T6 Electrical / Electronic Systems — practice test

High-yield T6 topics

The material that shows up year after year. If you're short on time, start here.

• Voltage drop testing, done with the circuit loaded. Know the working limits: about 0.5V max total on the insulated side of the cranking circuit, 0.2V on the ground side, roughly 0.1V per connection, and 0.5V/0.2V again on the charging circuit. Expect a scenario where a cable reads 0.1 ohm cold and still cooks under 500A - the ohmmeter answer is the wrong answer.
• Battery state of charge and load testing. 12.6V open circuit = 100%, 12.4 = 75%, 12.2 = 50%, 12.0 = 25%, and you remove surface charge before you read it. Load test at half the CCA rating for 15 seconds; must hold 9.6V at 70F, and that number gets adjusted down when the battery is cold. Specific gravity 1.265 full, more than 0.050 spread between cells condemns it.
• Parallel battery banks. Four Group 31s in parallel add CCA, not voltage - it is still a 12V system. One shorted or sulfated battery drags the whole bank down, which is why you disconnect and test each one individually instead of condemning the set. Same logic on cranking voltage: below roughly 9.6V while cranking, you are chasing supply or resistance, not the starter.
• Charging system output and AC ripple. Load the alternator to rated output and expect regulated voltage around 13.8-14.2V hot. One open diode costs roughly a third of rated amps and puts AC on the DC bus - more than about 0.5V AC (or a chopped sawtooth on the scope) says diode/stator, not regulator. Know remote voltage sense versus internal sense and what a corroded sense lead does to output.
• J1939 network basics. 250 kbps twisted pair (500k on newer trucks), 120 ohm terminators at each end of the backbone, so you measure about 60 ohms across CAN H and CAN L with the key off and batteries disconnected. 120 ohms = one terminator open, 40 ohms = an extra one hung on the bus. Know the 9-pin Deutsch color codes (green vs black vs yellow) and that stub lengths stay short.
• Fault code structure and gauge drive. J1939 reports SPN plus FMI; older J1587/J1708 uses MID/PID/SID plus FMI. FMI 3 = voltage above normal/shorted high, 4 = voltage below normal/shorted low, 5 = current below normal or open circuit, 2 = data erratic. Most dash gauges are driven over the datalink, so a dead gauge with a good sender points at the cluster or the bus, not the wiring to the sensor.

Where techs lose points on T6

Good technicians miss these — not from lack of skill, but because the question is built to catch them.

• Continuity and resistance readings that 'prove' a cable is good. A corroded ground strap will read a couple tenths of an ohm on a DMM and still drop a full volt at 400A. If a question offers both an ohmmeter check and a loaded voltage drop, the voltage drop is the answer nearly every time.
• The 60 ohm CAN reading. Techs remember 120 ohms because that's the resistor value and pick it. Across the bus you should see about 60 ohms because the two terminators are in parallel - and you take it with power off, not key-on. Reading it live is a distractor answer.
• Adding batteries to fix cranking. More batteries in parallel raise CCA, not voltage, and a bank is only as good as its worst battery. Watch for the answer that says 'replace the battery set' when the stem gives you enough to isolate one bad cell or a bad cable end.
• Test lights and LEDs. A standard test light on an ECM or module-controlled circuit can damage the driver - use a 10 megohm DMM or a loaded logic probe. And a turn signal that flashes fast after an LED conversion isn't a failed flasher, it's low current through a thermal flasher; the fix is an electronic flasher or a load resistor, not a bulb.

A study plan that works for T6

Roughly two weeks of real preparation, in the order that actually builds on itself.

1. Days 1-3: Ohm's law and voltage drop until it's reflex. Series/parallel current and voltage behavior, then walk a real truck: drop-test both battery cables, both grounds, and the charging circuit while someone cranks. This is the biggest task area and it feeds every other one.
2. Days 4-6: Battery, starting, charging. Memorize the SOC voltage table, the half-CCA/15-second/9.6V load test, and cranking voltage limits. Do a full charging output test with a carbon pile and check AC ripple. Learn solenoid pull-in vs hold-in windings, the magnetic switch, and the neutral/clutch interlock path.
3. Days 7-8: Lighting and gauges. SAE J560 seven-way pin and color assignments, the blue aux circuit that carries constant power to trailer ABS (and PLC per J2497), flash rates, and why a bad trailer ground makes brake lights dim when the turn signal is on. Then dash gauges, sender-to-gauge behavior, and key-on sweep self tests.
4. Days 9-10: Related electronics and full practice tests. Drill SPN/FMI vs MID/PID/SID/FMI, FMI 3/4/5 meanings, J1939 wiring and terminator math, ESD and pre-welding module precautions. Then run timed 50-question sets and review only the ones you missed - if you're missing general diagnosis items, go back to day 1.

Sample T6 questions

Straight from the bank — answers highlighted, with the explanation underneath.