If It’s Not One Thing It’s Another

Tracking down the cause of one problem can lead to other issues. A speedometer repair on this BMW evolved into something deeper.

Tools used:

90 amp clean and stable power supply

INPA with laptop, Launch EasyDiag with Android tablet

Bluetooth Oscilloscope and Android tablet

Multi-meter and amp clamp

Imagine this 2001 BMW 740i in very good condition that’s had some past TLC but had other recent shops “have a go at it.” This adventure all starts with a question: “Can you get the speedometer working?”

This is a first time customer that was advised from another customer/friend about; “having the correct tools.”

For the uninitiated technician that’s never seen one of these models, this is an example of a complex and early CAN adapted vehicle.

The approach and customer interaction

The conversation should begin with some background and specifically; who owned it, how and where the vehicle came from. One piece of information came from a paid invoice from a BMW dealer that diagnosed a right rear wheel speed sensor defect. It is assumed the speedometer uses the R/R speed sensor data to operate the speedometer.

Hint: Never assume and keep it simple.

A short road test may be the best way to see speed sensor data with the vehicle owner, while attempting to prove some answers to multiple theories. This model does have a DLC (OBD II) very close to the cup holder mounting of the center console. The customer did note “This is where the other shop plugged in a tool.” DME and EGS data can be accessed at the OBD II connection.

Road test with the customer

Quick and simple is the tool that offers wheel speed data that is current and is visible with screen captures. One choice is a tablet/Android device that offers wheel speed data in real time. With recorded speed at the EGS and no instrument speed, there was a problem that needed some investigation. The images were “stitched together” to offer current EGS data and give the customer the insight of what one controller has access to.

Simply explained for the customer is how the wheel speed data is transferred (shared) via CAN from one (multiple) controller(s) to another. The OBD II connection is not going to offer all of the information needed to solve the primary question.

Hint: A defective ABS system will allow the rear wheels to spin up at road speed on the hoist. An operating ABS system will NOT allow the rear wheels to spin at road speed. Here’s why.

If the vehicle has the rear wheels spinning and the fronts are not, the ABS assumes the vehicle is in a skid or front wheel lock-up.

It’s no different when driving on loose gravel or snow. If any of the wheels are determined as “locked” or “stopped” while rotating and one or more of the other wheels are still in motion, the ABS activates.

This BMW has this behavior on the hoist with the rear

Tags: featured, full-image

Category: Electrical