Speedome­ter er­ror an easy, in­ex­pen­sive fix

Winnipeg Free Press - Section F - - AUTOS - JIM KERR

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


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