90 Caprice vss location?

Is it located on tail housing or behind speedometer tried to order one from advance auto but its showing something that goes behind speedometer. Had code 24. Also if a code comes up for the ignition module but when I reconnect negative cable its cleared is it telling me of a future problem.

Earlier B-Cars have a pickup behind the instrument cluster and used a standard Bowden cable to drive the speedometer. Later vehicles, IIRC from about 1988-on, used a VSS mounted on the trans in place of the cable drive thimble.

If you look at the tailshaft on the left (driver's) side of the vehicle you should see either a cable attached or a VSS with electrical connector installed (likely the latter).

Either way, it is important to diagnose the problem rather than just start replacing components. The error code is telling you that there is a problem in the VSS circuit, not necessarily with the sensor itself. There are electrical connectors, a buffer/signal splitter box in some applications with cruise control, and there is a plastic gear which drives the transmission mounted VSS which can fail. Buying a new sensor may be a complete waste of money and time.

What would I need to do to pinpoint the exact problem the speedometer was working fine until this problem it doesnt move unless I floor the gas pedal. That code 24 also seem like it changed the way my trans shift.

Depending upon the transmission you have, the diagnosis will be different. Both transmissions are shifted hydraulically, and that is controlled by the governor and the throttle valve assembly.

A TH200-4R uses a governor which also drives the speedometer. It is accessible through the transmission pan and above one part of the lower valve body. If the trans is shifting oddly there may be a problem with plastic gearing driving the governor assembly.

A 4L60 uses a governor is driven by the gearing from the tailshaft. The VSS / speedometer cable thimble is also driven by gearing from the tailshaft. If the trans is shifting oddly and the VSS/speedometer is not working, there may be a problem in the tailshaft housing, such as debris or contamination which has damaged the plastic governor and speedo/VSS gears.

In either transmission, the TCC lockup is controlled by the ECM which relies upon the VSS to determine when to lockup the converter. If the only shifting problem is with the TCC lockup, the VSS system is the first suspect. If the transmission shifts oddly in all ranges, look toward the governor and TV system as well as the VSS.

Through 1990, it is located on the back of the speedometer head. It wasn't until 1991 that the sensor was moved to the tailshaft, with the advent of the 'bathtub' Caprice and its digitally controlled speedometer.

Ok appreciate that would I be able to reach it from the top of th dasboard

Good info, Si. That nails it down a bit. The problem may simply be the Bowden cable or the plastic driven gear itself. The comment that "it changed the way my trans shift" is a bit of a concern, however. I'm guessing this is a civi Caprice or non-HD/SEO with the 200-4R, but it might also be a 4L60. There would be a different diagnosis for each, depending upon what the OP meant by shifting differently.

FWIW, I recall the F-cars changed to a trans mounted VSS much earlier. My '86 TA has a VSS on the tailshaft of the 700-R4, while Camaros used the speedo mounted VSS until the 1990 M/Y. Odd that the B-cars would hold onto the speed mount longer, but that does make diagnosis easier. Repair, on the other hand, might not be as easy. There is always the VSS buffer module (yellow box) to consider. On the F-cars the buffer was on the back of the cluster, except Firebirds which mounted it near the right relay panel. I have no clue where it would be on the B-cars, but I'm guessing that YOU do.

Ok appreciate that would I be able to reach it from the top of th dasboard

You don't need to go underneath the dash to get it, in fact you really can't do that. You have to pull the speedometer head up through the front (steering wheel side) of the dash.