Rick's Free Auto Repair Advice

No lights, power windows, heat, or A/C

Why don’t my lights or power windows or heat work?

In the old days of auto repair all electrical problems could be solved with a new switch, wire, socket, or solder. Now we’re in a digital age and car makers have opted to switch electrical loads via digital communications. So you wind up with no lights, no power windows, no heat, or no A/C, you should certainly check the fuses, bulbs, and switches. Then you can check the relay in the fuse box. But if that doesn’t solve the problem, you may have a digital problem. Every car maker uses a different setup and they even use different systems between models of the same brand. But they all have some things in common. There are actuators (the things we used to call switches) and there’s at least one data line. Some vehicles contain several data lines, each dedicated to a different kind of digital traffic. And, the digital signals all end up at a switching point. That’s where relays take over the job of turning power on and off.

I’m going to give you a brief description of one system used in late model Chrysler minivans. Follow along with the wiring diagram. Just click on the illustration it pull up the PDF.

wiring diagram, door locks, power window, A/C, blower motor, brake lights

Click here to download a PDF

Every action starts with an actuator and ends at the Totally Intelligent Power Module (TIPM). Chrysler called it the intelligent power module (IPM) in earlier years. Think of the TIPM like a PBX system from the old days. The TIPM receives a digital command to turn on the headlights and it activates the headlight relay.

What goes wrong? Well, car makers have never been good about protecting the wiring harnesses they install in vehicles. Every day a car maker issues a new technical service bulletin telling dealers where the latest “rub through” is occurring in a harness. Now think of those same wiring harnesses carrying low voltage digital signals. Yeah, it’s enough to scare you. Then imagine a malfunction in module that’s supposed to send low voltage digital data down the bus. After the malfunction it starts sending 12-volts down the line. All of the sudden, none of the other modules can sense the low voltage digital data and everything gets screwed up. It happens more often than you think.

How can a DIY’er fix this? Well, the car makers did one smart thing. When the system malfunctions, it sets a trouble code. That’s the good news. The bad news is that the code is a “U” code. I’m not aware of a single DIY priced scan tool that can read a “U” code. Worse than that, “U” codes don’t turn on your check engine light. So the only option you have is to either pull fuses one at a time to each node module until the bus “comes back up.” Or, you can physically disconnect each node module from the bus. Of course, you’ll need a high quality wiring diagram and a really good shop manual to locate the right fuses and the find where the modules are hidden.

Now for the technical stuff. If you’re a techie, you’ll love it. If not, skip it.

The LIN Local Interconnect Network is used to move non-time-critical data between actuators and a higher speed bus. It’s known as a “body function network.” For example, air conditioning systems, doors, seats, column, climate control, switch panel, intelligent wipers, and sunroof—none of these need a high speed response. That’s where LIN comes into play. It’s perfect for automotive use because it operates on 12-volts DC and is inexpensive to implement. The maximum bus speed is 20K and the maximum cable length is 40 meters. Remember those old dial-up 56K modems? Well, this is less than half that speed. LIN is designed to use MASTER and SLAVE nodes. Each MASTER can have multiple SLAVES. Only one message is allowed on the bus at one time. So if you turn on the wipers and try to open the sunroof at the exact same time, the bus will only allow the first message to get through to the MASTER.
The MASTER nodes connect to the higher speed Controller Area Network (CAN) bus. The CAN network runs on twisted pair, shielded cable, or ribbon cable. There are three variants of the CAN network. BASIC CAN runs at a maximum of 250k. FULL CAN (also called extended frame CAN) runs at speeds up to 1Mbps (close to the speed of a slow DSL connection). The third CAN-FD is designed to run at up to 15Mbps. BASIC CAN has a minimum speed of 10k.
CAN is designed to proved message crash avoidance if two messages are sent at the same time. Rather than re-transmit the crashed message, as is done in an ethernet setup, the message frames in CAN contain a header (message identifier), basically a label telling the node how important the message is. The message with the higher priority label will get through first. So, if you’re trying to accelerate and slam on the brakes at the same time, chances are the ABS messages will get priority. Auto makers can use a slow version of the CAN network the same functions as the LIN network above, but CAN costs more. So they use CAN for high-speed data requirements like engine management, anti-lock brake systems, and stability control. CAN networks also run on lower voltages with a high of 4.75-volts and a low of 0.5 volts.

© 2012 Rick Muscoplat

Posted on by Rick Muscoplat


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