a deal on vdo tach?

[white-eagle]

i wanted a tachometer and mentioned it to someone.  they said they could get me a deal on a new one.  i got a 3.5 inch tach with hourmeter and instructions for connections.  hook it to the alternator, set it for engine type and cylinders, and 12v for the lights to work.  easy.

didn't work.  i'm not sure which connection on the alternator, so i hooked it to the DC connection on the dn50 mounted on the back of the engine.  that's the big connector in the center.  measures 13.6 with the engine running.  maybe the tach needs to know it's a 2 cycle with 8 cyl.  only goes to 2c/4 cyl, then back to to 4 cycle.

vdo phone listed is out of business??

so, i need to know what screw to connect to on the alternator if not the dc.  and does anyone know how many pulses off the system.  i don't see any other posts, nor are pulse rates listed in the eagle manual i have.

[Len Silva]

I believe you want to connect to the "R" terminal. I can't help you with t6he pulse count.

[brando4905]

I saved this info from the yahoo GMC group----

Let me clarify, this is a 1973 Buffalo with 8V71 engine in a 4905a coach generator (stock , gear driven). Can someone please help me find the number of pulses/revolution is output by generator so I can PROGRAM my VDO multi voltage tachometer/hr meter. The parameters of the guage range from 0-200 pulses per engine revolution.. My guess is that it pulses between 15 and 30 per rev. Anyone els's guess is encouraged as a starting point before I wire it up, Thanks, Glenn

Glenn,  you'll need to check your manual to determine the gear ratio used in the alternator drive.  2 different ratios have been used on 8v71s the 2.75 ratio is more common on 24 volt installations and results in 16.5 pulses/revolution.  The 2.29 ratio is typical on 12 volt installations and makes 13.75 pulses/revolution.
Regards
Jerry 4107 1120

[white-eagle]

Thanks guys, that's the info i wanted to get.  Len, i was afraid of that, that i would need to use the "r" connection.  i already had the connection out to the rear box from the dc, but i don't see anything showing on the meter.  The "r" for relay connection doesn't have any wires connected to it which means i need to climb inside the rear axle area to get to it.  Now i'm going to have to get greased up again!

Brando, thanks for saving the info.  i'll let you both know how it works soon.

[bobofthenorth]

Tom its been a long time since I replaced my VDO tach but I think I recall that the pulse settings weren't used with the "R" alternator terminal.  I believe the "R" output is actually an AC waveform.  As I recall on mine there were some DIP switches that needed to be set to select the input type and you only used the pulse settings for pulse input.

But all of this could be BS because my memory was never that good.   

[JackConrad]

Our Tach that feeds from the R terminal had to re re-calibrated when we changed from the gear drive 50 DN to a belt driven truck type alternator.  On the back of the tach are several DIP switches and a small potentiometer. When I called Precision Speedometer (where we purchased our tach) they asked how many pole the alternator was and the diameters of the engine and alternator pully.  Although close, we will need to get final adjustment by confirming RPM with a digital handheld tach.  Jack

[white-eagle]

and the solution is:

The R terminal works.  just for test purposes, i climbed under and managed to tie a wire around the r terminal, then ran it to the back bumper area where we had 12 v power and ground, and the rear start/stop buttons. i crimped a connector on, connected the wire from the relay monitor terminal ("R") on the alternator to the #8 pin on the tach, no connection on the #7 pin, then power + to #4 and grnd to #3. when i turned it on, i held the adjustment button in on the tach, per instructions, until it said pulse, then entered the 13.75 that i got from brando's post
at that point the tach reset the hours to zero, saved the pulse setting to it's memory, and we were ready to check the rpms, or burn the sucker up. 
i started the engine and it showed idle about 800.  Reached over and moved the throttle up till we had some wind blowing from the fan and it measured about 1900 rpms.

Now the easy part begins.  Finding a wire that runs to the front, finding a place to cut a hole, or mount the tach, and connecting the power and lights to something already in the dash.  I'm sure i'll be posting something soon about why my air guage doesn't work when i turn the lights on, or some such question.

thanks again to the mak board for bailing me out.

[RTS/Daytona]

 For Detroit Diesel Delco gear driven 50DN 270 Amp 24 volt alternators "R" Terminal hookup
                         The "R" terminal output is 12 volt -
Most 6V92 / 8V92-->(2.94 (53/18) gears & 6 pulses per rev=17.66 pulses/rev)
Most 24 volt - 8v71->(2.75 (55/20) gears & 6 pulses per rev=16.5  pulses/rev)
Most 12 volt - 8V71>(2.29 (55/24) gears & 6 pulses per rev=13.75 pulses/rev)           

[Dreamscape]

Pete, Very valuable information!

Now Tom, when you get that sucker all wired and installed we expect a full report!

Paul

[white-eagle]

For Detroit Diesel Delco gear driven 50DN 270 Amp 24 volt alternators "R" Terminal hookup
                         The "R" terminal output is 12 volt -
Most 6V92 / 8V92-->(2.94 (53/18) gears & 6 pulses per rev=17.66 pulses/rev)
  Most 12 volt - 8V71>(2.29 (55/24) gears & 6 pulses per rev=13.75 pulses/rev)           

Pete, my "r" terminal measured up to 12 volts.  putting in 13.75 seemed to be a believable result, but are you saying that it should be 17.66?  i guess i can try that number instead of the 13.75 and see if the rpm's shown seem as believable.  like i said, i got around 800 rpm's at idle, then revved it up to 2000.

[RTS/Daytona]

the output of the "R" terminal is a full wave rectified sine wave - it's the FREQUENCY of that "inverterted sine wave" that is proportional to the rpms

the 12volt output reference is the style of output voltage - 12 volt verses 24 volt - the "R" terminal is half the voltage output of a 24 volt 50DN - yes the actual voltage may easily go to 13.2v - 14.1v volts as the output voltage of the 50dn increases -

but it's not the voltage that the tach is measuring - it's the frequency of the inverterted sine wave

the pulse per revolution is determined by the 50DN gear ratio (53 teeth driving 18 teeth = 2.94)  times the number of poles in a 50DM (3 poles) x 2 because the full wave is rectified and inverted

so  2.94  x  3  x 2 = 17.66 pulse per revolution


You can take the instructor out of the classroom - but you can't take the classroom out of the instructor

Pete (IBM INSTRUCTOR 30 YEARS)  - Retired

P>S> - if you do the math (I Did)  you got 800 rpms with 13.75 at idle  - you'll get 622 rpm if  you set the tack to  my 17.66 --- trust me  :-)

[luvrbus]

Tom, Pete is on it your idle speed should be 650 rpm and the governed speed should be 2100 for the 8v92 with automatic


good luck

[white-eagle]

Pete,
of course i trust you!  We've shared beverages and toasts on NY Eve at the bus rally  .  No need for me to do the math.

So i guess i'll change the pulse number.  Now i'm trying to find the wire in the front to see if it comes from the "r" terminal.  there just doesn't seem to be comment like "Blue #67 front panel section g goes to alternator relay monitor terminal."

what's the tachograph wire in the dash.  i looked up on google, but tachograph seems to be a clock.  if that's not relevant, then skip the explanation.

Cliff: thanks for the governed speed.  one reason i want this tach is yesterday, i caught myself doing close to 80 with a tailwind and heavy foot.  i'd like to know what the engine speed is at 80mph.  i'm also assuming the governor will keep me from blowing this up with too heavy foot.

[RTS/Daytona]

tachograph was a RECORDING type speedometer - keeps tabs on the drivers speeds

[luvrbus]

Tom, 2250 rpm doesn't bother the 8v92 that is were mine is set over that for long periods will cause problems.   

good luck