Speedometer acting weird

[cmarsh90]

At a track event over the weekend my dad's 91 NSX with just under 100K on it exhibited some weird behavior.

Several times after being on track for 10 mins or so the speedometer quit working (meaning it went to zero and stayed there), additionally when this would happen the red line would drop to about 7K. Re-starting the car did not seem to fix the problem. Additionally after the car sat for 20 mins or so it would act fine for awhile then the problem started again on track after 10 mins or so. This was on a very hot day (90s), but everything else seemed find (namely temperature), other than the lower redline the car seemed to perform fine, although at one point the check engine and TCS light came on. Once when driving slowly around the pit area the speedometer jumped around erratically.

So based on a problem from before I'm thinking it could be the Main CPU relay, but I'd like more info from the experts on if this diagnosis is correct, thanks.

 

[zahntech]

PGM-FI Main relay? not so much no....You need to jumper the connector below the glove box and get the codes out of the engine ECU and the TCS too most likely you will find a code 17(vehicle speed sensor) in the Engine computer. The vehicle speed sensor or VSS is sent a 5 volt referance voltage and then as the wheels spin the VSS sends a 0 to 5 volts wave form to the computer as the vehicle goes faster the wave form speeds up and the computer makes a calculation as to determin the speed of the car. sounds like your VSS is failed but a proper diagnosis will be required to be sure..JZ

 

[cmarsh90]

Can you give me the location in the car of the vehicle speed sensor and the part #? Thanks.

 

[fox32223]

What I can tell you is that I had a 92 Legend that exibited the same exact behavior that you are describing. I replaced the VSC module that was mounted on the transaxle.

Since these two cars were designed about the same time, it's possible they may share some of the same technology.

 

[PaulL]

On a related topic ... could a bad VSS cause speedo readings that are 3 to 5 mph too high?

On edit: The VSS location is shown on page 13-10 of the service manual. It's No 7 on the pic below.

 

[zahntech]

3-5 mph faster than actual speed is fairly normal for most cars, tire sizeing can also effect acuracy.

 

[RogerHuey]

Had same thing on 91 with 34 K miles.Also if gonig slow noticed i had lost power steering.The problem was a loose connection at speed sensor.Simply located and push down tight and no problem again.That was about 9 mons ago.It was hard to locate though.I had only had my car for several weeks.Problem learned of on this site.And it cost nothing.