Computer Controlled Cranking Circuits: Part 8

Final Discussion

In Part 7 we digressed a little to discuss an introduction to digital logic circuits to get an idea of what goes on with logic circuits inside control units because they are becoming a major factor in vehicle electronic systems. On-board computers containing diodes, transistors and integrated circuits that create the logic circuits used everywhere on today’s vehicles. Digital circuits introduce their own set of issues such as the most difficult to diagnose, specifically intermittent problems. One minute a circuit works then the next minute it stops working. That could be another topic of discussion for later.

Fig. 8-1

Now back to the big picture. In Figure 8-1 we see another control unit added to the cranking circuit which we call the IPM (Integrated Power Module). It contains many logic circuits, transistors, diodes and integrated circuits that all have to work together to perform its many functions. Most manufacturers do not supply a schematic or block diagram of the internal circuits of the IPM. We have to deduce what is going on inside the IPM by seeing how it interfaces with the cranking circuit.

Check Computer B+ and B- Pins.

To insure the IPM has what it needs to function, verify that all IPM B+ pins have good B+ voltage. The voltage should be very close to battery voltage when the vehicle is cranking. Normal cranking voltage should be in the range of 10.00-11.00 volts. If the voltage on one of these B+ pins drops by a volt or more from the other pins suspect a defective wire or corroded connection in that wire.

Verify that all IPM ground pins connected to the IPM have a good ground voltage reading of 0.050 volt because there are logic circuits inside the IPM and they may fail to function normally if the ground circuit voltage drop is higher than 0.050V. If a ground circuit is corroded and begins to develop even a small voltage drop, it can adversely affect logic circuit operation inside a control unit. I have seen situations where a shop replaced an on-board control unit, they though were bad only to find the new control unit works the same as the original because of a poor ground connection.

As a general rule computer ground circuits should have a voltage drop no higher than 0.050 volt (50 mV), while general electrical circuit grounds can be from 0.02 (200 mV) – 0.10 volt (100 mV). To ensure that ground voltage drop readings are accurate it is necessary to ground the DMM to the -BATT terminal which is defined as the negative terminal connected to ground. If battery terminal access is difficult to access the metal case of the generator can substitute as DMM ground.

It is extremely important to take these B+ and B- voltage measurements on the IPM while the vehicle is cranking because that is the only time when each pin on the IPM is working the hardest, that is, passing the most electron current. A voltage drop may not show up if a circuit is not working hard because max circuit electron current is not flowing.

The IPM and Cranking Circuit

Notice how the IPM connects to the cranking circuit. IPM Pin 22 provides B+ to Starter Relay Pin 86 while cranking. After the engine is running, the B+ voltage to relay Pin 86 drops to zero volt. This indicates there is a solid-state switch (transistor driver or solid-state relay) controlling the voltage on IPM Pin 22.

This is a safety feature to prevent the starter relay from accidentally energizing during engine run.

Recently a vehicle manufacturer had a recall notice sent out. The problem occurred during normal driving when the starter motor could be engaged while driving. Go back and look in Part 7, at Figure 7-2 and notice that relay Pin 86 is hard wired (hot-at-all-times) to B+. All the starter relay needs to be activated and engage the starter solenoid, in this vehicle, is to have a ground at relay pin 85. The wire from relay pin 85 is connected to PCM Pin 8. If this wire were to become shorted to ground while driving, due to the wire harness rubbing against the chassis, the relay would engage the starter solenoid because relay Pin 86 has B+ all the time. The recall was to re-route the wire harness so as to not rub against the chassis. The safety solution is to program the IPM to supply B+ to pin 86 while cranking then remove the B+ voltage to pin 86 once the engine is running. Now if the wire between relay pin 85 and PCM Pin 8 were to be grounded, the starter relay could not be energized and engage the starter solenoid because pin 86 is zero volt.

IPM Pin 15 provides B+ to Relay contacts Pin 30 during cranking. The electronic circuits inside the IPM that does this remains a mystery. The B+ supply to relay Pin 30 could be a direct straight through circuit connection inside the IPM or a solid-state driver is involved to switch the voltage from 0V to B+ for cranking. We won’t know unless the manufacturer provides a schematic or an explanation. A simple voltage measurement of B+ at IPM Pin 22 and Pin 15 during cranking confirms the IPM is functioning during cranking for these functions.

Primary B+ is applied to IPM at Pins 36 and 28. Diode D4, inside the IPM, is for polarity protection to all IPM circuits getting (hot-at-all-times) B+ from Pin 36. Primary B+ is also supplied to IPM Pin 28 when the ignition switch is CLOSED. Diode D6, inside the IPM, is for polarity protection to all IPM circuits getting B+ from Pin 28. Diode D6 and D4 would probably not be shown on the manufacturer’s schematic diagram but are shown here to remind the reader that most control units are protected against reverse polarity which would occur should someone connect jumper cables to the battery in reverse polarity.

Without polarity protection diodes, onboard control units would instantly be destroyed by incorrect use of jumper cables during jump starting. Notice diode D1 in the PCM is also a polarity sensing diode. Many computers were damaged in the early days due to incorrectly connecting (reversing) jumper cables to a vehicle with a dead battery. But it would be wise to remember that any time you connect jumper cables between a charged battery and a dead battery in a vehicle that won’t crank, you are careful to follow battery polarity. Always connect both negative terminals together first. This will remove any static charge that is higher in one vehicle than in the other vehicle. Then follow industry standard jumpstart procedures recommended by the vehicle manufacturers.

The CAN Bus network is shown at the bottom of Figure 8-01 and connections to the PCM and IPM with pin numbers selected for purposes of this illustration (not actual pin numbers). The PCM and IPM would be referred to as “Nodes” in CAN Bus terminology. Nodes can both transmit information and receive information from the CAN Bus network. That is how the PCM and the IPM communicate with each other with programming that causes them to function at various times depending on the vehicle’s situation. This is a separate topic and cannot be explored in this discussion.

The reason I even mention it here is because today’s automotive and truck technology contains a lot of electronics and digital technology that was unheard of even 10 years ago. The challenge for the automotive or truck technician is a constant struggle to keep up with advancing technology. With each new model year, advances in technology change and require learning new concepts and techniques. As a result, it has been my observation that many technicians become so focused on keeping up with the latest innovations in technology that they forget some of the most important factors that should be considered in any vehicle today, regardless of the advances in technology found on a particular vehicle. My case in point is illustrated in Figure 8-2. Are we overlooking something?

What Are We Overlooking?

See Figure 8-2 below.

Fig. 8-2

In Figure 8-2 a generator (alternator) has been added to the illustration. Of course, it has been there all the time in our discussion but it was ignored (left out of schematic) as we focused on current electronics technology concerning the PCM and IPM. We shouldn't’t ignore the generator because it provides the electrical power to operate all electrical and electronic circuits on the vehicle. The generator is the heart and soul of a vehicle’s operation. The electrical energy produced by a generator is referred to as “charging voltage” because it is the generator’s voltage that is critical. The generator must produce a specified charging voltage under all modes of vehicle operation in all conditions the vehicle is operated.

As the generator performs, electron current produced by the generator does not have to be measured. It’s the charging voltage that tells the story of a generator’s operating condition, not the generator electron current. Measure the charging voltage. Here’s why.

A generator experiences maximum vehicle electrical-electronic system load when all vehicle circuits are turned ON. In a worst-case driving scenario (summer/winter extremes, night time, max heat or AC, max lighting, low engine rpm, etc.) charging voltage decreases slightly due to a heavy electrical load. But if the charging voltage drops too much it is a problem. Most technicians are unaware of correct charging voltage values in various modes of vehicle operation. Electronic circuits are designed to function best at specified voltages. Fig. 8-3 illustrates measuring the charging voltage.

Fig. 8-3

Measure the charging voltage as shown in Figure 8-3 or across both battery terminals. It is the charging voltage that supplies operating voltage to all electrical and electronic circuits on a vehicle during engine RUN. Good vehicle performance requires a proper charging voltage.

Normal range of charging voltage:

The charging voltage increases in cold weather and decreases in hot weather. Why? This is necessary to meet battery recharging requirements. In cold weather batteries don’t recharge easily. In hot weather batteries recharge very quickly. The charging voltage has to change with ambient temperature to prevent undercharging in cold weather and overcharging in warm weather. As a general rule, vehicle charging voltage in cold weather for a 12 V system will range from 14.9V to 15.1V. In warm weather the charging voltage will be a lower ranging from 13.0V to 14.0V.

Effects of lower charging voltage:

Lamps are dim when charging voltage is low because electron current from the voltage source (generator) is low. Electronic circuits are designed to function best at specified voltages and low charging voltage affects performance of onboard control units. Electronic circuits in vehicles are designed to function normally at 13.0V. below 13.0 V some degrading of electronic circuit performance becomes apparent. In fact, some passenger cars will not control fuel injectors if the battery voltage drops to 9.1 V while cranking. Any electronic circuit controlling solenoids will begin to experience difficulties activating the solenoid on a timely basis which can contribute to transmission late shift problems.

Effects of higher charging voltage:

Lamps are extra bright when charging voltage is high because the higher voltage pushes more electron current through the lamps which contributes to premature lamp failure. Electronic circuits begin to experience higher than normal electron current which increases circuit heat causing premature solid-state component failure. The solder connecting electronic components to the circuit board can become so soft that the components experience an intermittent connection when the circuit is turned off and the circuit board cools down again. At other times, components can fall off the circuit board and accumulate inside the case of the onboard control unit which becomes a throwaway unit. Higher than normal charging voltage also boils water in the electrolyte which evaporates and must be replaced with distilled water to maintain battery life.

A technician should monitor the electrolyte level in each cell. Keep electrolyte level to the bottom of the vent well by adding distilled water as necessary. Then measure the charging voltage under maximum load and minimum load to ensure the changes in charging voltage are within acceptable limits. Veejer Enterprises Inc. has developed an electrical flip chart that allows a technician to quickly evaluate the minimum and maximum values of charging voltage during critical times of vehicle operation and the generator charging voltage test procedure can be accomplished in 60 seconds.

This charging voltage test is incorporated into an electrical flipchart called FIRST THINGS FIRST-Pro for a single battery 14 V vehicle and a flip chart for a dual battery 14 V electrical system call FIRST THINGS FIRST-2. Also, our book VEHICLE ELECTRICAL TROUBLESHOOTING SHORTCUTS explains generator operation and charging system testing in detail in Section 6.

The more you know about measuring the charging voltage and the precise readings obtained when testing the vehicle and various modes of operation the more reliable the vehicle becomes and the longer the battery or batteries will last in the vehicle.

If you would like more information about multiple battery systems for 12 V and 24 V battery packs there is a publication available from Veejer Enterprises that explains multiple battery packs and testing procedures. It is called “12-24 Multiple Battery Systems." BUY NOW

This concludes our discussion of computer controlled cranking circuits but rest assured we have considerable training information available that expands understanding of these concepts.

Isn’t it time you became the "go to electrical tech" in your shop?

Continue Reading...


For Technicians

Learn about our technician training programs in electrical systems troubleshooting and electronics repair.

Learn More ›

For Shop Owners

Our training programs can train your technicians to be more proficient in electrical repair - saving labor hours, down time and fewer good parts replaced.

Learn More ›

For Teachers

We have the electrical-electronics curriculum that your students will need when they enter the job market. Be "THAT" teacher that showed them how!

Learn More ›