How can one calibrate a tachometer ?
 

Good Evening everyone, well after some time i managed to get the parts i need to get the tachometer working

Diesel tach.
Tach amp
New pickup sensor cable.

wired it up and no go.

it sits on 0 rpm, until i rev the engine and the needle won't move until i think the engine is doing 2000 rpm or more :/

Does anyone know how to calibrate it ?
om617 turbo engine 4 spd manual

https://www.youtube.com/watch?v=nBfHjedhKRs


Here's the measurement across the pickup sensor wire after it was installed into the car.

Is the pickup to flywheel gap adjustable on this engine? It sounds like the signal isn't strong enough.

A reluctance type sensor ( wire wrapped around a magnet ) generates AC voltage when a steel tooth sweeps by. If the gap is too large, the signal will be too weak. Others might know what voltage you should expect at idle / XX RPM

Does your meter have a frequency setting? This could be used to calibrate actual engine speed to indicated engine speed. If you know how many teeth are on the flywheel, you can calculate frequency at XX RPM.

What did catch my attention. If the head unit is otherwise properly grounded when installed. I would take in my case a wire with a couple of alligator clips and run a ground from the metal plate back to a real ground. There is a slight chance the head unit is not being properly but only partially grounded in your video.

Or just take you meter and read ohmage from that metal plate back to a good ground. It should look like a short circuit. Or really high resistance. A semi ground has to be exiting currently or there would be no function at all. This is a very long shot as the two wire plug that is installed looks like it has a ground. Plus a power feed.

If I am not mistaken the W123 cluster relies on a metal strap to provide ground to the right pod from the left pod. The left pod gets its ground through the round connector.

I have to admit I have only seen a 240 D cluster that only has the big clock and no tach.

However I have worked extensively on a 124, and the 124 has a similar setup. The 2 pin plug is power and tach signal and the single lead is the unswitched power to the clock. Both instruments rely on that strap to get ground from the left pod.

I had an 83 240D that had the EGR controller and it had a pickup on. the damper. There is one pin on the damper that triggers the sensor. So the output would be one pulse per revolution. Is this how the 617 is set up?

The later 603/606 engines have a sensor on the ring gear at the bell housing. The gear has 144 teeth so you get 144 pulses per revolution.

And of course the gas engines of this era get their signal from the ignition coil. So that is (number of cylinders/2) pulses per revolution.

When you changed the caps, did you remove the face? The most likely explanation is the simplest... the needle was positioned incorrectly. Pluck it off, set it back.

Yes, i took the face and needle off to do the bad caps.

I replaced the face ( only 1 position to screw it in )

and then put the needle back on. I let the tach sit at resting position and put the needle on pointing towards 0 rpm :/

Yes, i will check the ground with a multimeter :)

the picture below is one i took with the balancer off when i was changing the front crank seal. but you can see the yellow metal bracket where the pickup wire sits. The original grey coloured is currently in the position and no i don't think it's adjustable ?

Did the tach work post damper install?

If a factory spec exists for the sensor to tooth gap, use that. If not, I'd expect it to be in the 0.010" range with 0.020" the likely upper limit.

what is a damper install ?

Vibration Damper more or less = Crank pulley = thing that bolts to the crankshaft snout.

are you talking about the harmonic balancer ?

If so, The tach has never worked because it's a m110 to om617 engine swapped car.

I got a diesel tach

tach amplifier, and now my 3rd ****ing cable

1st cable was original, it was reading 85 Ohms resistance
2nd cable was broken, see the picture below, the end was snapped off at the factory or something as the broken off piece was not even in the box when it arrived

3rd cable is the current one, it fits, but reads 72.3Ohms.

It is 1.0mm away from the metal Pickup Pin on the harmonic balancer.


check out from 1 minute 20 seconds in, at 1 min 30 seconds he mentions about the cable resistance.

https://www.youtube.com/watch?v=5glxTyJbgTo&t=1m30s

1mm seems like way too much gap for a magnetic pickup. As 97SL mentioned earlier up in this thread, the gap is typically in the thousandths of an inch range (fraction of mm). If the tap is too wide, you'll get no reaction out of it until the RPM is high enough keep it triggered. The voltage pulse is tiny, getting the right amplitude out of it is all down to the gap.

I dont quite understand, is the harmonic balancer from a diesel?

The diesel needs a groove cut in the pulley. The groove causes a disruption in the magnetic field which the pickup senses.

I mounted a (300d) tach in my 6.2 diesel Blazer (<- comes with a bracket for connecting a magnetic probe for timing & rpm). I fab'ed a bracket to mount the MB pickup. Worked like a charm, , , , ,

-c-