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#4 fuse blowing when I accelerate hard

Started by Casey, 28 November 2015, 09:45 PM

Casey

Car is a 1979 300SD - really not sure how to diagnose or fix this - the #4 fuse blows occasionally - I have finally figured out that it is when I accelerate hard.  If I floor the gas pedal from a stop light, it will blow while revving up in the first gear, before upshifting.  Or if I'm already going, and floor the gas while in 2nd gear, it will blow after a couple seconds before upshifting to 3rd at high RPM.  I tried putting in higher-amp fuses (red and blue instead of the usual white), and they all blew under the same circumstance.  What could the problem be?

jmperkins

I googled # 4 fuse and w116 and saw references to auto trans kick down solenoid switch which is supposed to be located under the accel pedal.

I suggest you google as i did as you will see some good ideas as to how to proceed. Such as disconnecting the wire to thr solenoid and taping the wire and if you still have the symptom you will needvto look elswhere.

Good luck,
Joe

Casey

Quote from: jmperkins on 28 November 2015, 10:31 PM
I googled # 4 fuse and w116 and saw references to auto trans kick down solenoid switch which is supposed to be located under the accel pedal.

I suggest you google as i did as you will see some good ideas as to how to proceed. Such as disconnecting the wire to thr solenoid and taping the wire and if you still have the symptom you will needvto look elswhere.

Thanks - this makes sense, I forgot about the kickdown switch... I am currently parting out a wrecked 1974 450SE which hopefully has the same solenoid.  Assuming I can locate it, I will try swapping it out tomorrow to see if that does the trick.

Casey

Determined it is definitely blowing when the kickdown switch is pressed, immediately upon pressing it the fuse blows.  I tried the known good one from the 1974 450SE (which, interestingly, is NOT the same part, and the same thing happened.  Tried one that I know is good from a 1980 300SD and it also blew the fuse.  If I understand things correctly, the problem is probably the solenoid that the kickdown switch is connected to.  I scoured my manuals but I don't have a Transmission service manual - can you tell me how to locate the solenoid?

Casey

After a LONG time, I finally found a picture illustrating where to find the solenoid:



Any idea if they are interchangeable between a 450SE and 300SD?

jmperkins

Hi casey, remember that google is your friend as using words ' kickdown switch solenoid  sd300' points you to several discussions about  the solenoid such as 'a black widget at the rear passenger side (probably looks like this) is not a speed sensor, that is the kickdown solenoid. This gets energized by a switch located under the accelerator pedal when at WOT. This increases WOT shift points but isn't required to make the trans work.' Wot btw is wide open throttle!!! Trans manuals are hard to come by as i understand

Joe

Casey

Quote from: jmperkins on 29 November 2015, 09:48 PM
Hi casey, remember that google is your friend as using words ' kickdown switch solenoid  sd300' points you to several discussions about  the solenoid such as 'a black widget at the rear passenger side (probably looks like this) is not a speed sensor, that is the kickdown solenoid. This gets energized by a switch located under the accelerator pedal when at WOT. This increases WOT shift points but isn't required to make the trans work.' Wot btw is wide open throttle!!! Trans manuals are hard to come by as i understand

Your Google must work better than mine!  It took me a couple hours of searching to find the above image.  :-[

Casey

Replaced the solenoid with one from a 6.9, worked great!  Thanks for the help.

jmperkins

Wonderful news! Congratulations, Casey, and happy holidays!