TESTING & DIAGNOSIS Question: code 44 Lean Exhaust Tip: LOSS OF POWER WITH TBI ENGINES DIAGNOSTIC CHARTS > CODE 42 - ELECTRONIC SPARK TIMING > CIRCUIT DESCRIPTION When system is running on ignition module (no voltage on by-pass line), the ignition module grounds the EST signal line. The ECM expects to see no voltage on the EST signal line during this condition. If the ECM sees voltage on this line, Code 42 will be set and the system will not go into "EST" mode. When engine RPM for EST is reached (about 400 RPM), and by-pass voltage is applied, EST should no longer be grounded in ignition module, so EST voltage should be varying. If the by-pass line is open or grounded, the ignition module will not switch to "EST" mode. EST voltage will be low and Code 42 will be set. If the EST line is grounded, the ignition module will switch to EST, but because the line is grounded, no EST signal will be present. Code 42 will be set. Code 42 sets if there is an open, or a short to ground, in the EST or by-pass circuit. DIAGNOSTIC CHARTS > CODE 42 - ELECTRONIC SPARK TIMING > TEST DESCRIPTION NOTE: Test numbers refer to test numbers on diagnostic chart. 1. Code 42 means the ECM has seen an open, or short to ground, in EST or by-pass circuit. This confirms Code 42, and confirms that fault causing the code is also present. 2. Checks for normal EST ground path through ignition module. An EST circuit No. 423 shorted to ground will also read less than 500 ohms (will be checked later). 3. As test light voltage touches circuit No. 424, module should switch, causing ohmmeter to "overrange" if meter is in the 100-200 ohms position. Selecting 10-20,000 ohms position will indicate above 5000 ohms. The important thing is that the module "switched". 4. The module did not switch and this step checks for: Service Manual: CCC TBI TESTS W/CODES Print Date: 10/4/2017 1988 Chevrolet Pickup 7.4L Eng VIN N V30 Page 1 of 13 Printer Friendly View 10/4/2017 http://www2.prodemand.com/Print/Index?content=tabs&module=true&tab=true&terms=tr...
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TESTING & DIAGNOSIS DIAGNOSTIC CHARTS > CODE 42 ......1. EST circuit No. 423 shorted to ground. 2. By-pass circuit No. 424 open. 3. Faulty ignition module connection or module. 5.
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