Please help diagnose an ABS oddity

[JHZR2]

Hi,

My 91 BMW 318i has a perfectly operating ABS system. I try to engage it every winter, and it does as designed. No issues, and the car starts, stops and runs great.

However, after sitting for nearly 10 days while I was away for the holidays, I started it today to move it in the driveway. THis car has all the dummy lights come on as a check when starting... The first start all came on but the ABS warning light.

I shut the car off, and restarted, again the same issue, no ABS light. Finally, I shut it down and restarted it a third time - ABS light lit and went out as per the usual.

I _thought_ I had noted this same behaviour once before, when it had sat for a week or so. I thought it was strange, but when I restarted the car, it was on,m so I couldnt tell for sure - this time I know it happened, and twice.

In day to day operation (it is my commuter car), the dummy light lights right up, no issue, every time. The ABS dummy bulb is only a year old, as I replaced it whan I re-did my instrument cluster, and everything else is fine.

Any ideas? it is a bosch system, and as I said, all systems work 100% in this vehicle, no issues...

Thanks!

JMH

 

[brianl703]

10 days sitting, I bet the first and second starts dropped the system voltage too low for the ABS controller to be happy with it.

Get the battery load-tested and/or keep it on a charger when it's parked.

 

[oily boyd]

X2.

You're the man!

 

[JHZR2]

The battery is new within 2 months - a BMW battery... replaced the old one because it was time, not for any real reason... tested the charging system and verified parasitics at that time, and they were at the correct spec per the factory.

I started the car and immediately shut it off when the ABS light didnt go on. Twice. Only on the third time did it come on... wouldnt the voltage after two cold starts be (infinitessimally) smaller after two cold starts compared to the first???

Thanks for the insight!

JMH

 

[brianl703]

It depends on how much power was needed to start the engine and how much power the alternator put into the battery while the engine was running.

It's very possible that the alternator put enough of a charge into the battery that it was more charged at the 3rd start than it was at the 1st...

I assume the light normally comes on when you turn the key to the ON position before you crank the engine.

If that's true, then the light should always come on when you turn the key to the ON position prior to starting the engine, regardless of battery condition, unless it's REALLY low.

It is my suspicion that the voltage drop while cranking the engine is causing the ABS light to turn off as soon as the engine is cranked in some situations.

 

[JHZR2]

Interesting, now that said, and given that there are only normal, factory level parasitic loads (I forget but very low mA load, there is only a clock and radio presets), is this an indicator that my battery is in bad condition?!?

As I said, its new, and cranks really well...

Thanks again!

JMH

 

[brianl703]

You may want to get it load-tested. Autozone does it for free. A bad two-month-old battery isn't unheard of.

 

[TexasHonda]

There is a relay that controls the BMW 318 ABS. Relays can get "stiff" and intermittent w/ cold and age. A new ABS relay might cure this intermittent fault.

I had a 94 318is that developed an ABS fault in the printed circuit foil that connects the wiring harness to the ABS unit. It was a crack in the foil causing the problem. This might also cause a temperature-sensitive fault. Fix was to jumper a wire across the foil fault.

good luck

 

[g20ooh]

If that doesn't get it , vibration " cured " higher than desirable but still functioning ground path resistance issues .

TexasHonda thats a neat good -to -know .