Ran with diagnostic connector cap off for a week

[tim]

I thought the car would explode or something if you did that, but we can chalk that one up into the mythbusters column because I drove it like I stole it all week and no problems. I changed the oil last week and reset the light, but forgot to put the cap back on. Just noticed it today when I was showing my sister how to change her oil.

[zygoteer]

In all probability you may well get away with it, as the contact integrity of those cap pins is not brilliant when its fitted correctly anyway.

The cap is supposed to short the TX data line to +ve and the RX data line to 0V (or maybe the other way round - must check sometime).
... anyway the purpose of this is so your ECU does NOT get spurious (from electrical noise) signals on those lines, which are the RS 232 communications ports, then try to respond to the communications interrupt while its doing important time-critical stuff like the ignition/fuel control, which for obvious reasons is not desirable.

Worst case scenario is that the RS232 drivers on the ECU PCB gets incorrect voltages and go 'pop' - but this is probably unlikely.
The effect of such a failure is simply that the diagnostics box at the stealer (or a laptop as I use) won't work due to no communications .... and as I understand it, this is actually fixable (by the right person with no vested interest in the extra profit from selling you a new ECU).

[uscharalph]

In all probability you may well get away with it, as the contact integrity of those cap pins is not brilliant when its fitted correctly anyway.

The cap is supposed to short the TX data line to +ve and the RX data line to 0V (or maybe the other way round - must check sometime).
... anyway the purpose of this is so your ECU does NOT get spurious (from electrical noise) signals on those lines, which are the RS 232 communications ports, then try to respond to the communications interrupt while its doing important time-critical stuff like the ignition/fuel control, which for obvious reasons is not desirable.

Worst case scenario is that the RS232 drivers on the ECU PCB gets incorrect voltages and go 'pop' - but this is probably unlikely.
The effect of such a failure is simply that the diagnostics box at the stealer (or a laptop as I use) won't work due to no communications .... and as I understand it, this is actually fixable (by the right person with no vested interest in the extra profit from selling you a new ECU).

Really?

[Bill R.]

Really?

[zygoteer]

Surely the text in the middle is a mistake ?
It says pin 18 is shorted to Battery+ AND ground !!!!

Not quite sure why pin 18 is mentioned at all, as my cap does not connect to it, but maybe there different caps for the 20-pin connector.

My info (pic attached, which shows both the 15-pin & 20-pin variety) says this pin is the ECU programming pin (an 'enable' or maybe extra voltage for the EPROM ?)

That said, I don't think my ECU actually has an on-board programming capability.

Perhaps others have information on this ?

[Bill R.]

"and pin 18" then it makes sense. Its straight off the bmw manual.

Surely the text in the middle is a mistake ?
It says pin 18 is shorted to Battery+ AND ground !!!!

Not quite sure why pin 18 is mentioned at all, as my cap does not connect to it, but maybe there different caps for the 20-pin connector.

My info (pic attached, which shows both the 15-pin & 20-pin variety) says this pin is the ECU programming pin (an 'enable' or maybe extra voltage for the EPROM ?)

That said, I don't think my ECU actually has an on-board programming capability.

Perhaps others have information on this ?

[zygoteer]

OK that makes sense now.

Hmm ... and I always thought I was fluent in 'typo-nese' !

but its nice to know that BMW apparently have a dyslexic keyboard that is occasionally similar to mine