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Is there a way I can test the Vehicle Speed Sensor?

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Is there a way I can test the Vehicle Speed Sensor?

Postby ocalhoun » Tue Sep 11, 2018 5:17 am

I'm rebuilding a salvage 2013 Transit Connect with automatic transmission and gas engine (US model), and everything was going great until I was finally able to drive it off the trailer. It reversed off the trailer just fine ... and then I found out that none of the forward gears are working, only reverse & park. In forward gears, it acts like it's in neutral -- engine revs freely, and the van doesn't move whatsoever.

I've read that this may be caused by a faulty vehicle speed sensor (VSS), because the transmission may refuse to shift between forward gears if it doesn't know how fast the van is moving. I think I've located the sensor (on the back of the transmission just above the axle). It's still plugged in, and there's no external signs of damage (though it has a bit of extra heat insulation around it along with a ziptie -- is that factory, or does it mean someone has screwed around with it before?)

So I'm fairly sure that either:
A) I have a bad speed sensor, or
B) My transmission is super- f***
Obviously, I'm hoping for (A), but I'd really like to be able to test it before ordering any parts that may end up not fixing the problem ... so is there any way I can test the speed sensor, given that I'm not able to drive in a forwards direction?

One idea is to pull it with another car down the road for a short distance with the ignition on and in neutral, then see if the speedometer registers anything.
Another idea is to put it nice and high on jack stands, so both front wheels are well off the ground, pull the parking brake (and maybe even chock the rear wheels) and then put it in reverse, which does work, hit the gas and again see if the speedometer registers anything. (Is the speedometer supposed to work in reverse, though? It should, right?)

But are there any better/safer ways to test the speed sensor?
(And while we're at it, are there any other things I should check that could cause the transmission to not work in any forward gears? Anything else I can fix short of 'replace the whole transmission' would be great! It was previously involved in a front-end wreck ... is there perhaps some safety interlock switch that prevents the transmission from going into forward gears in the event of an accident? I've heard of switches that will shut the engine off if there's an accident, so maybe that's possible? If there was such a thing, where might it be located, and how would I reset it?)
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Re: Is there a way I can test the Vehicle Speed Sensor?

Postby liteace » Tue Sep 11, 2018 3:57 pm

Best thing to do is get a Ford diag tool plugged in and see what the transmission is doing or not doing, and also see if there is a VSS code, you could jack up one side put it in drive with the engine running as spin the wheel to see if the speedo needle moves. I cant see if having anything to do with the VSS as when you select a gear the vehicle will be stationary as will the VSS unless the PCM has completely lost the VVS signal, has the vehicle got ABS brakes ?
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Re: Is there a way I can test the Vehicle Speed Sensor?

Postby ned » Tue Sep 11, 2018 5:33 pm

I might be wrong but isn't the vss built into the abs and the plug you have found on the gearbox the reverse light switch?

I don't know much about automatic but surely they have a selector on the gearbox so I would start there, get someone to select reverse, neutral and drive while you look at the selector and see what happens.
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Re: Is there a way I can test the Vehicle Speed Sensor?

Postby ocalhoun » Tue Sep 11, 2018 6:04 pm

ned wrote: get someone to select reverse, neutral and drive while you look at the selector and see what happens.

Oh, already did that. The gear selector seems to move normally through each gear.

I might be wrong but isn't the vss built into the abs and the plug you have found on the gearbox the reverse light switch?

I'm pretty sure that's not the case -- this sensor is listed on a bunch of different parts websites as the vehicle speed sensor, and it looks just like what I've found on the back of the transmission; same metal clip on the side and all.
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Re: Is there a way I can test the Vehicle Speed Sensor?

Postby ocalhoun » Tue Sep 11, 2018 6:09 pm

liteace wrote:Best thing to do is get a Ford diag tool plugged in and see what the transmission is doing or not doing, and also see if there is a VSS code,

Heh, that would be nice, yes ... but probably cost more than just buying a new VSS and hoping for the best. I might be able to borrow an OBDII reader, but I'm not sure if it will be able to read such codes.
unless the PCM has completely lost the VVS signal,

That's the theory ... I hope.
Pretty much the only thing in between me and the competing theory that the whole transmission needs to be replaced or rebuilt.

It does kind of make sense... With no speed sensor signal, the transmission has no way to know the speed; for all it knows, the van could be sitting still ... or it could be coasting along at 70mph. So it would make sense that it doesn't want to shift into 1st (or any other gear) on the off chance that the vehicle is currently moving at high speed and that could cause transmission and/or engine damage, or cause the front tires to skid and lose control.
has the vehicle got ABS brakes ?

I'm pretty sure it does ... I know it has the RSC traction control thing, which I think also implies ABS.
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