You don't have to tear the entire wiring harness apart. You already measured 5.0 volts, (4.99 volts), at some point along the yellow wire. Everything up to that point has to be okay. Continue working your way toward the sensor to find the first point that 5.0 volts is no longer there. That is when you crossed over the break.
To add to the story of the dash gauge, this drawing is a representation of six of the dozens of computer modules on your car. There is a pair of "data buss" wires connecting all of them. On some car models the data buss is a single wire. When it's a pair of wires, they'll always be twisted around each other, called a "twisted pair". Stray magnetic interference would induce voltages that would confuse the computers. By twisting the wires, that interference affects both wires equally, and the voltages induced cancel each other out.
Some models today use up to three separate data busses. There is so much information being passed along them that it would take too long to share critical or time-sensitive data. An example would be for the air bag system. The deployment of the air bag is timed to milliseconds, and there isn't time to waste until it's time for that computer to send and receive information.
In this story, the Engine Computer knows all the operating parameters of the engine, including load, rpm, fuel volume, throttle direction and rate of change, along with all the data from its sensors. It controls the cruise control servo, so it needs to know when the brake pedal is pressed, when steering wheel switches are pressed, and it needs to know road speed.
The Transmission Computer needs to know engine load and speed, and coolant temperature. It already knows road speed from its dedicated vehicle speed sensor.
All of these computers know a lot of data by themselves, then, rather than having duplicate sensors for each computer, they get some of their needed information from the other computers.
Each computer takes turns broadcasting its information out onto the data buss, then it can be seen by all the other computers. Starting with the Engine Computer, when it is its turn, it sends out sensor data and all engine operating conditions such as rpm, charging system voltage, and which relays and solenoids it has activated. Among that data is coolant temperature.
The Transmission Computer, along with all the others, looks at that data and picks out what it needs and ignores the rest. It needs to see throttle position and engine load along with the road speed it already knows, to calculate shift points. It needs to know coolant temperature to know when it is okay to engage the torque converter's lock-up clutch. It ignores information related to the knock sensor, ignition timing advance, and the number of milliseconds the injectors are being pulsed open.
A few milliseconds later, the Transmission Computer gets its turn at sending out data. It sends road speed, which gear it's in, whether torque converter lock-up has occurred, and data related to the amount of wear on the clutch plates.
Much of that data is ignored, but another computer can be added to the data buss. That is the scanner we use to read this data and to read diagnostic fault codes.
The Body Computer also is able to read all the information on the data buss, but it too just picks out what is needs and ignores the rest. It has its own sensors too to get information that it shares on the data buss. Fuel level is one of those. In this story, the Body Computer picks out the coolant temperature reading from the message it saw from the Engine Computer. It analyzes that digital signal, interprets it, then translates it into a different digital signal. When its turn comes to broadcast its information onto the data buss, most of it is ignored by the other computers, except for the instrument cluster. That is the only one capable of understanding the digital signal related to coolant temperature. Logic would dictate the instrument cluster could take that data right from the Engine Computer's message, but to do that for many dozens of gauges, warning lights, and other functions would make the cluster more complicated than it already is. Since that data is already being processed by the Body Computer, it is sent to the cluster, but in a modified form that will be ignored by the other computers, otherwise they would be getting two different coolant temperature messages from two different sources. The transfer of data from the Body Computer to the instrument cluster is not time-sensitive, meaning it doesn't matter if it shows up right now, or a fraction of a second from now. Compared to engine performance, or the critical timing of air bags and anti-lock brakes, Body Computer data can be sent on a low-level data buss of its own, or it can just wait until its chance to broadcast comes along on the higher-level data buss.
Once a packet of data is transmitted onto the data buss by the Body Computer, the instrument cluster takes that and analyzes it, then turns it into the voltages that run the gauges and warning lights. This is where the coolant temperature gauge responds to what the Engine Computer is seeing from the coolant temperature sensor.
All of this data sharing became more and more involved over the years. On older cars up to around the mid to late '90s, the instrument cluster wasn't such an unnecessarily-complicated computer module. That was when, instead of interpreting digital signals, they just used their own, very reliable circuits and sensors to run the gauges. That was when there was a second coolant temperature sensor just for the dash gauge, and it always had just one wire connected to it. Those gauges were never meant to be particularly accurate. Their main purpose was for the driver to notice when something was out-of-the-ordinary. One coolant temperature gauge could run at half-scale, and another one could run at 3/8, and both engines could be running perfectly.
Coolant temperature sensors for Engine Computers have to be very accurate, but with all the computer circuitry involved before the gauge's pointer is placed to the desired position, there are just too many variables to expect the gauge reading to be right on the money. The only way to know the exact coolant temperature, or any other sensor reading, is with the scanner. That takes the information from the data buss exactly as it was transmitted by the computer, and interprets that so we can read it.
Monday, January 20th, 2020 AT 3:54 PM