[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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