How the Tach and Speedometer works
Thread Starter
Drifting
Joined: Sep 2007
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How the Tach and Speedometer works
I am wondering if anyone knows how the speedometer and tach works on the c5. I know the vss wires sends 4000 pulses/mile but at what voltage? And from what I read it is a sin wave and over 30000 pulses/mile coming out of the sensor, but once it is sent out of the comptuer to the speedo is it a square wave? Does the tach work the same way? Like 1000 pulses per minute = 1000 rpm?
Tech Contributor
Joined: Jan 2007
Posts: 19,450
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From: Dyer, IN
I am wondering if anyone knows how the speedometer and tach works on the c5. I know the vss wires sends 4000 pulses/mile but at what voltage? And from what I read it is a sin wave and over 30000 pulses/mile coming out of the sensor, but once it is sent out of the comptuer to the speedo is it a square wave? Does the tach work the same way? Like 1000 pulses per minute = 1000 rpm?
Tech Contributor
Joined: Oct 1999
Posts: 41,055
Likes: 9,817
From: Charlotte, NC (formerly Endicott, NY)
Here is what the Service Manual says about the operation of the IPC:
Speedometer
The IPC displays the vehicle speed based on the information from the vehicle speed sensor. The PCM converts the data from the vehicle speed sensor to a 4000 pulses/mile signal. The IPC uses the vehicle speed signal circuit (4000 pulses/mile) from the PCM in order to calculate the vehicle speed. The speedometer defaults to 0 km/h (0 mph) if a malfunction in the vehicle speed signal circuit (4000 pulses/mile) exists. The speedometer displays either miles or kilometers as requested by the activation of the English/metric button on the driver information center (DIC). The associated indicator (MPH or KM/H) illuminates.
Based on this description I would have to say the signal to the speedometer from the PCM is pulsed and not a sine wave.
Tachometer
The IPC displays the engine speed as determined by the PCM. The IPC receives a class 2 message from the PCM indicating the engine speed. The tachometer will default to 0 RPM if the PCM detects a malfunction in the engine speed sensor circuit.
Since this is a class 2 message it probably is more complex (a framed digital word) than the pulsed speed signal.
Bill
Speedometer
The IPC displays the vehicle speed based on the information from the vehicle speed sensor. The PCM converts the data from the vehicle speed sensor to a 4000 pulses/mile signal. The IPC uses the vehicle speed signal circuit (4000 pulses/mile) from the PCM in order to calculate the vehicle speed. The speedometer defaults to 0 km/h (0 mph) if a malfunction in the vehicle speed signal circuit (4000 pulses/mile) exists. The speedometer displays either miles or kilometers as requested by the activation of the English/metric button on the driver information center (DIC). The associated indicator (MPH or KM/H) illuminates.
Based on this description I would have to say the signal to the speedometer from the PCM is pulsed and not a sine wave.
Tachometer
The IPC displays the engine speed as determined by the PCM. The IPC receives a class 2 message from the PCM indicating the engine speed. The tachometer will default to 0 RPM if the PCM detects a malfunction in the engine speed sensor circuit.
Since this is a class 2 message it probably is more complex (a framed digital word) than the pulsed speed signal.
Bill
Thread Starter
Drifting
Joined: Sep 2007
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Thanks for the info. So the tach is a little different. I was looking at how to hook up an aftermarket tach and it looks like you just hook the signal wire up to the Red connector, pin 10, white wire @ PCM. Does this wire send out pulses or can aftermarket tachs read the class 2 message?
Race Director
Joined: Oct 2002
Posts: 16,477
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From: Gone
CI 6-7-8-9-10 Veteran
St. Jude Donor '03
The white tach wire is a pulse...but I believe you need to set to the (aftermarket) tach to the 4cyl setting to read correctly.
At least that's how it works when connecting shift lights.
At least that's how it works when connecting shift lights.
Tech Contributor
Joined: Oct 1999
Posts: 41,055
Likes: 9,817
From: Charlotte, NC (formerly Endicott, NY)
I did a little more checking in earlier versions of the Service Manual. The IPC gets two engine speed signals. One is a pulsed signal like the speedometer circuit and the other is through the Serial Data Bus. The Serial Data Bus also uses pulses but the pulses are a digital code Vs a simple pulse stream. You wouldn't be able to use this signal.
If you connect to the pin on the PCM that you mentioned you should pick up the pulse stream which a tachometer may be able to read. Not sure you will be able to get around the tach delay the stock tachometer has. That delay could be in the tach circuit or in the pulse stream from the PCM itself.
Bill
If you connect to the pin on the PCM that you mentioned you should pick up the pulse stream which a tachometer may be able to read. Not sure you will be able to get around the tach delay the stock tachometer has. That delay could be in the tach circuit or in the pulse stream from the PCM itself.
Bill
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