BACKYARD MECHANIC: Speedometer error an easy, inexpensive fix

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This article was published 22/11/2013 (4620 days ago), so information in it may no longer be current.

QUESTION: I recently switched a 263 six-cylinder engine with a power glide cast iron transmission to a 1978 305 V8 with a 350 tranny. The only problem now is my 1960 Pontiac speedometer does not match with the 350 transmission. When I am travelling 60 m.p.h., the gauge is showing 83 m.p.h. An auto electric company will make a compatible set of gears for a huge amount of money. Is there an easier way to solve this problem? Right now I am using a GPS. Eckler has gears that go in the transmission end of the cable. Is this a solution or is there another cure without spending a fortune?

ANSWER: This is old-school mechanics and there is a relatively cheap way of correcting the speedometer error you have in your classic Pontiac. You need to change the speedometer gear or gears at the transmission. Right now the speedometer is reading about 38 per cent too fast. You need to install gears that will slow the speedometer cable by the same 38 per cent.

At the transmission, disconnect the speedometer cable and then remove the bolt and clip that hold the speedometer-driven gear support into the transmission extension housing. With the clip removed, the driven gear support can be pulled out (there is an o-ring seal on the outside edge, so it may seem snug). The speedometer-driven gear can then be removed and you should count the teeth on this plastic gear. You need a larger gear with 38 per cent more teeth.

GM makes many different driven gears. I am familiar with about 17 different gears starting with 18 teeth and ranging up to 45 teeth. Each gear is also identified by colour for quick identification at the factory. It has been some time since GM used mechanical speedometer drives but many of these parts are still available. They are also available from the aftermarket or through transmission shops.

If you already have a gear that is 32 teeth or larger, you will also need to change the drive gear. Again, this is relatively easy. Remove the driveshaft and the four bolts holding the transmission extension housing onto the transmission. The driven gear is a plastic gear held in place by a clip on the transmission output shaft and it can simply be unclipped and slid off. There are eight different drive gears available ranging from seven teeth to 20 teeth. A little math and you should have a speedometer that is close to reading correctly. If you feel a little unsure about doing this, a transmission shop can change the gear for you in a few minutes.

QUESTION: I have a 1997 Chrysler Intrepid with a 3.5 engine. The motor surges at slow speeds and at high speeds after it warms up. It appears to be better for a few minutes while it’s cold. This has been happening for the last four months, or 4,000 km. Things that I have done to correct this problem are: replaced the exhaust gasket, the “check engine” light was on so replaced that module; however, no other codes show up, replaced the fuel filter and replaced the accelerator sensor on top of the engine.

None of these made a difference. The car does not stall but is a constant aggravation to drive. The transmission shifts perfectly. It has lots of power, but this surging is always there. It has only 144,000 km on it and is extremely well-maintained. All codes are updated on this car. When it is put on the computer analyzer it does not show a problem but the problem is evident to all Chrysler mechanics that looked at it. Would you possibly have any suggestions?

ANSWER: The most common cause of a surging condition like this is an intake vacuum leak. Check all hoses and connections carefully but you may want to also check for a leak at the intake manifold gaskets, as they have been known to leak where the upper manifold bolts onto the lower manifold.

If you can’t find a vacuum leak, suspect the EGR valve. The valve may be physically staying open slightly and this would create a surge without necessarily setting any trouble codes. When you remove the valve, you may be able to see if the valve is mechanically sticking or blocked open with carbon. Replace the valve if these problems are present.

Jim Kerr is a mechanic, instructor of automotive technology, freelance journalist and member of the Automobile Journalists’ Association of Canada.

kerr.jim@sasktel.net