[Vwdiesel] Cruise control on a diesel

[Roger Brown]

HWY9FERGS at cs.com wrote:
> In a message dated 9/20/2005 8:06:24 AM Pacific Daylight Time, Miser writes: 
> 
>>How does cruise control work on a diesel? Can I make one? 
>>Roooogggger!
>>Miser
> 
> You're probably wondering what they do for a speed input.  The 85 Jetta (RIP) 
> has a set of magnets on the halfshaft hub. I think this may have been stock, 
> because the controls were on a stock looking turn signal stalk. Anyway, the 
> magnets' fields get picked up by a little coil sensor mounted adjacent to them.  
>  Lots of aftermarket CC's did this, even for gas cars. They put them on the 
> drive shaft or wherever.  Later models didn't seem to use the magnets.  The VW 
> and other diesels came with a W terminal on the alternator which can be used 
> either for a ignition signal substitute, or for a tach, or both I suppose, but 
> now that I think of it, the 85 has W terminal and a tach, so maybe the W 
> terminal can't handle feeding both, thus the magnets.  Anyhow,  in summation, an 
> aftermarket unit with magnets will work in any case, but if the unit needs a gas 
> car ignition input, you would do best to have an alt with the W terminal.  I 
> can imagine also that some of the magnet style units might find the signal 
> from the W terminal usable, but I can't be held responsible for damage due to 
> improper/misbehaving electrons.   I trust this has been sufficiently confusing.  

At least on the units I have looked at, the W terminal signal is way to high a frequency to be used to feed a cruise control. 
They are usually set up with calibrations for differing number of pulses per mile, as from a Vehicle Speed Sensor (VSS).  The 
one I used went up to about 8000 pulses per mile.  Assuming you are using a 4-cyl gas engine turning about 3000 RPM at 60 MPH, 
that equates to about 6000 ignition pulses per mile, so using an ignition/tach signal works well, I think I set mine to around 
5000-6000/mile.  Problem with the W-terminal input is that is is about 12-13 times as fast since the signal is the unfiltered 
output of the alternator, so you would be getting 70,000-80,000 pulses per mile and I doubt the standard cruise control units 
could handle that high a frequency without pre-conditioning the signal.  Dakota Digital makes a programmable frequency divider 
that can generate a more "ignition-like" signal, or the optical pickup unit I designed makes an exact "ignition" signal that 
requires no calibration.  Mine drives my cruise control and the tachometer with no problem.

-- 
    Roger