VDO Tach runs fast; Diode makes it worse

[aeromech]

» Tabular list of translations | always
» List of translations starting with the same letters
» MAS | Mass | Maß | Masse | Massé | Maße | mäße | massefrei




ADJ

massefrei | - | -
massefreier | massefreie | massefreies


massefrei floating {adj}electr.
off-earth {adj} [Br.]electr.massefrei verbinden [ugs.] to connect without ground connection [floating] [Am.]electr.
_________________
Lead Mechanic: San Diego Air and Space Museum
Licensed Airframe and Powerplant Mechanic
Licensed Pilot (Single engine Land)
Boeing 727,737-200-300-400,757,767
Airbus A319,320,321
DC9/MD80
BAe146
Fokker F28/F100
VW type 1 1962,63,65,69,72
VW Type 2 1971 (3 ea.) 1978, 1969
VW Jetta
VW Passat
Capable of leaping tall buildings in a single bound

[mikedjames]

The real problem is the long 15 foot wire from the front to the back of the bus compared with the shorter 3 foot wire from a front engined car.

The explanations about 'ringing' make some sense.
As well as the diode

Other ways to stop ringing used in electronics are to put something like a 100 ohm resistor in series with the wire at the coil end to reduce the ringing.

Or use the diode with its band pointing to the distributor and a resistor to 12 v going to the signal on the tacho to pull it up to 12v

Points ------> diode stripe pointing left ----Z--> resistor about 500 ohms----> 12 V

connect point Z to the tacho terminal




You may also find that using different wire routed differently may change the behaviour.

Find somebody with an oscilloscope and get them to probe the signal at the dashboard end.
You will need solid pulses at all RPM.

[pittwagen]

I have had this same tachometer in my 79 CA bus for about 2 years. I originally bought the tachometer from CIP1 in March of 1987 and ran it in my 73 Sports Bug for years.

There is absolutely nothing in the instructions, dated >05/84, to reference a need for any diode. The instructions indicate the tach is compatible with breaker points and electronic ignition systems. My Sports Bug uses breaker points and a blue coil. My bus has the standard VW electronic ignition system that is part of the CA emissions package.

No needle bounce, nothing. It just works!

FYI.

[SGKent]

It says 12v/233 hz 4 zyl./takt

here is the deal.
12V
233 HZ to read full scale at 7000 RPM
4 Cyl engine
Tachometer

The Tach is a 7000 RPM tach (see face). It takes 233 HZ (pulses) PER SECOND to cause it to read at 7000 RPM. VDO publishes this online if you get their manuals and read on the calibration of the gauges. There may be a small adjuster on it on a side to allow fine tuning only if say 233 HZ were 6700 RPM. They give instructions for fine tuning one using a signal generator.

So here is the math:

7000 RPM / 2 revolutions per cycle = 3500 complete engine cycles per minute X 4 cylinders = 14000 ignition pulses per minute / 60 seconds in a minute = 233.33 pulses a second.

If that tach is hooked up to

12V+
12V -
Distributor green wire side of coil

then it should read very close to dead on.

If when you do that it bounces then you are getting extra signals in the 15' run and a diode should stop it because it will not cut off the pulses, just the returns (unless the tach electronics wants to see a sine wave in which case your torrid should help). If it goes flat then you are either getting noise in the signal wire that looks like pulses to the tach or you are hooked up to the power side of the coil by accident.

here is an old post that may help too:

[Krautski]

For accuracy's sake "takt" is actually stroke. As in 4 stroke.
_________________
BUS FOR SALE

[aerosurfer]

Thanks Steve, everyone else

I put on the ferrite donut just before the tach. It seems to have gotten the reading closer to my Tach/dwell. It still reads high; 3500 on the gauge is showing about 3200 on the portable one. 2200 is just over 2000.

I figure I'll check it regularly, and wait to stumble on another tach
_________________
Rebuild your own FI Harness..My Harness

77 Westy 2.0L Rockin and Rolling Resto!

72 Sportsmobile (sold)
79 Tran$porter... Parts car money machine (gone)

[SGKent]

[mkriglet]

Does anybody have a wiring diagram for this exact tachometer? The terminals are. +, 1, -

[aeromech]

Power
Points
Ground
_________________
Lead Mechanic: San Diego Air and Space Museum
Licensed Airframe and Powerplant Mechanic
Licensed Pilot (Single engine Land)
Boeing 727,737-200-300-400,757,767
Airbus A319,320,321
DC9/MD80
BAe146
Fokker F28/F100
VW type 1 1962,63,65,69,72
VW Type 2 1971 (3 ea.) 1978, 1969
VW Jetta
VW Passat
Capable of leaping tall buildings in a single bound

[mkriglet]

Thanks, I just temporarily hooked it up like that. It works good. I got this tach in a box of parts and I want to use it in my bus. I'm glad it works. Thanks again for the help.

[aeromech]

Sure but keep in mind that because of the distance in a bus between the coil and the tach, sometimes installing a diode in-line with the points wire is required to smooth out needle bounce
_________________
Lead Mechanic: San Diego Air and Space Museum
Licensed Airframe and Powerplant Mechanic
Licensed Pilot (Single engine Land)
Boeing 727,737-200-300-400,757,767
Airbus A319,320,321
DC9/MD80
BAe146
Fokker F28/F100
VW type 1 1962,63,65,69,72
VW Type 2 1971 (3 ea.) 1978, 1969
VW Jetta
VW Passat
Capable of leaping tall buildings in a single bound