Mounting AFM - Where is best?

[Coastcard]

Having now found all the bits and pieces and fabricated a mounting bracket template for installing the flapper AFM in my Land Rover 110, I was wondering...

Most AFM installations I can recall are attached to bodywork, independent of the engine. My AFM bracket attaches to the redundant alternator bracket on the Range Rover engine (alternator is mounted other side on 110) - thus directly to the engine.

Are there any rules for this? Is the AFM vibration sensitive hence why on bodywork? Anyone know the answer - is engine mounting a no no.

[katanaman]

At a guess I would say mounting the flapper type AFM on engine is a bad idea because of vibration. Not so sure on the hotwire but personally I still wouldn't do it. You could also be introducing a lot of heat into the meter which might not do it any good either.

[Coastcard]

Thanks Marki, heat should not be an issue as it is remote enough from any major heat source and the bracket is quite long. The AFM is protected (heat/vibration?) to some extent by rubber gromets in the mounting as well. Just not sure this is enough on the vibration stakes.

[ramon alban]

Thanks Marki, heat should not be an issue as it is remote enough from any major heat source and the bracket is quite long. The AFM is protected (heat/vibration?) to some extent by rubber gromets in the mounting as well. Just not sure this is enough on the vibration stakes.

I believe the rubber mounting grommets' primary purpose is to isolate the body of the AFM from Chassis Earth.

This way the only earth that the AFM sees is the Efi/Engine loom earth connection which is at the stud below the rear of the LH rocker cover.

If the AFM were connected to chassis earth this can affect/over-ride the sensitive air volume and temperature signals going to the ECU because of the variable battery charging currents passing along the main engine/battery earth strap.

Regarding vibration, I dont think that the delicate components inside the AFM would last very long with the engine banging away at anywhere between 4000 and 24000 firings a second.

Particularly as at some particular frequency there would be a resonance that might literally make the AFM components sing like that opera singer who used to break wine glasses with her voice.

[Coastcard]

Yikes Ramon!

I was under the impression (maybe misguided ) that earth was earth. I only have a single circuit on my landy and as such the earth is common to everything. Engine is strapped to chassis and so is everything else. The stud at the rear of the engine I would suggest is no different.

That said, I think the safest way is probably a body mount.

Thanks.

[ramon alban]

Yikes Ramon!

I was under the impression (maybe misguided ) that earth was earth. I only have a single circuit on my landy and as such the earth is common to everything. Engine is strapped to chassis and so is everything else. The stud at the rear of the engine I would suggest is no different.

That said, I think the safest way is probably a body mount.

Thanks.

I guess I should have prefaced my suggestion that my knowledge is limited to the Rover SD1 Efi system. Sorry about that.

Never-the-less, I had perhaps wrongly assumed that a normal system had the Efi engine fixed on rubber engine mounts, the gearbox supports are rubber and the exhaust system fixed to the engine and gearbox is otherwise hung on rubber loops, all insulated from the chassis.

Thus the voltage sensitive Efi system has a single unique earth point at the stud on the engine and is therfor isolated from any other possible or unintentional earth paths which may have residual earth differentials due to corrosion or variable battery charging currents.

[kiwicar]

Surly there is a thunking great earth strap for the starter motor to the chassis so the engine is far from isolated from the chassis (or did I miss your point Ramon? ) .
I agree any screen earth would be seperate back to the ECU, and an earth return from a sensor back to the ECU, but the earth from the ECU will be via the chassis any way not through the engine. The AFM should also be earthed through the ECU to the chassis to avoid earth loops, star or daisy chain earth, never loops, or was my transmission theory lecturer a bigger dork than I thought?
Mike

[katanaman]

The earth potential on the engine could be different to the potential on the ECU which could be different to any other part of the car. It might be minimal but when your dealing with electronic signals it could be enough to through out the reading. All the sensors for this reason should be separated from the chassis so the earth reference is exactly the same as the ECU sees. This is why for instance the temp sensor is two pin and doesn't use the engine earth as the temp gauge sensor does. Would it work connected straight to ground? Yes it would but the potential for inaccurate readings is much greater not to mention the chance of introducing electrical noise into the system.

SOOOO after all that drivel I am with Ramon

[kiwicar]

I think in that case I am with you both and as what you say about the temp sensor ties up with what I was thinking when I wrote about the earth returns to the ECU. Now to go and sit in a darkened room and bathe my sore head.
Mike

[ramon alban]

I think in that case I am with you both and as what you say about the temp sensor ties up with what I was thinking when I wrote about the earth returns to the ECU. Now to go and sit in a darkened room and bathe my sore head.
Mike

Dont do that Mike!

Break out the digital voltmeter and on a low voltage scale connect it to both ends of the "thunking great earth strap".

Put on all the lights and make the battery and alternator work for their living.

There will be a small residual voltage across that earth strap.

How big or how small will depend upon how clean the earth connections are and how much current is flowing thro the strap.

That very small voltage differential will upset the response of the ECU to its input signals if one of the sensing components is strapped directly to chassis earth.

[katanaman]

I think in that case I am with you both and as what you say about the temp sensor ties up with what I was thinking when I wrote about the earth returns to the ECU. Now to go and sit in a darkened room and bathe my sore head.
Mike

I would highly recommend JD as the bathing ointment

[jwriyadh]

I have seen a discussion concerning the earthing of the AFM, (IIRC on the Morgan site) where even the contact of a water hose on the AFM had a bad effect on the operation of the device.

Yes the hose is rubber but is conductive enough to give a problem.

As long as all EFi components are solidly earthed to the back of the cylinder head then all should be fine.

jw

[Coastcard]

Glad we sorted that out then

These are about the only fine tolerances I have heard of on a Land Rover