My Speedometer is a Liar! Is my dealer?.. :-)

Maintenance/Repairs
,
1

My speedometer shows 15 mph at idle stop in Drive gear. Sometimes it fluctuates simultaneously with the engine. Then tachometer also fluctuates. I took it to the dealer, and they wanted to replace an instrument cluster (under the dashboard), but I declined. If it is a cluster, why does it fluctuate with the engine and tachometer? I took it to the shop, but it did not do it then (just like when we come to the doctor and suddenly nothing hurts :-). What do you think? Or do you want me to call your show and exchange a few jokes with you (before and after you give me a solution)? Thanks.

2

What year Sentra and how many miles on it? Have you owned it since it was new or did you buy it last week? Have you had any other electrical problems with your car?

3

Also, the dash cluster on most of the newer cars and trucks are 100% electronic. They basically show readings given by the ECU, and can give very erroneous readings if there are circuit failures within the cluster.

For your issue, replacing the cluster is usually the correct repair. However, sometimes they can be repaired if it is a matter of removing the cluster and cleaning the electrical connections on the back side. I did this successfully with two different cars when the gauges began to fluctuate. Hitting the dash stopped the fluctuations temporarily. Cleaning the electrical contacts on the back side stopped them for good.

4

OMG…why would you assume the dealer is lying? Do you think maybe the car has an issue?

5

Modern instrument clusters are easy to bench test and expensive to replace. Bench testing is the first step.

Without having access to the schematic, I’m going to offer a wild guess that the vehicle speed sensor and the tachometer have some ground or power circuitry in common and a problem in that circuitry is causing an induced or misdirected signal to the speedo, causing the erratic behavior when stopped, and also “dropping” voltage spurriuously from the tach circuit, causing the fluctuations.

Or…perhaps the alternator bridge recitifier or filter (the “voltage regulator” portion) has a bad diode or component causing erratic voltage. On second thought, check this possibility first. Post back with the results.