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How many pulses do I have???

N

nimbus

Member
I have had to replace the tacho on my V8 5Lt GXi-F and chose a VDO as someone said they were the best.
However the tacho is programmable (Is'nt everything)!! and requires to be fed with the number of pulses per revolution of the crank.
This comes from the ECM
I have had the Volvo experts on this and they still cannot give me any values.
Can anyone out there help, before I have to get out my oscilloscope and start looking at the pulse train, and this may not give me the correct answer.
 
The Tacho is digital and the indicator needle is i believe driven by a stepping motor.
You program it using the 5 digit display which normally functions as the "Hours" meter. there's a button on the back which is used for this.
I sourced it direct from the US over the internet.
Good point 'kimcrwbr1' if VDO have a distributor in Australia I will call them, the problem is this is a universal Tacho for all and any application so they may not have any records/lists to refer to.
 
The "standrd" interface for a tach on a V8 emgine would be 4 pulse per revolution. "Standard" in that most tachs for inboards, look at the ignion coil and "count" spark plug firings and "divide by4" for a 4 stroke V8. Volvo being volvo who provides the whole package including instruments may have "gotten creative".

Trying 4 pulse per rev is a safe first guess. A sanity check would be to start it up an check if the idle RPMS is near specs as the ECM controls that and the tach nas no reverse effect on the engine due to programming. Since the other choices are for 4 and 6 cylinder engines (and assuming true universality, 2, 10 and 12 cylinder engines).

What you are seeing with talking to Volvo is a trend in " corporate technology" these last 20 years which is to replace one highly skilled (and paid) person with a less experienced (often "fresh out") employee. For many years, the goal of "modern management" is to institure policy and procedures so that the skill and experience of the employees is not a factor. This is to eliminate the highly undesirable (from management's view) person with critical skills ( who they would have to pay and treat well).
 
You are absolutely right Bob in your observations, as the applied technology imported into vihicles both road and water (and dare I say air as well)
increases at an alarming rate, the overall general individual knowledge of the interopperative componentry of these "systems seems to decrease in direct proportion. And the scary part is that it is increasingly difficult to find someone who actually can understand what you want or are talking about.
With futher 'playing' over the last few days I think I have cracked it. For the VDO I have it seems that you must load the correct, or close approximate number, into what they call the pulse mode value.
Then use the Callibration mode to fine tune the indicated revs to match the measured revs (from another meter) pressing of the button causes the needle to increase or decrease. I chose the idle condition as this is supposed to be regulated at 500rpm by the ECM.
However the Instructions indicate that 'callibrate' can be used to actually set the rpm reading as an option. (if you don't know the exact value)
This does not work!!!! (would that be surprising) I have found that you must actually set a value that is close to the 'pulses per crank rev" and then use the "callibrate" mode to move the needle to the correct reading. They actually reccomend that a speed of about 2,000 rpm be used to do this.
If you don't get the approx value correct, then when you attempt to fine tune the reading, the needle will (in my case) move downwards to some internally defined limit, then start to move up again.
The value of 4 (for a V8) seems to give reasonable results, The pulses are supplied from the ECM so they could in practice be any value generated from crank pulses and the master clock oscillator.
 
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Re:" the overall general individual knowledge of the interopperative componentry of these "systems seems to decrease in direct proportion."

Spent almost 20 years of my career ( Electronics Design and Development Engineer) as a consultant at a world famous telecommunications lab here in NJ. A favorite expression from management.... "Hurry up and finish and toss it over the fence ( to the internal customer, i.e. partnering team member)". This lab had the practice of divving up larger jobs to various teams that often were managed by different management chains. It lead to some very "interesting happenings" at final system test.
 
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