RPM reading affected by DMF ?

**sierrafery** · 24th November 2020, 07:49 PM

Hi all, here's one i can't figure out: can a faulty DMF mix up bad the rpm reading ? The crank sensor and it's circuit + the ECM are 100% ruled out by myself but getting erratic rpm readings while cranking like 0-180-0-190-0 ... and so on fluctuating within seconds . Im trying to help a friend and i'd rather not make him remove the DMF untill i'm not sure that the DMF can cuse such thing or not. Anyone had this ? TIA.

**V8Ian** · 24th November 2020, 08:34 PM

Strewth, I think this is the first time you've asked for help, rather than offering it.

**DiscoDB** · 24th November 2020, 09:08 PM

Fery - you didn’t mention if the motor actually runs and if the revs are smooth or getting lots of bad vibration as well.

If the DMF or Harmonic Balancer has failed you would feel it.

If it is just a signal issue to the tachometer then I can’t see how it could be the DMF.

**sierrafery** · 24th November 2020, 09:53 PM

Originally Posted by DiscoDB

Fery - you didn’t mention if the motor actually runs and if the revs are smooth or getting lots of bad vibration as well.

If the DMF or Harmonic Balancer has failed you would feel it.

If it is just a signal issue to the tachometer then I can’t see how it could be the DMF.

Sorry, the engine cranks but doesnt start which i know is normal as long as the rpm input is not 250 rpm or above while cranking.

**discorevy** · 24th November 2020, 10:37 PM

wouldn't think the DMF could cause the fault unless the bolts were very loose.
What are you measuring the revs with ? , as you know the delay with nanocom could cause inaccurate readings.
You've no doubt ruled out bad shielding to CPS.
Have you pulse tested the injectors ?

**sierrafery** · 24th November 2020, 10:46 PM

Originally Posted by discorevy

wouldn't think the DMF could cause the fault unless the bolts were very loose.
What are you measuring the revs with ? , as you know the delay with nanocom could cause inaccurate readings.
You've no doubt ruled out bad shielding to CPS.
Have you pulse tested the injectors ?

I've used nanocom and double checked with hawkeye, both the same, i made a CPS harness from shielded wire with plug and terminals so i connected the CPS directly to the ECU to rule out the circuit then the CPS from my vehicle to the non-starting vehicle's ECU and cranked them both at the same time and then i've got good reading so the ECU is ruled out too... i'll try tomorrow to connect the faulty vehicle's CPS to my vehicle to see what i get, very strange

**p38arover** · 25th November 2020, 01:23 PM

What’s a DMF?

**Kaaaiju** · 25th November 2020, 01:43 PM

Dual mass flywheel

Originally Posted by p38arover

What’s a DMF?

**Bohica** · 25th November 2020, 05:37 PM

Originally Posted by sierrafery

Sorry, the engine cranks but doesnt start which i know is normal as long as the rpm input is not 250 rpm or above while cranking.

The TD5 Engine document may help, if you do not have a copy.

Discovery Series 2 Diesel Engine TD5.pdf

**pojir** · 25th November 2020, 09:44 PM

I'd try a different starter motor. I could imagine it having some partial short circuit in the coil - which would mess up the voltage in general. Same thin could happen to alternator - maybe try starting the engine without serpentine belt? Shouldn't do any damage when cold, without load and for a short period of time.

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