VDO Tachometer

[dan tonner]

Hi all;

 

I just recently got my new Savannah XL VG (80 Hp Rotax UL) completed and in the air. I am very pleased with the results and look forward to spending the next bunch of time with her in the air instead of in a workshop and hangar.

 

I ran into an unusual situation in that the ICP stock VDO Tachometer started performing erratically just before the first flight. It had been working perfectly through many hours of taxi trials and engine runs, but decided to start acting up when my test pilot arrived.

 

The gauge seems to read accurately up to about 3500 - 4000 rpm, but then the needle dances to maximum and then gives readings that seem all over the board when attempting to test prop pitch at full throttle. At one point during the initial flight, the gauge needle dropped to zero for a few seconds before continuing its erratic journeys across the face of the dial.

 

Unfortunately, ICP did not provide any manuals with the new tach but I believe (from data on the 'net) this gauge goes through a self-check each time the power is switched on. During the check, the needle on the gauge swings momentarily to maximum. All this led me to look for momentary power interruptions at the gauge - so far without finding any such problem.

 

Have any of you encountered similar (or unsimilar) issues with the VDO Tach?

 

Best regards, fly safely,

 

Dan

 

 

[dan tonner]

Hi all;

 

I just recently got my new Savannah XL VG (80 Hp Rotax UL) completed and in the air. I am very pleased with the results and look forward to spending the next bunch of time with her in the air instead of in a workshop and hangar.

 

I ran into an unusual situation in that the ICP stock VDO Tachometer started performing erratically just before the first flight. It had been working perfectly through many hours of taxi trials and engine runs, but decided to start acting up when my test pilot arrived.

 

The gauge seems to read accurately up to about 3500 - 4000 rpm, but then the needle dances to maximum and then gives readings that seem all over the board when attempting to test prop pitch at full throttle. At one point during the initial flight, the gauge needle dropped to zero for a few seconds before continuing its erratic journeys across the face of the dial.

 

Unfortunately, ICP did not provide any manuals with the new tach but I believe (from data on the 'net) this gauge goes through a self-check each time the power is switched on. During the check, the needle on the gauge swings momentarily to maximum. All this led me to look for momentary power interruptions at the gauge - so far without finding any such problem.

 

Have any of you encountered similar (or unsimilar) issues with the VDO Tach?

 

Best regards, fly safely,

 

Dan

 

 

[old man emu]

Step 1: Check all wiring for loose connections or shorting.

 

Step 2: Check for any chaffing of wiring.

 

KISS

 

Old Man Emu

 

And I don't mean osculation

 

 

[old man emu]

Step 1: Check all wiring for loose connections or shorting.

 

Step 2: Check for any chaffing of wiring.

 

KISS

 

Old Man Emu

 

And I don't mean osculation

 

 

[Kyle Communications]

there was something about putting a resistor in series with the tacho wire somewhere on the forum

 

 

[Kyle Communications]

there was something about putting a resistor in series with the tacho wire somewhere on the forum

 

 

[planesmaker]

Yep had a similar problem a few years back, put a diode in circuit from the sender. I'll see if I can find the information on it.

 

Tom

 

 

[planesmaker]

Yep had a similar problem a few years back, put a diode in circuit from the sender. I'll see if I can find the information on it.

 

Tom

 

 

[planesmaker]

This shows a resistor

 

 

[planesmaker]

This shows a resistor

 

 

[SDQDI]

Step 1: Check all wiring for loose connections or shorting.Step 2: Check for any chaffing of wiring.

KISS

 

Old Man Emu

 

And I don't mean osculation

My tacho played up on me not long after I got the hornet home and it took me a while to figure out that I had inadvertently half pulled the wire off while fiddling around with something else under the dash (can't remember now what I was doing) anyway I was standing out to the side with the engine running with my head under the dash and I finally spotted the loose wire, so I pushed it back on and for some reason automatically turned the dash power switch back to off (was a totally automatic movement I guess I thought I still needed to turn it on to check when I was actually turning it off) then I climbed back in and was totally bamboozled as to why my tachometer had gone from an intermittent wobbling to totally dead. It took me about 30 seconds before I spotted what I had done, a proper face palm moment.

Anyway, what I was trying to say is you are spot on in regards to checking loose wires first.

 

 

[SDQDI]

Step 1: Check all wiring for loose connections or shorting.Step 2: Check for any chaffing of wiring.

KISS

 

Old Man Emu

 

And I don't mean osculation

My tacho played up on me not long after I got the hornet home and it took me a while to figure out that I had inadvertently half pulled the wire off while fiddling around with something else under the dash (can't remember now what I was doing) anyway I was standing out to the side with the engine running with my head under the dash and I finally spotted the loose wire, so I pushed it back on and for some reason automatically turned the dash power switch back to off (was a totally automatic movement I guess I thought I still needed to turn it on to check when I was actually turning it off) then I climbed back in and was totally bamboozled as to why my tachometer had gone from an intermittent wobbling to totally dead. It took me about 30 seconds before I spotted what I had done, a proper face palm moment.

Anyway, what I was trying to say is you are spot on in regards to checking loose wires first.

 

 

[Old Koreelah]

My VDO Tacho/Hobbs has been acting crazy for months. Can't find any wiring issues so assume its internal. Ordered a new one.

 

 

[Old Koreelah]

My VDO Tacho/Hobbs has been acting crazy for months. Can't find any wiring issues so assume its internal. Ordered a new one.

 

 

[cscotthendry]

On Rotax engines, the tacho signal is generated with an inductive coil. If the coil is not loaded enough it can generate quite high voltage pulses which can cause problems with electronic tachos. The solution is quite simple. put a small value resistor (around 220Ohms ACROSS the outputs of the pickup (not in series). Typically, one lead of the pickup is connected to the signal input of the tacho and the other lead is connected to ground. However the pickup is wired to your tacho, putting the resistor across these two leads should solve your crazy tacho problem.

 

 

[cscotthendry]

On Rotax engines, the tacho signal is generated with an inductive coil. If the coil is not loaded enough it can generate quite high voltage pulses which can cause problems with electronic tachos. The solution is quite simple. put a small value resistor (around 220Ohms ACROSS the outputs of the pickup (not in series). Typically, one lead of the pickup is connected to the signal input of the tacho and the other lead is connected to ground. However the pickup is wired to your tacho, putting the resistor across these two leads should solve your crazy tacho problem.

 

 

[dan tonner]

Here's an update...thanks to Old Man Emu, Kyle, Tom, Old Koreelah and Scott for taking the time to write and offer encouragement and suggestions.

 

The problem persists..... all seems ok up to about 3500 -4000 RPM at which point the gauge begins to jump up - eventually reading full scale although the engine speed has only marginally increased. Keep in mind everything was working fine up until the day I had arranged for a ferry pilot to bring the plane across the head of the Bay of Fundy to her home base in Amherst, Nova Scotia - I suppose THAT should have been predictable! (*&^$#@) The pilot and I set the prop pitch to very coarse (15degrees) and he flew her home without the benefit of the Tachometer anyway.

 

Here's what I've done and the results: each trial was done separately from the others and afterwards the tach was reconnected normally and confirmed the issue persisted.

 

• switched the tach wires - (the tach gave no reading whatsoever)
   
   
  
• installed a 10 Kohm resistor (later, a 30 Kohm resistor) in series with the sensor wires (first one, then the other) - no change - tach continued to read erratically upwards from about 3500RPM
   
   
  
• installed a 1N4004 diode in series with the blue-yellow signal wire - either no tach reading at all or (when diode was reversed) gave no reading up to about 2000RPM, then gave steady readings above this....(prop is still set to very coarse - maximum RPM about 4500)
   
   
  
• installed a 230 ohm resistor in parallel across the signal wires - no tach response.
   
   
  

 

 

I measured the continuity across the solder joints on each of the 5 trigger coils: the four ignition trigger coils gave readings of 350 - 365 ohms (coils were not unplugged at the connectors). The RPM trigger coil gave a reading of 9 Mega ohms - the signal wires were disconnected at the time and I didn't have the presence of mind to take another reading with these connected to the tach - or even shorted to each other. (If it is a failed trigger coil, why do I see a tach response up to 4000 RPM?)

 

I measured continuity in the signal wires across the plastic gang connector at the firewall and found (to my surprise) an 85 ohm resistance here - I don't know whether this is significant or not.

 

These tests were done with the engine not running (of course) and all switches and ignition off.

 

With the engine running and the signal wires disconnected from the tach, I measured the voltage across the signal wires: I saw about .27 volts at 2000 RPM rising steadily to .5 volts as engine speed increased when measured on the A/C scale of my meter. The reading was a steady 2.3 volts at all engine speeds when measured on the D/C scale of my meter. (I don't know what this means - if anything.)

 

And here I am. No tach and very limited opportunity to borrow another tach or trigger coil to test, or even another 912 setup to which I could connect my tach.

 

I would like to be able to "re-set" my VDO tach but there was no manual supplied with the tach and I cannot find a manual for it on the 'net. (I am very reluctant to press the "factory-set" button on the back without some direction regarding what to do next.

 

Anybody make any sense out of all this???

 

Fly safely,

 

Dan

 

 

[cscotthendry]

Dan:

 

Measuring the resistance across the trigger coils without unplugging them won't really tell you anything as you can't know what circuitry is inside the ignition boxes that you're also measuring that way. Even though it's a pain, it will be more meaningful if you unplug the connectors on the ignition boxes and re-measure the trigger coils to compare with the tacho coil.

 

Having said that a multi-mega ohm resistance across the tacho pickup sounds high. I seem to recall som Rotax troubleshooting tips somewhere, either from the manuals or from Rotax Owner. Maybe have a look at those places for help also.

 

Another thing to try is to connect your ohm meter across the wires of the tacho pickup coil, at the gauge end and then start wiggling the wires between the pickup and the tacho. If there is a broken wire somewhere, this may show it up. For this kind of test, an analog meter is best as the digital ones jump around in the readings mostly and it can sometimes confuse the readings. On an analog meter you will see large movements of the pointer if you have a broken wire.

 

I'm not quite sure what you were measuring when you were measuring across the gang connector on the firewall, so I can't comment on that.

 

The DC measurements from the tacho pickup are meaningless. The AC measurements sound a bit low to me, but explain why you didn't see any tacho reading until a certain speed with the diode in the circuit. A 1N4004 diode is a silicon diode that requires about 0.7 volts before it conducts. When you had the diode one way, it completely blocked the signal in the polarity the tacho gauge needs, in the other polarity it blocked the signal until the signal reached the forward bias level required to make the diode conduct. This experiment only confirms the AC voltage readings you took.

 

Given that the resistor across the tacho leads did not work, my best guess would either be a faulty pickup coil, a broken wire in the path from coil to gauge, or a faulty gauge. My strongest suspicion would be a broken wire between the pickup coil and the firewall. This is where the maximum flexing occurs in the wires due to the engine moving on its mounts.

 

HTH

 

 

[dan tonner]

This is somewhat late, but will complete the thread....

 

1. Kudos to Scott Hendry. The RPM gauge pickup coil was faulty.
    
    
   
2. The pickup coil produces and delivers a pulse. That's all. It does not power anything. This being the case, I tapped into one of the ignition trigger coils with the RPM gauge feed wires. Problem solved.
    
    
   
3. I did check the continuity of the RPM trigger coil wires and there were no problemwith any from the coil outwards.
    
    
   
4. I managed to find a "used" coil at half the "new" price of $275.00 Canadian. (!!!!!)
    
    
   

 

 

Thanks to all, in particular, to Scott.

 

Fly safely,

 

Dan

 

 

[cscotthendry]

Dan:

 

I'm pleased you got a result. Also if what I added helped then that's good too.