Vehicle Speed Sensor Testing - Cruise Control

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Vehicle Speed Sensor Testing - Cruise Control

Postby sefiroxx » Sun Jul 28, 2019 7:32 pm

Working on some other stuff on my 5th and figured I might as well fix the CC. It was working before the last rebuild when I swapped in a tranny. Not wirkng now. Will do the diagnostics to see if something is cancelling but will focus on the VSS first.

I have 4 of them, One the original that failed a while back. I want to run a diagnostic on each of them (rather than swapping them and testing them , painful).

From the FSM, it's a 3 wire system. But nothing on the Internals. Guessing 1 pair is a 12v circuit and the 3rd wire is a either a control signal or a return signal from/to the CC computer.

The electronics are not wrapped around the speedometer cable, just clamped to the side. Mechanically, the speedometer cable likely spins a magnet/iron than induces a electromagnetic signal on the 3 wire setup causing some sort of change in the signal.

Anyone ever bench test one of these? How?
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Last edited by sefiroxx on Tue Jul 30, 2019 3:40 am, edited 1 time in total.
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Re: Behind Speed Sensor Testing - Cruise Control

Postby 93celicaconv » Mon Jul 29, 2019 12:32 am

Is your "5th Gen" an ST185 USDM? Or a JDM?
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Re: Behind Speed Sensor Testing - Cruise Control

Postby FC Zach » Mon Jul 29, 2019 12:41 am

sefiroxx wrote:Anyone ever bench test one of these? How?


There are instructions in my Chilton manual on how to test the speed sensor. If I remember when I'm home, I'll share those here.

Check all the pads for the switches at your clutch/brake pedal's returned position. If they've deteriorated and aren't there, the CC will never function. Glue a penny or whatever if missing. . something like this (not an alltrac, just an example I googled).
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Re: Behind Speed Sensor Testing - Cruise Control

Postby sefiroxx » Mon Jul 29, 2019 4:57 pm

:bang head:

Found it in the FSM. Not sure how I missed it (beverage)


[moderator, change title please from 'behind' to 'vehicle']
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Re: Behind Speed Sensor Testing - Cruise Control

Postby FC Zach » Mon Jul 29, 2019 9:34 pm

sefiroxx wrote:moderator, change title please from 'behind' to 'vehicle'


You can do it. . . Go to the first post and edit the subject from there (I may be confusing this forum with another, if that doesn't work).
1992 Toyota Celica AllTrac
1993 Yamaha GTS1000
1987 Mazda RX7 Sport
1979 Yamaha G1A, rotary powered with Sachs KM24
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Re: Vehicle Speed Sensor Testing - Cruise Control

Postby sefiroxx » Tue Jul 30, 2019 3:43 am

FC Zach wrote:
sefiroxx wrote:moderator, change title please from 'behind' to 'vehicle'


You can do it. . . Go to the first post and edit the subject from there (I may be confusing this forum with another, if that doesn't work).
Thx,

Could not change it in Tapatalk like I can on other forums. Was able to change it directly logged into the website.
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Re: Vehicle Speed Sensor Testing - Cruise Control

Postby Numbchux » Tue Jul 30, 2019 7:38 pm

Diagnostic mode is stupid easy to activate. Hold the stalk in "set" or "resume", and then turn it on. From there, the light will turn off if any of the cancel switches are activated (brake, clutch, ebrake, etc.), flash twice and 3 times for "resume" and "set" respectively (or maybe the other way around). While driving, the light will begin to flash above about 30mph. This makes it very easy to narrow down if the cruise unit is getting good signals. If your light doesn't flash in diagnostic mode, and you don't have a break in the wires to the sensor, than the sensor is bad.

I was getting intermittent speed sensor issues from mine last year (oddly enough, beginning with a round of cheap LED taillight bulbs...maybe a coincidence, but perfect timing, and another symptom disappeared when swapped back to incandescents). After trying several things and driving around to look for a failure pattern, I followed a suggestion that was posted here. The Auto transmission xT18xs use the speed signal coming from the dash (tied into the signal to the ECU), the Manual transmission ones have a separate speed sensor inline with the cable. This sensor is unique to MT T18 Celicas, and is discontinued from Toyota. BUT, you can grab that signal from the dash. I spliced into the speed signal wire at the ECU, and ran a ~18" chunk of wire over to the cruise unit and connected it in place of the signal wire from that bespoke sensor. My system works perfectly, now.
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Re: Vehicle Speed Sensor Testing - Cruise Control

Postby sefiroxx » Mon Aug 05, 2019 8:21 pm

Update

Sure enough, bad VSS. I had a replacement tranny that I put on and must of installed the VSS from our original owner without testing it.

So, have 4 of them. 2 bad, 2 good. One of the bad ones I'm sure is just a short in the wiring harness. The other is giving all sorts of numbers. Noticed the internal of the mechanical part is plastic while the others look like metal.

Will eventually run a shorted wire test on the bad ones and do some interchange of harness with mechanical part. (Might be easier to do rather than connecting the computer speed signal per above. Next time I pull the wiring apart, I'll do the has bypass hookup)
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