When disconnecting electrical connections, is it absolutely critical to disconnect the cable from the negative terminal of the battery beforehand?

I'm not asking to avoid doing proper procedure. I usually disconnect the neg battery cable or intend to do it, but once or twice I simply forgot when doing stuff in the engine bay. I was wondering what type of risks are involved. In regards to the airbag connectors, there's the chance of inadvertent deployment, and in other situations I'd guess there's a chance of damaging the PCM. So, should I always remove the cable from the neg battery terminal? Thanks.

PS: Hopefully, asking about it will help me to remember to do it. doh.

 

That depends! Can't hardly do a KEYON ENGOFF test with the battery disconnected. Sometimes it's needed to force the PCM to relearn after a sensor, etc. is replaced or just plain old safety.

On a older vehicle, disconnecting the battery may cause strange things to happen as it relearns, that why they sell little power devices that plug into the cigarette lighter plug to still power the KAM in the PCM. I have one that uses the OBD-II plug and a cigarette type plug for a jump pack since they stopped putting cigarette lighters in vehicles.

 

It's funny you mentioned the cigarette lighter gizmo - I stumbled across a mention of one in the manual just last night.

I asked about disconnecting the battery because it seems every procedure in the manual says to do it. Thank you for the reply.