Scope & Scan - Service Repair

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More studies of advanced diagnostics

Thursday, October 1, 2009 - 00:00
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Welcome back for part two of advanced circuit diagnostics. Go and grab last month's Motor Age so that you can use the graphic provided there to help you understand this month's live captures. Don't have the issue handy? You can download a copy at www.searchautoparts.com, keyword advanced diagnostics.

Let's take a look at some live scope captures taken from circuit defects installed on a test board. I have labeled these captures shown in Figure A with the circuit test areas A-E laid out the same as in Figure 1 from the first article. We will stay with the lettered test points laid out in that same graphic. Scope Channel 1 is at Point B, Channel 2 at Point C, Channel 3 at Point D and scope ground at Point F. The voltage measurements in the boxes at the top of Figure A are voltage levels at the cursor 2 points.

Notice readings in section 1 of Figure A. Here the circuit is without defect but also not complete or turned on. Circuit point B is at battery voltage as is point C. Point C is at battery voltage because no current is flowing through the incomplete load circuit to ground at point D. With no current flowing through point D, there is no voltage consumed or dropped across the load.

In section 2, we completed or turned on the circuit. As expected, point B is still at battery voltage, but point C is now low or close to zero as current is now flowing across the load to ground past point D. Point C is not at absolute 0V, because there is a small voltage dropped across the transistor in the ECU on its way to ground. In addition, there are a few, about 50mV, dropped through the scope to ground.

In section 3, we started out with a complete circuit at the cursor 1 point, but at cursor 2 the circuit was opened on the battery feed side to the load between points A and B. The voltage at point B of course drops. With no battery feed, the current at point D also drops to nothing. If we were only measuring current in this circuit, how would we know if we lost the feed side of the circuit or the ground path? We would not know.

If we were only probing voltage at point C when the circuit was on, would we know that point A-B lost its connection, or that point C was simply low as it should be? We would not know without seeing the current level also. When point C is low and there is no current flow, the circuit is open. When point C is low and current is high, the circuit is merely on and without defect.

In section 4, we started out with a complete circuit at the cursor 1 point but at cursor 2 the circuit was opened on the ground path between points B through E. Notice that the voltage rises at point C, cursor 2 and the current flow drops to 0. We have the same conditions we had in section 1, circuit off. If we were only measuring current flow would we know if the defect was on the battery feed side A-B or on the ground path C-E? We would not know which half of the circuit had the defect. By making the extra voltage measurement connection at point B we cut our inspection time down by half.

In section 5, we added an illegal resistance to the battery feed circuit between points A-B, causing the feed to drop from 12V to 8V. The current flow has also been reduced as a result from 1.8A to 1.4A. If we were only measuring current would we know if the illegal resistance was on the battery feed side or the ground side? We would not know. But with one more voltage probe at either point B or C, we now know which it is. Point A-B has the illegal resistance and voltage drop.

In section 6, we have a defective component between points B-C (resistance above 100k). With no current flow, good battery feed at point B and all voltage dropped to ground through point C, we immediately know the component or its connector is no good.

In section 7, we added an illegal resistance to the ground path circuit between points C and E, causing the ground path to rise from nearly 0V to 4V. The current flow has again been reduced from 1.8A to 1.4A. If we were only measuring current, would we know if the illegal resistance was on the battery feed side or the ground side? We would not know. But with one more probe at either point B or C we now know which it is. Point C-E has the illegal resistance and voltage drop.

Jim Garrido of "Have Scanner Will Travel" is an on-site mobile diagnostics expert for hire. Jim services independent repair shops in central North Carolina. He also teaches diagnostic classes regionally for CARQUEST Technical Institute.

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