[Vwdiesel] Cruise control on a diesel

[Hayden Chasteen]

in the A2s the vehicle speed sensor is on the back of the speedo.  It is a 
little plug in device with 3 wires that measures the revolutions of a disk 
on the backside of the speedo. Since all of these speedos are interchangable 
(within certain limits) you should just have to find one with the sensor as 
well as the gas version of the cruise and you are all set.  Hayden
----- Original Message ----- 
From: "Roger Brown" <r.c.brown at ieee.org>
To: <vwdiesel at vwfans.com>
Sent: Tuesday, September 20, 2005 12:16 PM
Subject: Re: [Vwdiesel] Cruise control on a diesel


> 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
>
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