I bought a new battery that ran flat after two weeks. Took it back to the garage and they disconnected negative from my battery whilst the engine was running to check my alternator but now I have brake light and ABS warning lights on the dash.

When I start the car they disappear as normal but reappear if I rev the car above about 1100 RPM.

He said the car was still fine to drive and they would disappear in a few miles but that has not happened and I have since read that a combination of these lights is an emergency and the car should not be driven.

Looking for potential cause of the fault.

I drive a 1999 Toyota Corolla.

  • Disconnecting the batt neg is th4e worst way of testing an alternator... This can cause massive spikes in the system and damage some or many electronic items including sometimes the alternator. Some, who frequent this forum, will chime in with "I have always tested alternators like that"... Well, sadly you have to get this checked and find out what the cause is.
    – Solar Mike
    Commented May 16, 2019 at 16:13
  • Could this be as simple as a blown fuse or are there computers/components regulating the brakes on a car of this age? I'm trying to figure out how much electrical equipment my brakes rely on or if the car is still safe to drive to different mechanics. Commented May 16, 2019 at 16:26
  • If it has ABS then it has an ecu at a minimum. Have you checked the fuses? But the behavior you describe is not likely to be a fuse - the lights come on to indicate a fault... This is more serious than just a fuse...
    – Solar Mike
    Commented May 16, 2019 at 16:29
  • I have not checked the fuses as I am unsure how to check the 50 amp ABS fuse it is encased in a large (compared to other fuses) plastic housing. I have done a visual inspection and it looks like the internal copper wire is still in tact. Commented May 16, 2019 at 16:37
  • Underhood there is a diagnostic connector, the underside of the cover is labeled. Use a fused jumper wire and jump the Tc and E1 pins then key on engine off watch the ABS lamp it should blink some numbers at you. If it flashes 2 time per second then there's no fault currently. To check the fuse you could disconnect the ABS connector and check for power on the first two terminals.
    – Ben
    Commented May 16, 2019 at 21:07

3 Answers 3


The same happened to me. Turned out to be a bad alternator. My alternator was generating 19v. Abs module is a complex system including a motor. When it sense overvolts, it shuts it down itself tl prevent further damage.


Just wanted to update what the problem was.

The day after I asked this question my battery was flat again. I bought a multi meter and there was a 5 amp drain on the battery when the car was off and locked. I traced the parasitic drain to the alternator itself. Also when the car was started it was charging the battery at 16v as aposed to the 14.5v it should have been, this is what was causing the ABS to malfunction.

I bought and fitted a reconditioned alternator which has rectified the issues with the battery and the ABS.


The shop most likely fried the ABS computer by disconnecting the battery while the car was running. Take it back and make them fix the problem. See this post on what happens when you disconnect battery while car is running.

  • Fried the alternator itself but was their fault. Commented May 18, 2019 at 14:24

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