GM 3.8L VIN 3 & 3800 VIN C Engines-Code 41 Diagnosis

GM introduced the 3800 VIN C engine with the 1988 model year. This is classified as the “Fast Start “system because the ignition module can identify cylinder position and properly select the appropriate ignition coil to start the firing sequence within one-third of a crankshaft revolution. Unlike the earlier 3.8 VIN 3 engine, a code 41 will not cause a no-start condition. On the 3.8 VIN 3 engine, code 41 was fairly easy to diagnose. In this situation, the engine will not start due to the cam signal not being present. Because of this, the module is unable to start the firing sequence, and the ECM will not pulse the injectors. Refer to Diagram #1. 3.8L VIN 3 engines are equipped with a 3 wire crank sensor and a 3-wire cam sensor. The crankshaft damper has a slotted trigger wheel with three evenly spaced slots. This signal is referred to as the 3X signal, which is used as an RPM input to the ignition module. The cam sensor is used for cylinder position identification and a single pulse is produced for every cam revolution or two crankshaft revolutions. Before the ignition module can produce spark from the ignition coils, or generate an RPM signal to the ECM, it must receive a signal from both the cam and crank sensors. It is the RPM signal (circuit 430) that enables the ECM to pulse the injectors.

3800 VIN C engines use a 4-wire crank sensor, and a 3-wire cam sensor. The crank sensor is triggered in the same manner as the VIN 3 engine, except this trigger wheel has two sets of slots on the crankshaft damper. The outer set has 18 evenly spaced slots. This part of the wheel is used to generate the RPM signal to the ignition module, which is called the 18X signal. The second, or inner, wheel has 3 unevenly spaced slots which is used to identify cylinder position. This synchronization signal enables the module to properly select the appropriate coil to start the firing sequence. Once the firing sequence begins, the ignition module will send a 5-volt square wave signal to the ECM based on the speed of the coil firings on circuit 430 (purple wire with a white stripe). The ECM uses this signal to determine when and how often to pulse the fuel injectors. If the ECM does not see this signal, the injectors will never get pulsed. The cam sensor signal is used only for proper fuel injector sequence by the ECM. If the cam sensor signal is missing on startup, the ECM will begin sequential injection in a random pattern and set a code 41. A cam sensor failure on the 3800 engine will not cause a no-start condition. Now, let’s examine how the 3800 cam sensor system works. Refer to Diagram #2.

The ignition module supplies a signal voltage of 4 to 8 volts to the cam sensor on circuit 633,or module terminal “J”. As the magnet that is attached to the cam gear passes over the sensor, this voltage is pulled low, or switched to ground, resulting in the beginning of the cam sensor pulse. Once the magnet has passed the sensor, this voltage signal is no longer grounded, and reverts back to its high or original position (refer to Diagram #3). At the very same moment that this signal is generated by the cam sensor, the module sends a similar pulse on circuit 630 from module pin “F” to the ECM. If this does not happen, code 41 is generated by the ECM and the “Check Engine” light is illuminated. Let’s examine how to diagnose the cause of this code. First-disconnect the cam sensor connector and measure the voltage at all three terminals of the connector with the key on, engine off. Pin “A” (brown wire with a white stripe) of the cam sensor connector should measure between 4 and 8 volts DC. Pin “B” (gray wire with a red stripe) is a ground. Maximum allowable voltage on a ground circuit is .050 volts (that is, 50 millivolts). Pin “C” (white wire with a black stripe) is the power feed for the sensor. You should measure pretty close to battery voltage at this pin. If the cam sensor is not receiving the correct voltage on pin “A” or “C”, look for an open or short to ground on those two circuits. Do not overlook a bad connection at the module caused by excessive probing from previous testing. If the wiring checks OK, then replace the ignition module. Next, back probe pin “F” (black wire) of the ignition module connector while it is still connected to the module. You should measure approximately 5 volts at that terminal. With the cam sensor still disconnected, using a jumper wire, repeatedly touch pin “A” (circuit 633) of the cam sensor harness connector to ground while monitoring the voltage on pin “F” of the ignition module. Each time you ground circuit 633, the voltage at pin “F” should read 0 volts (or very close to 0). If your results are correct, then suspect a faulty cam sensor. If the voltage stays high, check the connector at the module. If the connection is good, then the module needs to be replaced. If the voltage is always low, then look for a short to ground between the ECM and the ignition module. If everything checks out OK, and you cannot get a cam sensor pulse, do not overlook the possibility of a weak or damaged cam gear magnet. Early 3.8l VIN 3 engines used an aluminum cam gear with nylon teeth. Sometimes, replacing this gear with an iron or steel gear can cause a code 41 and a no start, particularly if the original magnet was used. It is good insurance to replace the magnet when you replace the gear. The 3800 VIN C engine uses a thin steel gear with a very strong magnet that is in a plastic holder. This magnet is inserted from behind the cam gear. If the magnet is installed in reverse, it could hit the cam sensor. Even if it does not hit the sensor, the polarity of the magnet will be reversed, and that will also cause the loss of the cam signal and set a code 41.