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How to Test BUS Voltage in Jeep WranglerPosted 6/10/2005By Michael Brown
The same two-wire DaimlerChrysler AG BUS that Chrysler has been using for years on its cars is now being used on jeeps and trucks, giving today's Jeep Wrangler a level of sophistication. On these vehicles, the BUS bias voltage is produced by the printed circuit board or instrument cluster. As with all DaimlerChrysler AG BUS circuits, the wires are twisted together in the wiring harness with 2.5 volts on each wire. Some manuals show BUS+ or BUS- for each wire, but both wires have 2.5+ volts on them, so be sure to check each wire individually to ground with a digital volt/ohm meter. A range of 2.3 to 2.6 volts is normal, though it may vary a bit when the system is awake. If the instrument cluster doesn't work, first determine which gauges don't operate or if the whole cluster is down. I like to start testing with the printed circuit board self-test. Check your manual for the proper procedure, as they may vary somewhat. 1) Put the key in the "unlock" position, the next position up from "lock." "Lock" is where you can remove the key. 2) Depress and hold the trip reset button and turn the key to the "on" position, but do not start the engine. 3) Release the trip reset button and the tests should start. 4) All the gauges and related lights will operate through their paces in two-second intervals. If none of the gauges operate, check the powers and grounds at the instrument cluster. You'll need to rerun the test if you missed any gauges. If any of the gauges don't actuate, then the instrument cluster is defective. If all the gauges appear to work normally with the self-test, move on to the BUS operation from the data link connector under the dash. Check for the 2.5 volts on each wire. If the voltage is off on one or both wires, unhook modules on the BUS one at a time while monitoring BUS voltage. When the voltage returns to normal, you've found your culprit. If all the modules are unplugged and the BUS voltage is still off, separate the connector near the instrument cluster to test the BUS voltage straight out of the printed circuit board. If the voltage returns to normal, there's a problem with the wiring harness. If it doesn't, a bad instrument cluster is the problem.
EBCM Causes Silverado's Gauges
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“ When the EBCM class 2 data wires were disconnected from the data line, the symptom was gone. Now we knew the EBCM circuit was causing the problem, so that left either a problem with the power/ground or the EBCM.”
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The vehicle: a 1999 Silverado. The customer complaint: At speeds over 40 to 50 mph, the antilock brake system (ABS) and brake indicators came on, and the fuel and volt gauge sometimes dropped low at the same time.
The technician had already verified the condition and checked for codes before calling IDENTIFIX. He found code C0237 (rear wheel speed signal erratic) stored as a history code but it had not reappeared for him.
Next, he road-tested with a scanner, watching the vehicle speed input through the powertrain control module (PCM) and electronic brake control module (EBCM) data. When the ABS and brake light came back on, data was lost on the scanner for both the PCM and ABS. Something was crashing the class 2 data line - but what?
On this truck, the instrument panel cluster, body control module, EBCM, PCM, and automatic transfer case control module are all on the class 2 data line at pin 2 of the data link connector.
The tech went to the splice pack (SP205) to determine which module was initially causing the problem. The splice pack joins all of the data circuits at one point, and is located on the lower left side of the dash behind the headlamp switch. The splice pack, a connector end with a jumper bar seated into it, is located on a short harness and often is taped to another harness.
When the EBCM class 2 data wires were disconnected from the data line, the symptom was gone. Now we knew the EBCM circuit was causing the problem, so that left either a problem with the power/ground or the EBCM.
The tech checked the connector at the EBCM for corrosion and terminal condition - it was OK. Next, he removed and cleaned the ground for the EBCM at the frame (see General Motors Corp. bulletin No. 04-05-25-002A). Using a headlamp to load the circuit, he load tested the power and ground to the EBCM. The power and ground tested OK, but there was still a problem. A new EBCM fixed this one.
 | Bill Siegmann is an IDENTIFIX GM specialist. He is ASE master and L1 certified, with 20 years of diagnostic and repair experience. |
This information is provided by IDENTIFIX®. IDENTIFIX® resources cut diagnostic time and provide repair solutions that increase the shop's bottom line. From Repair-Trac pattern failure quick fixes, to Diagram-Online wiring diagrams by fax, to the Repair Hotline staffed by 32 master techs who specialize in diagnosing complex problems by phone or fax, IDENTIFIX® helps techicians fix more cars in less time.
For more information on IDENTIFIX, call (800) 288-6210, 8 a.m. - 6 p.m. Central Time. © 2011 IDENTIFIX. All Rights Reserved.
© 2005 IDENTIFIX. All Rights Reserved. |
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