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The case of the erroneous ABS code

Are you sure the grounds are good?
Saturday, October 1, 2011 - 00:00

Are you sure the grounds are good?

Motor Age Garage 2001 Dodge Grand Caravan ABS brakes ABS lights fixing vehicle repair shop training technician training automotive aftermarket

I recently encountered an interesting electrical problem on a 2001 Dodge Grand Caravan with 3.3L engine and equipped with a Teves Mark 20 ABS system. A local repair shop had tried to determine why the dash's amber Antilock Brake System (ABS) and red brake lights were illuminated. According to information the shop provided, the shop had performed the tests for two ABS chassis DTC codes, C0110 Pump Motor Circuit Malfunction and C0267 Pump Motor Circuit Open/Shorted. The technicians there could not find anything wrong with the circuit voltage readings. At the bottom of the service procedure is the famous statement "Replace with known good part and retest," which is what the shop did. I was asked for a second opinion when the lights came on again.

I picked up the van after it sat overnight so I could observe it from a cold start. I first used a scan tool to check and clear the diagnostic trouble codes (DTCs). Sure enough, both C0110 and C0267 were present at Key On. The van had to be driven above 15 mph after clearing the codes before the ABS module would retest and turn off the lights. Driving the van around town turning corners, hitting bumps and braking hard didn't set any codes, so I headed to a local gravel parking lot to lock up the brakes and make the ABS cycle. The ABS module (CAB — Controller Antilock Brakes), worked perfectly, so I headed back toward my garage wondering what conditions would set the codes.

I turned everything off while driving so I could listen to the CAB work, checking for unusual sounds. It was hot that day, and I turned on the A/C and cranked the blower up to full speed. As I was braking for a stop sign, the ABS and brake lights came on. I thought this was strange, being the ABS pump was not working, but I also noticed the blower motor's speed slowed every time I put my foot on the brakes. With the van parked by the garage, every time the brakes were applied the blower slowed and the fuel pump whine changed. Then the cooling fans cycled on causing the blower to slow even more. I wondered if this could be tied to the ABS codes setting.

With the van in the garage, I hooked up Chrysler's DRB III scan tool to check the codes. The C0110 was present again, but no other codes were. I turned the ABS pump motor on using the DRB III. The pump showed no problems and sounded normal. Service information said the pump is monitored when the ignition is on and the van is being driven faster than 12 mph without the brakes applied. The CAB tests the pump after start up while driving above 12 mph (or 25 mph if the brake pedal is applied) and checks voltage every 7 milliseconds (ms). The DTC C0110 is stored if the CAB detects improper voltage decay after the pump is off; if the pump was not energized by the CAB, but voltage is present for 3.5 seconds; or if the pump is turned on with the CAB detecting insufficient voltage to operate it.

Thinking the problem would show on the scanner while driving and watching the CAB test the pump, I performed another test drive. No problems showed, so I headed back to the garage, again turning the A/C blower up. At the first stop sign, the dash ABS and brake lights came on. Now I thought that the alternator or a power supply connection might be the problem, but that was not correct as the CAB passed the tests while the van was setting still with the engine off. Driving the van and hitting the brakes had to be connected to the problem. I was not looking forward to doing voltage tests at the CAB connector, as it is not easily accessible.

The CAB module is located in the lower left hand corner of the engine compartment under the master cylinder and booster making it difficult to get to the connector for testing. I like doing the easy stuff first anyway, so I tested voltage between the battery ground post and the ABS Module System fuse No. 21 located in the Intelligent Power Module (IPM). The IPM is mounted between the battery and the left front fender under the hood. I read 12.4 V with Key On and measured the same on the ABS/Module Pump fuse No. 9.

With the van running and the blower on High, the voltage went up to 13.9 on both fuses. Running the ABS pump tests with the engine off or running showed nothing. The next step naturally was to test voltages at the CAB connector. But being so inaccessible I stopped for a few minutes to think about what all the symptoms were. The ABS codes intermittently set only while driving and just before coming to a stop. At this point I am not sure about the A/C blower speed change really being part of the problem, but then what about the fuel pump speed change? Another drive was in order to verify when the code set and if it had anything to do with the blower and fuel pump speed changes.

Test Two

During this drive, I had the blower on and made several stops without any codes setting. Getting curious, I turned on the headlights. At the next stop sign the ABS and brake light came on setting the C0110 code. The front and rear blowers seemed to change noise a little when the brakes were applied sitting still, but not as much as when driving. While at a stop, I turned on the wipers with the headlights and both A/C blowers still on to see if the code would set. But with or without the brakes on, no codes and no ABS light so what is happening? As I drove back to the garage the lights came on again, remembering the 12 mph requirement.

The Chrysler service procedure says to pull the connector between the ABS pump and CAB to check for the usual damage and/or corrosion. But what about the common denominator of the CAB only seeming to have a problem when there are extra body electrical demands? Before diving into the service procedure, it's time to look at a wiring diagram. The two things in common are voltage and load, so I tested the battery voltage at Key On Engine Off. With the headlights on, both A/C blowers on high and the wipers on the reading at the IPM's battery terminal was 12.2V. As the headlights were turned on the blowers noticeably dropped in speed as did the wipers.

Why did the blowers and wiper speed change when the voltage stayed the same? All of the circuits involved so far are supplied from the IPM, so I checked fuses No. 10 (front blower motor) and No. 12 (rear blower motor), which still read 12.2 volts with everything on. I used the DRB III to perform an ABS pump test while everything was on and the engine was off. The ABS pump tried to work but sounded bad and then shut off. I went back to the wiring diagram to look for a something in common with all these circuits. I like to draw a simplified circuit of a system when doing any diagnostics. Simplifying a circuit helps particularly with complicated circuits spread over several pages making it easy to get confused.

It hit me as I started to draw my simple diagram. I had performed the voltage test at the battery and IPM positive terminal with my ground connection hooked up at the battery's ground terminal. The often forgotten part of an electrical circuit is the ground side. Always remember, "any amperage that leaves on the positive side of a circuit has to return on the ground side of that circuit, it's a circle." I found that all but the CAB was grounded to the body in various places.

Where's the connection for the body ground to the negative terminal of the battery? Following the ground cable from the battery to the block was easy, but Dodge did not use a ground wire from the block to the body and there was not a visible connection from the body to the ground terminal anywhere in sight. Going back to the wiring diagrams, I found the CAB ground circuit connected to the negative battery terminal through splice (S141) in the wire running from the a stud labeled G100 attached to the body under the battery tray up to a crimp in the negative battery terminal. All the circuits grounded to the body return to the battery negative through this wire.

The data link connector, transmission control and powertrain module and one of the IPM grounds are connected to the negative battery terminal through the extra ground wire connected to the negative battery terminal's clamp bolt. All of the affected systems ground through splice S141 to the crimp at the battery terminal with the starter cable. Because the starter was not having a problem and neither were any other modules, I decided to test between the splice and the battery terminal.

I connected a jumper wire to the body ground stud and then connected it to the battery ground clamp at the battery post. With the car just idling, the ground jumper has 7.43 amps on it indicating current flow between the stud and the battery. The voltage drop between the body and the negative battery terminal measured just under a volt and at a glance might not seem that bad but should there be any?

One rule of thumb to live by when dealing with electrical diagnosis is to always test any circuit for resistance while loaded looking for amperage reduction and any voltage drops across the load. The voltage drop across the load should nearly equal applied voltage while the circuit is handling its rated load. If they do not read the same then there is extra and possibly unwanted resistance in the circuit.

I found the ground wire coming from the CAB and hooked my temporary ground wire from it to the battery and went for a drive. The ABS light and brake light did not come on and the blowers did not change speed.

The resistive ground caused a low voltage reading at the pump affecting the CAB's ability to perform. The service procedure does say to test for more than 1 ohm between CAB grounds and battery, but does not say to test voltage drop under load. I called looking for a cable with splice only to find that Dodge packages this as a front wiring harness selling for a lot more than the district wanted to pay. After hearing the price plus looking at the labor involved, the local school district decided that my alternative of a homemade ground would work just fine.

Tim Janello is an assistant professor for Sourthern Illinois University in Automotive Technology teaching baccalaureate students for the past seven years. He has 40 years of experience in the automotive repair field with Master ASE certification and L1.

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