Talk Morgan Forums Morgan Sports Cars Classic Morgans (pre-CX) Sticking rev counter on modern 4/4

HI. I have a three year old 4/4 and from new the rev counter sticks at the idle speed 50% of the time when I switch off. Neither Life's nor now Beamish Morgan, with the help of the factory have got to the bottom of this. Purely by chance after all this time I have discovered this evening that it only does it when the heater fan is on when I switch off. Has anyone else come across this or something similar, and most importantly has anyone got a solution I can suggest to the dealer and factory???
Cheers
Derek

Yep. Same on my 2012 plus 4. I didn't know about the fan switch... I'll try that. I have put it down to the dreaded dash pcb. I'm on my third modified board.

Thanks Nick. Factory is professing ignorance of the problem occurring. Nice to know I'm not alone!

It sounds as though a “back emf” from the heater motor is upsetting the rev counter. You would need a storage oscilloscope to isolate the issue. However, it could be worth checking the routing of the wires behind the dashboard to ensure that the heater motor wires are well separated from the power and signal feeds to the tachometer.

On my 4/4, I don’t have that problem but I do find that the rev counter does not zero properly at times - I have got into the habit of always waiting for the whole system to complete initialisation after turning on the ignition before I start the engine.

I have just had a look at the spare rev counter I have in the garage & wonder if the drive mechanism, for the pointer, is a stepper motor. Thus needing power to return the pointer to zero. So once the ignition is turned off the rev counter uses stored power (capacitor ?) to return the pointer to zero, but the process gets screwed up by the emf from the fan motor. If I get a rainy day I will run a test on the spare rev counter (VDO with Morgan wings)

I had a similar problem 3 years ago with my 4/4, now I wait 10 seconds for the ECU to cycle properly before turning to Start

Ive never had a problem since.

Just noticed my 2012 plus 4 did this on our last trip, never noticed it in the previous 3 years.
Didn't really concern me at the time but now my nearside indicators have failed (see separate thread) I'm beginning to think it could be the start of something bigger.

My +4 (2013) does it sometimes, I just turn the off then turn back on and resets

Will try the heater fan

That sounds very plausible.

One of the things I noticed is that if I stall the car and restart very quickly, the rev counter does not zero - it just starts from wherever it stopped and over-reads by the amount it stopped on.

I think there is some link to the heater fan... my second board was modified after failure (simply the diodes for the back emf being under spec as standard) and then started to misbehave in an alarming fashion... rev counter not working along with some other gauges, but in an intermittent way. That board had some blown tracks so I have modded my second board with new diodes but intact tracks and I just get the rev counter hang on switch off. Leaving the full ecu cycle makes no difference on mine