorting Bars
Never bypass shorting bars while the airbag unit is still attached to the harness. An accidental airbag discharge causing severe personal injury is always possible.
NEVER DEFEAT THE SHORTING BAR SYSTEM WHILE AN AIRBAG IS STILL CONNECTED TO THAT CIRCUIT. SEVERE PERSONAL INJURY CAN RESULT.
Typically shorting bars are located on the airbags themselves, on the lower steering column connector for the clock spring and on the main airbag module harness connector itself. Typically the only time you would need to defeat the shorting bars is when measuring for open circuits from the top of the clock spring (with airbag module removed) and the lower connection at the base of the steering column or when checking continuity to the ECU harness from one of the airbag connections. Again, the airbag must be removed for safety before these continuity/resistance tests are performed.
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When installing your test pot into female harness connections APPROPRIATE SIZE TEST TERMINALS MUST BE USED. I have seen way too many damaged female harness terminals in my diagnostic travels to recommend anything but the correct test terminals of the types.
If you do determine that an airbag connection has slightly high resistance by just a few Ohms I have had good success applying a product called “Stabilant 22A” liquid improver to connectors. Just a tiny drop applied with a wooden toothpick cures many intermittant high resistance connection problems.
As always, make sure the airbag system “proves out” after
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.!
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