Originally Posted by Sailorbob8599
Thanks for all the responses folk, unfortunately the BMW D12 engine has neither an oil pump or even a filter. It relies on the old "splash" system which saves on filters for sure ... so I guess it will have to be an electric
meter, that will be activated whenever the ignition switch is in the ON position.
Hmm... something doesn't jell here, in your first post, you described the engine as having an oil pressure idiot light.
Assuming for a moment that is does actually have such a light, this light will illuminate when you initially turn the key on and will extinguish once oil pressure is developed.
If so, I strongly suspect it will have a normally closed oil pressure switch that switches a ground to the idiot light. That is, there is a ground present until
oil pressure is developed. In this case, the Hobbs meter circuit as described A64pilot and endorsed by Gordmay will be inappropriate. In this instance you will either have to add a normally open switch to the engine or use a relay to reverse "the sense" of the existing switch so that a relay contact becomes the normally open contact to activate the Hobbs meter.
Quite straightforward, the positive side of the relay coil goes to the key positive and the negative side of the coil goes to the existing oil pressure switch. Thus both the idiot light and relay operate in parallel. Once oil pressure is reached the light extinguishes and relay relaxes. A normally closed contact on the relay then switches a ground to the Hobbs meter. The positive side of the Hobbs meter is feed from the positive of the key switch so that the only time the Hobbs meter is activated is when the key is on and oil pressure is present.
Of course, you initial post may be in error and there really is no oil pressure in the engine; but I got to say, I haven't seen a diesel
engine without an oil pump / oil pressure except at antique farm engine display days