Dead Gen 1

[Batfink]

Using a test light can --be very unfriendly to electronics (ecu , etc )
use a digital multi meter ( can be quite cheap )
A spike in voltage CAN happen with an old type test light as with old style jumper leads without spike protection
happens rearly but the cost of another ECU would bring tears to your eyes
harky

Never had an issue with a test light.. as the globe is only 5W this equates to an equivalent of a 0.4 Amp fuse.... I think most vehicle electronics can handle .4 amp current????
I earth light to vehicle frame and yes I have blow a few globes on tester but better than blowing something else...

Jumper leads can dump 40-80 amps which WILL blow electronics