Trip/Speedo/Odometer

TxDoc · July 9, 2002

A thread on another site made me wonder how these do work. The speedometer and odometer have no solid connection like a cable and gears? Correct? What about the trip odometer? If you go to bigger tires, are they all off?

What about the trip odometer. It seems to click and reset some kind of mechanical gears and is not an LCD display (on the pre 99 trucks-tell me if the newest are different).

I have seen SkyJacker, or one lift company selling a device to make your speedometer work right with larger tires. I am assuming this is an electronic device and not a "gear" like on old speedo cables.

Basically, what is mechanical and what is electronic, now?

GnatGoSplat · July 10, 2002

The 99+ trucks have fully electronic speedos and odometers. The odo/trip odo are digital, so they are 100% electronic. The odometer is eletro-mechanical. It has a mechanical dial consisting of electromagnets and fixed magnets. It's exactly the same as a voltmeter. The speed signal is picked up by the VSS (Vehicle Speed Sensor). It consists of hall-effect sensors and a moving reluctor. This speed generates a small AC signal that the PCM picks up. The PCM translates this signal to digital pulses. For most GM cars and trucks, there are 4000 pulses per mile. This 4000ppm signal is sent to the instrument cluster where the circuitry translates the number of pulses into a voltage. The voltage then drives the speedometer's mechanical meter. The odometer takes this signal directly and computes the miles accumulated.

In the case of a truck with an electro-mechanical odometer, the circuitry in the cluster varies the speed of a small electric motor which drives the gear-driven odometer movement.

Yes, to make the speedometer work right with larger tires would require an electronic solution!

Sorry for the length. Hope I made sense.

DaveZee · November 26, 2004

The 99+ trucks have fully electronic speedos and odometers. The odo/trip odo are digital, so they are 100% electronic. The odometer is eletro-mechanical. It has a mechanical dial consisting of electromagnets and fixed magnets. It's exactly the same as a voltmeter. The speed signal is picked up by the VSS (Vehicle Speed Sensor). It consists of hall-effect sensors and a moving reluctor. This speed generates a small AC signal that the PCM picks up. The PCM translates this signal to digital pulses. For most GM cars and trucks, there are 4000 pulses per mile. This 4000ppm signal is sent to the instrument cluster where the circuitry translates the number of pulses into a voltage. The voltage then drives the speedometer's mechanical meter. The odometer takes this signal directly and computes the miles accumulated.

In the case of a truck with an electro-mechanical odometer, the circuitry in the cluster varies the speed of a small electric motor which drives the gear-driven odometer movement.

Yes, to make the speedometer work right with larger tires would require an electronic solution!

Sorry for the length. Hope I made sense.

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DaveZee · November 26, 2004

I have a 2002 Duramax Diesel. Recently the odometer and gear select have been out and I am unable to view the mileage and gear indicator.

I tried to tap on the plastic shroud and it would cause the odometer and gear selector to appear.. However, this would last for a short time, and then it goes out.

I checked the fuse, and replaced it, but still have the same problem.

Any suggestions on troubleshooting the electronic odometer/gear indicator?

I would like to trace the wiring from the fuse to the back of the instrument panel. Should I remove the dashboard and trace the wiring to see if it is loose in connection to the instrument panel?

Can you give instructions on removing the dashboard?

Thanks for any help you can give.....

My truck is out of warranty, although I spoke to service about this before it ran out. At the time, the lights were on when I brought the truck in. They found nothing wrong.

Thanks very much!

Dave

budhayes3 · November 26, 2004

Awesome explanation GnatGoSplat!

mcreyaufmiller · November 26, 2004

So let me get this straight, the dohickeys send a jolt to the thingiemabob, after that the thingiemabob told the widget to turn and make the little needle move. Am I right?

Just want to make sure I understand.

Mike