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Can someone tell me what could be wrong/am I just stupid

So I just got a 1998 Ford Taurus from my dad. Now it’s got two D’s on the shifter and I didn’t really know what that meant. So I drove on the highway with it in Drive (I looked it up later that I should have been driving in overdrive) and my headlights fade in and out, my battery light will flicker occasionally and my inside car lights will turn off and on. Now I got the alternator checked and battery, tightened the battery terminals and took it to a shop where they couldn’t find anything wrong. Is it simply because I drove it in the wrong gear this whole time or should I take it somewhere else to get looked at? Any advice appreciated. I’m sorry if I’m just dumb.

Driving in the wrong “D” gear won’t cause any problems. At least not the problems you’re describing.

My first thought was a bad or dying battery. Sounds like you’ve had that checked, though.

Do you have any more history on the car? Has it had any other issues you’re aware of?

I’ll also mention that any 21 year old car is going to have some issues. Some more mysterious than others.

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driving in Drive instead of Overdrive will not cause your issues.

A weak or failing alternator will, a bad battery can, and dirty, loose, or corroded battery cables can cause this.

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The body control module may be starting to fail.

https://www.rockauto.com/en/catalog/ford,1998,taurus,3.0l+v6+dohc,1304615,electrical,body+control+module+(bcm),2888

Tester

If you have a volt meter to use to test the battery voltage, before the first start of the day the battery should measure about 12.6 volts. Immediately after starting the engine, 13.5-15.5 volts. What voltages do you measure?

I concur w/the posters above, the D vs D driving experiment you did is almost certainly unrelated to the symptoms.

Two letter ( D 's ) on the shifter ? Could the OP be mistaking the letter ’ O ’ for a D because of wear by handling of the shifter for all these years .

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I remember some older vehicles having a “D” and a “D” with a sort of circle around it for overdrive. I don’t remember what vehicle that was though.

Just shift P-R-N and then stop on the next one, whatever it says!

Ford/Merc used to have it as D1 and D2. GM was usually D and then a D with an O around it for overdrive. At any rate the first D is perfectly fine for highway but just a lower gear and will use more gas. It was recommended for pulling a trailer or hilly driving. No harm no foul. Suspect the problem is either an alternator or as Tester suggested the BCM. You shouldn’t use overdrive though on hilly conditions etc. where the thing is lugging or shifting in and out. Even in high head wind conditions. I was told by the dealer that the overdrive clutches in the transmission are much lighter and won’t take the strain of heavier use.

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Did anyone check to make sure the alternator belt or serpentine belt isn’t slipping? If you have one serpentine belt that drives everything, one of the other pulleys might be starting to seize.

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One of my exes used to occasionally drive her parents’ Ford LTD, and one day I noticed she was driving it in D rather than OD. I’ve also seen the same setup in a U-Haul with a Ford chassis.

Since I haven’t seen anyone mention it yet: The non-overdrive D is for towing.

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Mom had a 65 Galaxie and after N it had D then a D with a circle around it then L. She bought the car used and there was no owner’s manual. I never knew what gear to drive it in.

I’ve always wondered the purpose for a separate overdrive select. It seems like all the driver should have to do is select D, then let the transmission decide what gear to use after that. It’s an automatic transmission after all. If the driver wanted to choose the gears themselves they’d have purchased a car with a manual transmission.

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The transmission does decide what to do. A problem may arise going down a along, steep hill. Locking out overdrive limits speed increase when descending the steep grade.

But automatic transmissions w/out overdrive already have that capability. For example on my truck I can select either 1,2, or D. If it had 4 gears, and the 4th was the overdrive, which of these gear selection displays makes more sense? Either way I could select the third position for steep hills, no difference in function.

1, 2, 3, D

or

1, 2 , D , O

It’s so you can tow a trailer with the overdrive disengaged.

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Like I said the issue is with trailer towing for one. You need to avoid the higher overdrive gear for the sake of the transmission. The other is hilly conditions or bad weather conditions such as ice. Sometimes the higher gear will free wheel and not give you engine braking like you would get in the D1 position. This can be a problem in slippery conditions where you don’t want to brake but don’t want the car to free wheel down the hill faster to make it harder to make the turn at the bottom. Yeah I’ve got one in mind that we call dead man’s curve and have negotiated it hundreds of times in bad weather and I’m still alive. D1 just locks out the higher overdrive is all and is the same gear that cars used to have without the overdrive so you can drive all day long in it with no problem.

I understand why you’d want to lock out the final gear sometimes but I don’t understand why that requires labeling it any differently than the other gears. If I want to lock out the final gear on my 3-speed truck I just select 2 rather than D. “2” provides gears 1 and 2, just not 3.

Trying to compare transmissions that have 6 , 7 or 8 speeds and using your old 3 speed truck is a little silly.

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With the old 4 speed automatic transmissions D or Drive is direct, 1:1 just like your truck. OD or Overdrive is an overdrive ratio. It is acceptable to use either position based on conditions. This is different than being locked in third gear, drive is an automatic gear selection.

On my 3 speed truck, if I select the 2 position, that doesn’t lock it in 2nd gear. It starts out in first, then shifts to second. It just never shifts to third is all. It’s just like being in D, but without 3rd gear. I don’t understand why the driver should have to know or worry about whether the final gear ratio is 1 or less than one.