How to test an RPM sender output with a multimeter

Your Fluke meter is simply showing an AC voltage. Does this increase from idle, if so I suspect your tach is simply a form of voltmeter that will show an increasing reading with revs. Not sure how linear this instrument will be with engine speed. At least you have an output from the sensor and assuming the wiring to the tachometer is all sound it really leaves the gauge as the prime suspect if the overall system is faulty. Parts, I would imagine are very rare. How does your electrical generation work ? If by DC generator (dynamo) - it is a 1970 engine- you are a little stuck. If however by alternator, possibly a later up dating, why not fit a conventional tacho gauge ? Normally there is plenty of adjustment via onboard (DIP) switches to accommodate the number of alternator poles and pulley ratios to make calibration easy and relatively accurate. Good luck whatever you decide to do. Richard
 
Bejasus said:
only 2 terminals on the sender olewill and one of those goes direct to ground.



Bejasus
Click to expand...

Not had any experience of the Ford but the Cummins engines used such in the late sixties through till late seventies and it was 99% the generator that was at fault, which btw was AC. I would also think your engine is to old to be fitted with any type magnetic pick up for which ideally you would need a scope.
There is two connections so would I would be sure it's a permanant magnet in there. Disconnect signal connection that goes to the tacho just in case this is grounding out. Then connect voltmeter across the two terminals either way round it won't matter as been AC and as said look for voltage increase with revs. No need to worry about Hz, you will not get a correct reading without a generated voltage and you have no data to compare with.
 
Ok, so it is an AC output which increases with rpms. I am suspecting tacho or wiring issue, so next step is to rewire tacho to 12v and ground and run wire from the signal generator direct and check it out. Pretty sure the signal generator is of the permanent magnet type. If the tacho is faulty, I will replace and probably take the signal direct from the Balmar alternator if I can.


Bejasus
 
Some info on the side for 'sender searchers' : My 4108 (1978) has a VDO sender, 2 terminals, isolated from chassis. It is attached to the engine by a left hand threaded nut which is upside down from what you think you are looking at, so spanner (7/8") it to the left looking from above to remove.
The p/n of the sender is 340-808-001-004G although that doesn't seem to be stamped on it anywhere (unless it's under the paint).
Mine has 86/2/3 and 9.73 stamped on the underside. The two terminals are labelled "Z" and "G" which are designated 'odometer' and 'sensor' respectively, although they are simply the output of an AC 4-pole generator which the only printed specification says "19.3 volts at 2000 RPM open-circuit"
I bench-ran mine on a motor specified as 1425RPM and the output was around 14volts, so it may be reasonable to assume it generates 1 volt per 100 revs, hopefully reasonably linear. I am in touch with VDO in USA, if they give any more info I will update this thread.
 
VDO only seem to respond positively if there's a sale in the offing, so no info on output/rev. However, having got the sender back on the engine, it may be worth mentioning that the retaining nut is dual-threaded and works in the same way as a rigging bottlescrew to draw the sender onto the crankcase.
 
Apologies for resurrecting, but more info for people who may have tacho probs....
My 4108 tacho was erratic, varying by 100s of revs. The sender had been removed and examined (see previous post) and priced 184 euros!!
When the erraticism stopped, it finally read low, so it had to be revisited.
The voltage and frequency were measured. Frequency : what the meter shows is cycles per second as a decimal of kilocycles ie - 0.050 Kc/s at 1.500 revs. 50 x 60 =3000 cycles per minute which is twice the RPM indicated. That can be used to confirm the accuracy of the tacho.
Voltage - the VDO setup uses voltage however. at about 1 volt per 200 revs displayed.
The AC waveform is rectified inside the head and the movement is simply a 20volt meter.
In my case one of the rectifier diodes gave up after having been intermittant for a while.
Pics to follow.
 
servicing a tacho head.

DSCF3035_zpsaqg1owaa.jpg


You can (just) see how I prised the bezel ring open for about 1/2 its circumference in order to pop it off ,releasing the glass and seals. Remove 4 bolts at rear to drop out the movement. In this pic the rectifier bridge has already been snipped off - that's the wee grey block. I replaced it with a bit I had cut off a scrap circuit board, bigger but electrically the same. I couldn't pull the indicator needle off its shaft, hence the need to leave the stumps of the original component to solder onto. The hardest bit was getting the bezel back on again - a bit like fixing a puncture on a bike!
Testing is simple - just apply a voltage less than 20v to get a reading. It should show the same value whichever way round you connect the pos/neg if you get different readings you've got the same problem I had.
DSCF3045_zpssle6uuon.jpg
 
following on from the simple 2 -wire setup above, if you've got one of these,
DSCF3049_zpsmjw1mim2.jpg


it will normally use three wires, two (+ & - ) to power the internal circuitry, and the "W" terminal which connects to raw AC from the alternator stator winding prior to rectification.
DSCF3050_zpsvkcr47bg.jpg

If included during manufacture, it will be marked "W" on the back of the alternator, but it may be added at a later date and simply be a wire poking out of a hole at the back.
The "G" terminal appears to be internally connected to the "W" terminal. but I did not test the alternator connection to it. G stands for Geben which is 'giver' (sender) in German (where VDO is made) The 4 pin interface seems to be standard and the 4th pin may be used when a transducer is fitted. So this type doesn't care about voltage, only frequency.
I hope these ramblings may be of use to someone sometime.
 
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