[DeTomaso] More cruise control issues

Christopher Kimball chrisvkimball at msn.com
Mon Aug 31 14:12:38 EDT 2009


Well, then I guess I'll have to figure out a way to use the magnets (which I'd rather avoid) or the speedometer cable (which I prefer).

Thanks,

Chris



> Date: Mon, 31 Aug 2009 10:26:36 -0700
> From: asajay at asajay.com
> To: detomaso at realbig.com
> Subject: Re: [DeTomaso] More cruise control issues
> 
> -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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