[DeTomaso] More cruise control issues

asajay at asajay.com asajay at asajay.com
Mon Aug 31 13:26:36 EDT 2009


-That- brought back memories.

I used an old CompuCruise computer in an older car of mine that had a  
manual transmission.  The Tach signal was only -one- source of input  
for the cruise control.  The others were the brake switch and the  
third was a set of magnets attached to the drive line and picked up by  
a nearby sensor.  -That- was the speed sensor.

The tach adapter can NOT be used as a speed sensor.  After all, you'll  
get the same rpm in 1st gear, second gear, third, etc.  i.e. 2,000 rpm  
in first is one speed, while 2,000 rpm in 5th is totally different.   
You -can't- use the tach as a speed sensor.

The tach sensor was designed for two purposes, one to monitor the  
engine -was- running and two, if the rpms increased by xx amount over  
yy time, then it would cut out the cruise control to prevent the  
engine from over-revving.

The brake signal was of course to cut out the cruise if the brake was applied.

The magnet sensors are the only reliable way to measure vehicle speed.  
  Many OEM installs do this one of two ways.  They either have a  
similar speed sensor assembly mounted at the hub (toothed wheel with  
pickup like a crank sensor), or they put a device in-line with the  
speedometer cable (more old style if I recall now).

So, I think the tech is correct, you'll need to install some kind of  
speed sensor.

Asa Jay


Quoting Christopher Kimball <chrisvkimball at msn.com>:

>
> Today I called MSD for some clarification on the wiring of my new   
> electronic cruise control, and the tech I got said a few things that  
>  made me question his expertise, so I thought I'd bounce some ideas   
> off the forum to see what y'all think.
>
> Although most of the wiring is now clear to me, the guy said the VSS  
>  wire must somehow be attached to the drive train.  He said magnets   
> attached to the u-joints or a speed sensor in line with the   
> speedometer cable were my only options.
>
> I asked if there wasn't some wire I could take from the MSD AL6 box   
> I'm using, or the MSD tach adapter I also use, but he said no, it   
> had to be from the drive train due to the fact the signal has to do   
> with speed and not the tach RPMs.  It seems to me the RPMs would   
> register speed exactly like the drive train would, the faster you   
> go, the faster the RPMs in any given gear.  Keeping the RPMs   
> constant would naturally keep the speed constant, too, so  I see no   
> reason why some kind of signal from the tach wouldn't work as a   
> speed signal.  Any thoughts?
>
> Have any of you had success with the electronic cruise control, and   
> if so, what did you use for a VSS signal?  Is there a metric signal   
> generator that will work with our metric ZF speedometer cable?
>
> Thanks for your help.
>
> Sincerely,
>
> Chris
>
>
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