Classic cars forum & vehicle restoration.

peter scott · Joined: 18 Dec 2007 Posts: 7211 Location: Edinburgh

I've had cause recently to remove the intercooler to access heater hoses on my "old classic" Mondeo turbo diesel. If I run the engine without the intercooler in place then the rev.counter remains at zero. The turbo is still blowing but not into the inlet tract.

Just out of curiousity do any of you guys who are into modern technology happen to know why this should be? I always thought modern rev. counters used a Hall effect sensor on the flywheel. I really can't imagine that the instrument responds to manifold pressure which would not be exclusively proportional to engine speed anyway.

I am pretty sure that I'm not just disturbing the wires to the sensor because the effect is totally repeatable with and without the intercooler.

Peter Shocked

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https://www.nostalgiatech.co.uk
1939 SS Jaguar 2 1/2 litre saloon

Uncle Alec · Joined: 14 Jan 2008 Posts: 734 Location: Manchester

Is the alternator still driven? One way of powering a tacho on a diesel is to run it from the alternator.

peter scott · Joined: 18 Dec 2007 Posts: 7211 Location: Edinburgh

Hi UA,

Yes the alternator is just as normal. Everything is normal except that the turbo output is not connected to the inlet manifold. No other items are disconnected in any way. The intercooler only has input and output connections. Having said that, there is a mass airflow (MAF) sensor between the air filter and the input of the turbo, so it will probably detect higher than expected flow rate through the turbo as the turbo output is unrestricted.

Peter
_________________
https://www.nostalgiatech.co.uk
1939 SS Jaguar 2 1/2 litre saloon

Peter_L · Posted: Fri Mar 18, 2011 4:32 pm Post subject:

Peter... Did you ever find the answer to this conundrum.

I haven't a clue, but.... is there the remotest chance that the Rev Counter is grounded via the bit that you took off ?

(Ground to either the Rev Counter or the revolution sensor ?)

peter scott · Joined: 18 Dec 2007 Posts: 7211 Location: Edinburgh

hi Peter,

It's definitely not anything to do with breaking electrical circuits. I'm pretty much convinced that the MAF (mass airflow) sensor is telling the the limited intelligence of the car that it doesn't understand what's going on so RPM shall not be displayed.

When the intercooler is not in position the MAF will be seeing a much larger airflow than normal because the turbo is just blowing out to the atmosphere instead of into the engine.

The effect is totally deterministic and, as far as I can see, the MAF sensor is the only one that will see a change from normal.

Peter
_________________
https://www.nostalgiatech.co.uk
1939 SS Jaguar 2 1/2 litre saloon

Peter_L · Posted: Fri Mar 18, 2011 8:51 pm Post subject:

.......... so, if you forced a reduction in airflow past the sensor, the RPM should spring into life.