Back to Forum
Post a Response
    BMW/728/1998/E38/ Speedo dead Archived Message

    Posted by George Georgiou on March 28, 2008, 2:22 am

    Main problem is Speedo not working . The dashboard was sent off for testing by BBA, came back no fault found.

    Launch shows rear left abs sensor signal fault. Unable to clear. Looks like abs sensor changed recently. New bmw sensor fitted but no change. All other sensors read speed ok on test drive but rear left continues to show zero speed.

    Have been informed that speedo works by using abs signal from rear left abs sensor, so this looks like reason why speedo not working.

    I was asked to come in and scope ABS sensors for signal. Tested all sensors for resistance. Measured directly into the two rear sensors across the two pins and also checked the 'old'replaced sensor, but all showing 11.5 Mega Ohms approx . Surely this cannot be right!!.

    I removed rear sensors and looped wiring at each sensor end and measured both rear wiring looms back to abs connector (abs connector removed from abs unit) , both looms showed 1 ohm approximately, so rear looms are 100% OK and I am definitely getting stupidly high ohms when measuring with sensors connected back at abs connector (connector removed from abs unit), same readings i.e. 11.5 Mega Ohms on all four sensors, front and rear.

    I cannot find any signal at all when rear wheels jacked up and vehicle put into drive to simulate rotation, either directly on sensors at rear or at the other end on abs connector with sensors connected. I have scoped each pin independently with respect to earth and also across each pair of wires , absolutely no signal just random noise. Yet we know that 3 out of 4 sensors show a reading on scanner when test driven!!!

    What am I doing wrong? Any ideas would be helpful. I have scoped sensor signals and measured resistance of about 1000/2000 ohms on other abs systems with 2 wire sensors. These must be inductive sensors because only use 2 wires, so how can they work if resistance is so high?

    Thanks

    George

    Message Thread:


Copyright © uk autotalk