BACKYARD MECHANIC: Speedometer gives some peculiar readings

Hey there, time traveller!
This article was published 19/03/2010 (6016 days ago), so information in it may no longer be current.

QUESTION: My 2004 Buick LeSabre speedometer sometime screws up in a couple of ways. Occasionally when I start the vehicle, and before it moves, the speedo will jump up randomly to show a speed of, say, 80 km/h. When I move the vehicle, the speed display will start at 80 and show I’m doing 130 in a 50 zone. If I restart the engine, the speedo will drop about 15 km/h. If I do this four or five times it will then register zero and work fine for a day or a week.

The speedo also sometimes refuses to back down sometimes, for instance if I am travelling at say 100 km/h and slow down to 60 then it still shows 100. Do you think the problem lies in the gauges or the sensor?

ANSWER: The problem could be a loose needle on the speedometer but more likely it is caused by binding of the bushings in the electric motor that moves the gauge needle. A speed sensor wouldn’t cause this problem.

It is possible to disassemble the speedo assembly and replace the motor with another used one, although dealers don’t do this. There are instrument-repair shops dealers use, so ask where the nearest one is located if you don’t want to tackle this problem yourself. While the electronics of the instrument cluster could be at fault, they seldom cause your vehicle symptoms.

QUESTION: I have a 2001 Impala with keyless entry. When I shut the car off and leave the key in the ignition, after approximately one minute, the horn will blast five times. Remove the key, exit the car, lock it (manually or remote), and it will sound again — always only five times. Periodically, the security light will come on and stay on until I shut the car off. Four times recently, nothing happens when I try to start it. After five to 10 minutes it then starts.

A dealer found some ignition switch codes and told me a worn switch and tumbler assembly would cause it. I had the switch and tumbler changed, but after two days the horn is at it again. The immobilizer checked out good. It’s getting a little annoying to have the horn go off almost every time you shut it off or try to start it. I’m thinking it has something to do with the factory security system, but the dealer can’t find anything. Have you ever come across this and can you please steer me in the right direction?

ANSWER: The horn in your Impala is controlled by the body computer, which grounds a relay to make the horn blow. Because the horn action is fairly consistent, I would think the body computer is causing it to blow. If there is an additional security system besides the immobilizer on the car other than the factory body computer system or a remote start system, they should be disabled, as it could be causing the horn to blow, too.

The dealer can reflash the program in the computer, but your description makes me think it may be a faulty body computer and it will need to be replaced and reprogrammed.

The content-theft system, the keyless entry system and the trunk trap alert system also all signal the body computer to blow the horn. None of the system descriptions match exactly your horn symptoms, but I would check the trunk trap system first.

The trunk trap alert option was offered only in 2001. A motion sensor in the trunk detects movement in the trunk, causing the horn to blow and release the trunk lid. A faulty sensor in the trunk could cause the system to blow the horn, and so could a body computer that had the feature activated when the car doesn’t have this option.

You can defeat the option by putting a jumper wire in place of the sensor connector under the package tray in the trunk. This is the only repair, as the sensors are no longer available.

Jim Kerr is an experienced mechanic, instructor and member of the Automobile Journalists’ Association of Canada.

kerr.jim@sasktel.net