Why measure driveshaft speed?
Engine RPM tells you what the motor is doing. Driveshaft RPM tells you what's actually reaching the rear end. Comparing the two, or tracking how fast driveshaft speed rises, shows wheelspin, converter or clutch slip and how hard the car is accelerating. On an Elite ECU, driveshaft RPM can be used to adjust fuel, ignition timing and boost control, among other things. It's also essential if you run driveshaft RPM control through a Haltech REM, where the ECU manages power against a driveshaft speed target.
The hardware
The collar
Magnets are carried in a split collar that clamps around the round section of the tail-shaft yoke. The sensor counts them as they pass. To pick a collar, measure the outside diameter of that round section. The two sizes we see most often are 1.875 inches and 2.125 inches, and we also stock other sizes:
- 1.812 inch (46 mm) split collar, 8 magnets
- 1.875 inch (47.63 mm) split collar, 8 magnets
- 2.125 inch (53.98 mm) split collar, 8 magnets
- 2.187 inch (55.55 mm) split collar, 8 magnets
How many magnets?
Two magnets is the minimum we'd use. That's enough for a basic speed reading, but the ECU only gets two updates per turn, so the signal is coarse at low speed. If you're running a driveshaft control strategy, 8 to 12 magnets gives much better resolution, and the control reacts to changes sooner and more smoothly.
Mounting the sensor
The sensor bracket usually bolts on using one of the pinion support bolts. Many aftermarket differential carriers and pinion supports have a threaded provision so the sensor screws straight in. Either way, it has to be rigid. A bracket that vibrates or flexes will give you a noisy signal exactly when you need it most. Set the gap to the magnets per the sensor maker's instructions and check it with the collar rotated through a full turn.
Setting it up in ESP
The function has four tabs. Here's what each setting does and how we set it.
| Tab and setting | What it does | Our advice |
|---|---|---|
| Wiring: input | The input the sensor is connected to | It must be an SPI (synchronized pulsed input). Plan for one when you lay out the harness |
| Wiring: Edge Select | Which edge of the signal is counted | Falling if the input is pulled to ground when a magnet passes, Rising if it's driven high |
| Wiring: Sensor Type | How the input reads the signal | Hall Effect |
| Wiring: Pullup | Supplies a voltage the sensor can pull down | Enable it if the sensor has an open-collector output and there's no external pullup |
| Display: Warning Maximum | Driveshaft RPM above this makes related gauges in ESP flash red | Set it just above the highest driveshaft speed you expect at the finish line |
| Calibration: Number of Teeth | Pulses per driveshaft revolution | Equal to the magnets in your collar, typically 2, 8 or 12 |
| Diagnostics: Operating Min | Raises DTC C1049 when driveshaft RPM drops below your Minimum | Disabled by default; use it if you want to flag a lost signal |
| Diagnostics: Operating Max | Raises DTC C1050 when driveshaft RPM exceeds your Maximum | Disabled by default; a good overspeed check |
If engine protection is active, you can assign a DTC severity level to either diagnostic so the ECU responds automatically rather than just logging a code.
Checking your calibration
Once it's wired and set, compare driveshaft RPM against something you know. A simple check is to drive at a steady speed in top gear with no slip: driveshaft RPM divided by the final drive ratio should match wheel RPM, and divided into engine RPM it should give your gear ratio (with lockup engaged on an automatic). If it reads a fixed multiple out, such as double or a quarter, the Number of Teeth setting doesn't match the magnet count.
Troubleshooting
| Symptom | Usually |
|---|---|
| Reads zero all the time | Wrong input, pullup needed but disabled, sensor gap too large or magnets facing the wrong way |
| Reading is a fixed multiple of the real value | Number of Teeth doesn't match the collar |
| Spiky or jumpy at speed | Bracket flex, a loose collar, or the signal wire routed near ignition or alternator wiring |
| Coarse, steppy reading at low speed | Too few magnets for the job; step up to an 8 or 12 magnet collar |
Driveshaft RPM FAQ
Is driveshaft RPM the same as vehicle speed?
They're related, but driveshaft RPM is measured before the rear end and tires, so it doesn't depend on tire size or ride height. If you want road speed as well, see vehicle speed sensors.
Can I use the same idea on the input side of the transmission?
Yes. A transmission input RPM sensor measures the clutch or converter side and helps spot slip.
Can you help set up driveshaft control?
Absolutely. It's one of the more involved strategies to get right, so book a free consultation and we'll talk through your combination.
