The Chrysler Ratchet

August 17, 2009 7:49 PM

Why do early to late 90s Chrysler/Dodge vehicles with an automatic transmission make a very distinctive ratcheting sound when slowing down?

This has been driving me crazy for years... I first remember hearing it when I was a freshman in high school. and still to this day when i'm walking through a parking lot and hear that sound I immediately know there is some dodge type vehicle near me. I've noticed it on Lebarons, New Yorkers, Dynastys, Caravans, Basicaly every dodge/chry that is not a truck... to my knowledge no other make of vehicle makes this sound. What is this sound and why is it there?
I hear it much less now days and I have an 07 Caravan and it doesn't make the sound.

posted by jmsta to grab bag (7 comments total)

Is it the same sort of sound you hear in reverse on a manual transmission? It could be that they used straight-cut gears.

It could also just be some quirk in their torque converter.
posted by Netzapper at 8:22 PM on August 17

Those are solenoids in the transmission opening and closing rapidly as the trans downshifts when coming to a stop. The solenoids direct transmission fluid pressure to the assembly that controls gear selection. Shift quality can be controlled by pulsating the solenoid to cause a gradual gear engagement rather than a sudden shock.

Just about every transmission that bolts to their smaller 4 and 6 cylinder motors does it. Neons, PT Cruisers, Stratus, Caravan, all of them.
posted by Jon-o at 8:23 PM on August 17 [3 favorites]

Could it be the 'normal sound' referred to on this page? It suggests that the sound comes from "transmission fluid control solenoid valves opening and shutting rapidly".
posted by Paragon at 8:24 PM on August 17

Exactly. That transmission was slightly before its time. If one could have a favorite transmission, it's mine. Chrysler got a little over ambitious with the design and didn't make it strong enough for some of the applications they put it in. (Especially minivans used for work-type purposes.)

(Also, I *think* they only put it in v6 cars originally- might have been in some later 4 cylinder cars. Certainly the cars from the 90's that make that noise were all v6 cars. I think they quieted it up for newer applications.)

Most normal transmissions use valves to modulate fluid pressure- quiet and mostly works. But its hard to control and leads to oddball shifts. This transmission, as others say, uses solenoids flipping on and off to modulate the various pressures.

http://en.wikipedia.org/wiki/Ultradrive
posted by gjc at 5:26 AM on August 18 [1 favorite]

Jon-o has it. Those early 90s Chryslers and Dodges had their solenoids outside the transmission, which made them easier to hear. In later years they relocated the solenoid pack into the transmission and the transmissions got quieter.

So says my father, a mechanic of some 30 years experience, 15 of which were working at Chrysler.
posted by KevCed at 5:28 AM on August 18

The transmission is the A604. Chrysler put it behind almost every engine at one time or another. It was the first computer controlled automatic transmission and even featured a fuzzy mode that allowed the transmission to adapt to your driving style. As such it lacked the up until that time conventional valve body. All functions are controlled by the noisy solenoids which were activated by the engine computer.

It's a brilliant transmission really. An engineering marvel on par with the slant six. It was plagued with problems because people would use Dextron instead of ATF+ and like all transmission in car based minivans it was over rated in minivans. Weirdly they performed fine behind even the chopped 318 in Dakotas.
posted by Mitheral at 8:11 AM on August 18

Thanks for all the great information!!!!
posted by jmsta at 8:26 PM on August 18