Fuse #3 blows irreguarly - Cause found... but now what?

[Kopie]

I feel like I'm getting really close to a happy Pagoda!  I have this fuse problem after sorting the rest of the transmission problems.  I can drive for sometimes a week and sometimes a day before the fuse blows.  I always know almost immediately when it blows because I can hear the clunk when the car downshifts coming to a stop.  My questions are:
1.  Can anything other than a short cause this problem?
2.  Can I take one of the switches (kickdown or idle) out of the equation to begin a process of elimination?
3.  Are these any insulating washers in this circuit that could be failing which would cause this intermittent short?
I swear I think it would almost be easier if it blew every time i turned the key.

I am definitely open to any other suggestions.  I can say that when it's working the car is an absolute joy to drive and I'm hooked! I can also say thank you very much to all who have helped me get her to this point.  I can't begin to say how many problems I've been able to solve through reading the forum.  This site is invaluable and i'm so glad to be a member.  I firmly believe that anyone who owns one of these cars and finds this site as helpful as I do should take the time to join and contribute.  Sorry to get off topic, but I feel the need to say my thanks.

[jeffc280sl]

Hi Kopie,

The wiring diagrams for the 280SL show a dotted line coming from fuse 3.  It goes nowhere.  The wiring diagram for the 280SE indicates the only circuit involved with fuse three is the kick down switch and solenoid valve for the auto trans.

When the fuse blows are you actively using the kick down switch?  In other words are you slamming the accelerator pedal down to engage the kick down switch to force the transmission into a lower gear?  If not, I would suggest a failure in the kick down switch where it is shorted to ground.  This would indicate a short in the blark/red wire from fuse 3 to the kick down switch.  Should be easy to trace, everything is under the dask, the switch is located behind the accelerator pedal. 

Its odd though. The kick down switch is normally open.  When activated it provides 12 v to the solenoid which in turn asks the transmission for a lower gear.  If the fuse blows there is no power to ask the trans for a lower gear.  You indicate that when the fuse blows the trans clunks into a lower gear.  This is not possible according to the 280SEwiring diagram.

I'd start by examining the kick down switch.  Take it out, apart if you can and clean it.  then inspect the wiring for shorts.

Good luck

[Kopie]

Jeff,
Thank you for the reply.  I should have been more clear.  The clunk I was referring to was when coming to a stop.  If the idle switch doesn't lower the fluid pressure, the downshift to second is a noticeable clunk.  You're right in that the the kickdown switch is non-functional when the fuse blows.  I have taken out the kickdown switch and cleaned it and used a continuity meter to test its operation which appears to be normal.  I've also done the same with the idle switch.  There is no obvious pattern to the fuse blowing.  Kickdown performs normally as well as the idle switch when the fuse is good then all of a sudden it blows and I know it when i come to a stop.  I suppose it must be a short in the wiring somewhere along the way.  The frustrating part is that I don't know where exactly these wires are in the harness.  It occurs to me that one way to explore this might be to run a wire at each leg one at a time and wait to see if it still blows.  Could take a while, and i still wouldn't actually find the shorted wire, only a workaround.  I'm guessing the wiring diagram for the 280SL you were looking at was for a manual transmission car.

[jeffc280sl]

Hi Jim, 

You're welcome.  The 280SL wiring diagram in the Haynes Manual does not have the kick down switch wiring nor the idle speed adjuster so the information that I have concerning those circuits comes from the 280SE wiring diagram.   In this case there is only one circuit on fuse #3.  It is the kick down switch and auto trans solenoid.  The idle increase circuit in the 280SE is fed from fuse #2.  Are you saying that when fuse #3 blows the idle speed adjuster also stops working?  They should be on separate fuses.

If the kick down switch looks good I'd move down stream to the solenoid and the wiring on the tranny.  Do you have the Haynes Manual because it shows a nice view of the tranny wiring block?

[Kopie]

What I'm calling the idle switch is the one on the throttle body that activates the 3-way solenoid on the transmission in the opposite direction from the kickdown switch, the purpose being to lower the pressure in the transmission when the throttle is in idle position.  It allows smoother downshifts when braking and slowing down.  I'm sure these are the only things on this circuit.  My real problem is reproducing the short so I can eliminate it.  I think I'll try just unplugging the kickdown switch as i know i won't damage anything in the transmission without it.  I'm not sure about the "idle" switch.

[wwheeler]

Here is something that happened to me that may or may not be relevant. I had problems with my trans shifting being erradictly hard and soft (it is on a current post). Turns out the trans solenoid was bad but it was intermittent. What was also happening was that my idle switch on the venturi would foul the points after about two months. It would only work every so often once the points were fouled. I would take the switch apart and clean the points. After that the switch worked great for another two months. I repeated this four times with the exact same result.

I inspected the wires and connections to the extreme with a multimeter and could find nothing. I ended up replacing the solenoid and have never had a problem with the switch since. My point here is that maybe the solenoid is shorting causing the fuse to blow just like it caused my points to foul. Maybe?

[Kopie]

I hope that doesn't turn out to be it.  I guess it should be a last resort after eliminating everything else first.  Any idea how much the solenoid sells for?  How did you finally determine that it was your problem?

[wwheeler]

Call the Classic center. It ain't cheap, but if you have to have it.....

Over a period of days, I kept checking the operation with the trans tunnel cover plate removed. Sometimes with a friend looking at it while in operation, and sometime with the ignition on and manually activating the switches via the gas pedal. I could see that the action was lazy to return to neutral or into position and not strong like the new one is. This was of course after making sure the power supply/ground was good. I also checked the rod and sockets to make sure they weren't binding and causing an excessive load which could also be your problem.

[Kopie]

Well, after going for a couple of weeks with no problem, I manually downshifted to third to accelerate and then shifted back to fourth and the fuse blew immediately.  I've now come to find that it won't blow from normal driving in fourth, but only if I downshift manually.  I can't understand how these could be related.  Anybody have any ideas?  I can't even see where anything electrical in the shifter is related to fuse #3.

[Peter van Es]

Is the shifter making a short somewhere?

[wwheeler]

Maybe the neutral switch is shorting or out of adjustment???

[Shvegel]

Every now and then you get a ghost in the machine. In cases like this I used to put a small light bulb wired across the fuse and placed somewhere I could see it  so I could see exactly when the fuse blew.

[Kopie]

Sorry I've been away for a while.  Busy with work!  After all the times I have studied this wiring diagram, I never noticed that the starting lock switch is connected to the kickdown switch.  It makes since that if that switch is malfunctioning, it could cause fuse #3 to blow even though the starting lock switch is fused at #5.  I noted early on that it wouldn't start in neutral but never figured it could be related.  Maybe I'll get to solve two problems with one fix.  I'll post as soon as I get some more garage time. Thanks!!!

[Kopie]

I got a little more time to investigate and it is definitely related to the neutral safety switch (or it's wiring).  I was able to disconnect the cable from the safety switch and operate the switch with the engine running.  It caused the fuse to blow immediately.  Has anyone been able to take one of these apart?  I would just like to make sure it's the switch before I buy one to replace it if it's possible.  I did adjust the switch and it functions normally in as far as allowing the car to start in neutral and park and not in other gears.

[wwheeler]

Jim,

I did take my neutral switch apart but didn't take any pictures of the innards. I had a problem with the starter cutting out and thought that may have been the problem. Turned out to be bad wiring at the alternator. Not a whole lot to go wrong with these. I am planning on taking an old one I have and document it but have to much going on at the time. Not too difficult to do but you must grind off the rivet heads to get it apart. I used metric allen head screws and nuts to hold the unit together once it was cleaned out. The diameter of the allen heads are the same as the rivet heads so they fit into the counterbores nicely.

There are some springs and contacts inside that you need to document so you put them together correctly. Other than that, just clean the contacts and make sure there are no shorts. From what I remember, all of the contacts are brass inside. I used dielectric grease liberally inside to lube the sliding contacts.

Since it was apart, I plated the bracket (111s are different) but the arm cannot be removed and just painted it with a zinc look-a-like paint. Here's a picture of the finished product.    

[Kopie]

Wallace,
I think next garage time, I'll go your route and take it apart.  Yours looks great and sadly would shame everything else in my engine compartment.  Since you brought up plating though, it reminds me that I bought a Caswell kit a while back with intentions of plating various parts in there.  It's amazing how one thing leads to another and another....

[wwheeler]

The Caswell kit does do well for small batches. Like any DIY kit though, it has its limitations.