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  #1  
Old 07-23-2003, 11:14 AM
garage bohemian
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Exclamation 1988 420 SEL Transmission

OK, I just picked up a 1988 420 SEL from a family friend that passed away. I got the car for $1000.00. I decided to take it on the expressway to see how it rode. It rode beautifully, except for one thing. While cruising along at 60 mph, the tranny decides to downshift on its own. Obviously thats not good, are there sensors in the transmission that went bad? Or would I need a complete rebuild?
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  #2  
Old 07-23-2003, 03:23 PM
86560SEL's Avatar
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That was an excellent price on that car. I do not know about these transmissions, but I am sure that someone here should know. Good luck.
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1991 Lincoln Town Car Executive (33k miles)
1991 Cadillac Sedan DeVille (77k miles)
1992 Toyota Camry LE (177k miles)
1972 Chevrolet Caprice Kingswood Estate 9-passenger wagon (44k miles)
1973 Pontiac Grand Ville 4-dr hardtop sedan (94k miles)
1988 Mercedes 300SEL (was mine, now mothers)
(Prior MB's: 1974 240D, 1985 380SE, 1984 190D)
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  #3  
Old 07-23-2003, 04:33 PM
garage bohemian
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Well the car was sitting for about 3 years because of a tranny leak that my friend never got to fix. So I brought it to a reputable tranny mechanic and had that fixed, also had the fluid and the filter changed. But still the same thing, it downshifts for no reason.
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  #4  
Old 07-23-2003, 05:33 PM
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I'm not a tranny expert..

I would start by checking the fluid level, and the condition of the fluid..

A fluid, and filter change would be my next step.

Only then, would I start to look for something more complicated..
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Old 07-23-2003, 05:35 PM
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That's weird.. I posted my guess before garage bohemian posted for the second time...

Now I look like an idiot..
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89 300SEL Money Pit

92 Blown Buick Ultra Pimpmobile 220K and adding 1K per week

88 Wagoneer Slightly modified (Not for soccer moms)

04 Kia Sedona with every option... NICE
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  #6  
Old 07-23-2003, 06:58 PM
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Join Date: Dec 2001
Location: w. North Carolina
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I own a 1989 420SEL. I have not had your problem in my current car but in past owned cars, I have. My suggestion: test out the Engine Coolant Temp. sensor located at the top front of the engine. I had exactly your problem in an '89 GM car. Driving down the Interstate, the car would, without cause, shift up and down. The temp. sensor controls a lot of things in fairly modern cars. Hope that helps.
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  #7  
Old 07-23-2003, 07:19 PM
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The only electronic control is the kick down solenoid. It is at the passenger side rear of the trans and can be disconnected by unplugging the single wire. I would do this and see if it is eroneously downshifting electrically (that would stop it).

If not governor pressure should be tested. If the trans has over 150k and has intyernal leaks its best to disassemble clean, reseal and replace all the wear frictions and thrust washers. sort of what they call a rebuild.
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Old 07-23-2003, 07:31 PM
garage bohemian
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OK, ill try that. But if it boils down to me needing a rebuild, how much am I looking to spend?
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  #9  
Old 07-24-2003, 01:23 AM
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Quote:
Originally posted by garage bohemian
OK, ill try that. But if it boils down to me needing a rebuild, how much am I looking to spend?
I am guessing at least $1000. or more.
__________________
1991 Lincoln Town Car Executive (33k miles)
1991 Cadillac Sedan DeVille (77k miles)
1992 Toyota Camry LE (177k miles)
1972 Chevrolet Caprice Kingswood Estate 9-passenger wagon (44k miles)
1973 Pontiac Grand Ville 4-dr hardtop sedan (94k miles)
1988 Mercedes 300SEL (was mine, now mothers)
(Prior MB's: 1974 240D, 1985 380SE, 1984 190D)
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