I'm new to this site. I have a 91 honda accord that the transmission shakes or "shimmers" at 40 mph. It DOESN'T SHAKE WHEN IN "SPORT" MODE. I know it's not a tire or bent rim. Both axles are basically new " a year old or newer". I. I changed the fluid (not flush). I used actual honda ATF. It has been shaking before I changed it. I was hoping it would fix it but no luck. I'm a fairy good back yard mechanic. But I don't know much on Honda's or automatics. I've always had standard shift trucks. There are no flashing lights from the indicator lights, no check engine lights. It has about 278,000 miles bit has been maintained well. Any info would GREATLY APPRECIATED.

 

Have you recently done the timing belt(s) on this car?

 

Check the motor mounts.

 

no I haven't

 

they are in good shape

 

Torque converter shudder? Sport mode eliminates the lockup, right?

 

Could be mount.

 

When in normal D4, what rpm does the car shudder at?
If you manually downshift to D3 > 2 > 1 does the shudder follow engine speed or is it road speed only?
What is the rpm when selecting the S button at 40MPH?
I have not driven an 'S' button equipped Accord. In other makes/models of the era the 'sport' or 'hold' button would keep the trans in a given gear that was manually selected, and sometimes prevent upshifting into 4th or overdrive gear. This is why road speed to rpm are important to note.

If the car has over 100K miles have you ever had the EGR ports cleaned?
If the shudder takes effect at 40MPH while in 4th gear you may be feeling the lean misfire from clogged EGR ports. This shudder should still be present in higher road speed but with less violence. Or higher engine speed but same road speed. It would be rpm/load specific not road speed.

Transmission functionality is greatly affected by engine running condition. Bad engine tune, even if the engine appears to be running smoothly, will cause the transmission to shift erratically which may be construed as a shudder. One needs to monitor engine rpm to road speed to denote when the problem occurs.

Verify the basics.
Battery is holding a full charge when ignition is off,12.4-12.6 Volts.
Battery terminals are clean, corrosion free, and tight.
All bonding jumpers are clean and tight.
Verify fluids are properly filled.
Verify the throttle body to transmission cable is properly adjusted, no slack and no pretension. A loose cable will have reduced pressure to the trans which may cause a shudder at low throttle angles.
Verify the NM/NC shaft sensors on the right side cover are clean of sludge, these sensors are used by the TCU to monitor shaft speeds to calculate the correct upshift strategy.

Broken mounts don't really cause a given speed shudder, more vibrations into the cabin when at a stop or when accelerating in general. They tend to make a racket in general.

 

I don't really what the sport mode does honestly. But that sounds about right because I been thinking it's a torque converter but I took it to a "honda and import shop" and the guys there told me it was because it had 278,000 miles on it. But that was a bullshit answer

 

it has 278,000 miles and going up daily. I've took off the AIC and EGR and cleaned both with seafoam and carb cleaner. I sea foamed the engine and has newer plugs/wires/cap/button (with in the past 9 months) I don't know anything about throttle body for the transmission or the cables. I didn't think it had cables since it was a floor mount shift. The "sport mode" can be on front the start and it will shift through all gears just stays in the gears for a little longer maybe 10mph higher in each gear. Battery and all cables are fairly new no corrosion on them. New alternator. New distributor. New ignition control module and coil. New cv axles. New control arms. New speed sensor. I've replaced a ton of stuff on this old honda.