Chasing down the cause of electrical problems requires a logical approach.
At times the diagnostic business can be a lot of fun and quite rewarding. Not too long ago I got a phone call about a 2006 Ford Taurus with some electrical problems. The complaints were the ABS light was glowing, the driver's seat would not move, the SRS light was glowing and the horn would not honk. The suspect vehicle was at a body shop and the main body wiring harness had caught fire during the repair process. The body shop had replaced the whole body harness.
This required removing all of the seats and most of the interior of the car. On this model, this harness starts behind the left headlight, goes through the firewall to several connectors that sit behind and below the instrument cluster, travels along the left side of the body, branches off under both front seats, travels on to the rear of the car to the tail lights, rear ABS sensors and a few modules that are located in the trunkThe technician, I will say, had done a very good job of putting the harness and all of the interior parts back so the finished job looked as if nobody had ever touched the interior of the vehicle. When I arrived at the body shop to look at the vehicle, I hooked up an IDS scan tool and found more than 25 codes stored in memory.
Almost every module on the car had some sort of circuit code stored. This was from all of the electrical connectors that had been unplugged in the repair process. I proceeded to clear the memory in all modules and see what would come back.
The only codes that returned were B2293, the driver's front airbag circuit failure (SRS Module), and a B1165 right rear WSS circuit fault (ABS module). With a little testing, I found the clock spring had an open circuit and recommended the part be replaced, which I thought would fix the SRS code and also fix the horn problem.
The clock spring was replaced, which fixed the SRS light, but did not fix the horn problem. At this point the problems were the horn would not honk, the driver's seat would not move and the ABS light was on.
The First Look
We brought the vehicle to my shop for further diagnosis and to continue the repairs. With my scan tool, I could command the horn to blow, so I knew that the circuit was intact and that the problem would be in the control function of the circuit.
This was quite easy to find, because it was a fault in the ground side of the horn pad switch. The driver's seat problem was just as easily traced to a poor connection where the harness was plugged to the smart junction box. Now all that is left was the ABS light.
The scanner showed that a B1165 code was stored in the ABS module, which is a circuit code for the right rear wheel speed sensor. Consulting a wiring diagram, the circuit seemed to be quite simple. Two wires run from the ABS module to each wheel speed sensor. Because there are two wires, this should have been quite easy to figure out.
All that was needed was to grab my ohm meter and test the resistance of the wheel speed sensor. But wait, at the bottom of the wiring diagram was a little message box: "An active speed sensor is an integrated circuit that performs a 7/14th amp current source depending on whether a tooth or a gap is standing in front of the sensor." (The integrated circuit is a mini computer with no simple way to know what is going on inside.) The ohm meter was not going to be the tool of choice when testing these sensors. It was time to find out how this system works.
At times, navigating service information can be a real adventure. Going to the brake system and reading the description of operation, all I could find was something about brake fluid and how to handle it. Maybe I should go and see what the service information (SI) has to say about the B1165 code.
When I went there, I was directed to "pinpoint test E." Here I found some of the needed information about this ABS system. The wheel sensors were not the typical variable reluctance (VR) sensors that we were used to seeing.
Here it said that the wheel speed sensors were active sensors. I also found that each sensor had a battery power and a ground supplied by the ABS module. Because the B1165 is a circuit code, the SI talked about testing for shorts to power, shorts to ground and open circuits.
Now I was a little reluctant to use an ohm meter for testing circuit integrity, because I am a great advocate of testing circuits dynamically, which means to test the circuit in its natural working state. In this case, I wanted to follow the logic behind the code and use a testing method that would mimic the testing that the ABS module uses each time the key is cycled from the off to the on position. I reluctantly turned on the DVOM and went to work.
Testing Procedures
I started with the E-1 test. This test is for a short to power, but hey, this was a brand new factory Ford wiring harness, how could this be? Well I think it would be a good thing to verify the integrity of the circuit, so I grabbed a volt meter to see what I could find.
Unplugging the connector C135 from the ABS module, the volt meter was hooked between B- and each of the wires (pin 1 and 2) to the right rear wheel sensor and no voltage was found. The next test, E-2, was to test for a short to ground. An ohm meter was used to test the resistance between pin 1 and 2 of C135 and chassis ground. No faults were found.
The next test, E-3, was a resistance test of the right rear wheel speed sensor circuits. Here again, the ohm meter was used to test the integrity of the two wires, pin 1 and 2 at C135 and pin 1 and 2 of the wheel sensor plug. No faults were found. Test E-4 tested the resistance between pin 1 and 2 of the harness with both the wheel sensor unplugged and C135 unplugged.
This test looked for a short between the two wires that go to the speed sensor. Still no faults were found. Test E-6 tested the voltage output of the ABS module. This is where the testing process got a little interesting.
Now I will say that I am no fan of following a diagnostic chart or being led along by a mapped out diagnostic process, but there is a time and a place for such things. In this case, reading over the testing procedure gave me an insight into what the ABS module is doing when it does its self-test. But let's get back to the E-6 test.
This test said to plug the C135 plug to the ABS module and with the ignition key turned on use a volt meter to test the voltage at the wheel sensor plug. The voltage between the pins should be more than 9 volts. If the voltage was above 9 volts, I would need to replace the wheel speed sensor. If the voltage was below 9 volts, you need to replace the ABS module.
Well, my volt meter showed that the voltage between pin 1 and 2 of the WSS plug was 5.01 volts. That voltage was below 9 volts, so I put in a call for a new ABS module. The circuit passed the short to ground test, passed the short to power test, passed the internal short test and failed the voltage test.
I installed a new ABS module, plugged in all of the wheel sensors, turned the key to the run position and found that the ABS light was still glowing! Well, now what? I hooked up the scan tool in order to see what the problem was.
What do you know, the B1165 code still was there. How can this happen? The circuit passed all the tests to verify the circuit was working as designed. Wouldn't it be nice if we could use an ohm meter to test the integrity of the wheel sensor? Well, let's go back over the testing again.
Yes, the circuit passed all the tests again, but this time, when I tested the voltage at the wheel sensor, I back probed the RD/PK and PK/b wires at the ABS module plug. These are the two wires that go to the right rear wheel sensor, pin 1 and 2.
My volt meter read .112 volt, so what was going on here? I back probed the positive wire to the left wheel sensor and got 12.23 volts. Did that mean that the new ABS module I just installed has a defect in the right rear wheel sensor circuit?
These voltage readings did not make sense to me when I compared them to the test information supplied by the manufacturer.
Drawing on Experience
Over the years, I have found that a lab scope waveform can lessen the confusion when testing electrical circuits. This is because as voltage changes, it leaves a picture of what happened. I fired up my scope and hooked it between pin 1 and 2 at the C135 connector, turned the key on and saw the voltage do something strange (Figure 1).I had an idea. Why not remove the ABS module from the vehicle, take the old wiring harness, supply power and ground to the module and test the module off the car where its outputs cannot be affected by the vehicle? Maybe this would bring some resolution to this thought process. The module was hooked up and when power was supplied, the voltage to each wheel sensor looked exactly the same as the voltage signal did to the right rear wheel (Figure 1) when I had tested with the module hooked to the vehicle wiring harness.
Service information just told me to test the circuit with the sensor unplugged. Something was not right here. When I unplugged the sensor, the voltage on the positive wire dropped to 5 volts. I thought that the logic circuit in this wheel sensor had a great effect on the output voltage of the ABS module. Something was not right with the circuit, which was reflected in the code B1165.
With the ABS module back in the car with all 4 wheel sensors hooked up, I still had the B1165 code. Now this was getting a little annoying. I checked the voltage between pin 1 and 2 of the C135 connector with the right rear wheel sensor plugged in. When I turned the key on, I got a flat-line voltage of .140 volt.
Where have we seen this voltage before? Hey, I remember — it was on the negative side of the known good left rear wheel sensor. Was there a chance that the right rear wheel sensor polarity was reversed? But there was no way that the plugs could be reversed; every plug will fit together only one way.
Back to C135 at the top of the ABS module, armed with a pin out diagram of that plug, I saw that the pin 1 and 2 wires were the wrong colors. Pin 1 wire is a red with pink stripe. Pin 2 wire is a pink wire with a black stripe. Two wires that looked almost the same but were not the same.
The picture of the plug showed the pin 1 should be the pink with black stripe and pin 2 should be the red with pink wire. Both of these wires were very hard to tell the difference between. Removing the plug from the ABS module and reversing the two wires in the C135 plug fixed the problem. With the ignition switch in the run position, the ABS self-test ran with no codes set. Woo-hoo! The test passed!
Finishing Up
I hooked my lab scope to the right rear wheel sensor circuits, turned on the key and the waveform showed the problem fixed. I even gave the wheel a spin to show how this circuit worked (Figure 3).
The ABS module supplied a power and ground to each wheel sensor. The ground was elevated to .140 volt to remove ground side electrical noise from the circuit. This is known as a floating ground. The ABS module supplied a regulated voltage of 12.36 volts to the sensor. As the tone wheel passed the sensor, that voltage was toggled to 11.27 volts, or dropped about 1 volt, a very accurate way of determining wheel speed. The scan tool was hooked up and the vehicle driven for a few miles to see if the wheel sensors performed correctly (Figure 4). Lab scopes are great tools that will find electrical problems, but the final answer will be verified with the scan tool.This diagnostic problem proved to me again the value of using a scan tool to get data fast, the need to know how the system works, the need to follow the logic behind the code that was stored in memory and the need to follow a logical diagnostic process.
Albin Moore opened Big Wrench Repair in Dryden, Wash., in 1992. He has moved the business to specialize in drivability problem analysis, both with gasoline and diesel vehicles. Albin is an ASE CMAT L1 technician, and brings with him 40 years of analyzing and fixing mechanical and electrical problems.
