1950 Mercury Overdrive Blowing fuses
 

I am still having problems with my overdrive blowing fuses (7) just as it hits 28 mph. I have chased wires, verified components and have even taken the entire system out of the car.


I talked with Mac VanPelt several times and have followed his recommendations. However, at the beginning of my journey I mentioned that I had an MSD ignition system on the car. Mac said there was a problematic condition with MSD ignitions. I called MSD to find out what their recommendations would be…no problem, just install as usual….WRONG !!


I have been chasing circuits in my mind and can not figure out how the ignition cuts off momentarily and allows the primary side of the solenoid (connection 4) to go to the holding portion side of the solenoid (connection 6). What controls (grounds) the coil and takes pressure off the transmission to allow the system to work?



What have I missed; moreover what don’t I understand?

Re: 1950 Mercury Overdrive Blowing fuses
 

It seems you have done about everything that you could do, save one. I would temporarily go back to a stock ignition system and see if the problem persists.

Re: 1950 Mercury Overdrive Blowing fuses
 

How to connect HEI or Pertonix Ign with Borg Warner R10-R11

https://vintageautogarage.com/blog/h...warner-r10r11/


This link may help.

Re: 1950 Mercury Overdrive Blowing fuses
 

The solenoid has 2 contacts and 2 windings if it is a 2 wire solenoid, the 4 wire one has more contacts inside
One of be solenoid windings is heavy wire — takes a lot of amps and is only intended to operate for a second— while doing the heavy work of moving the solenoid then the contact is opened and the lower amperage winding holds the solenoid —- if that internal switch doesn’t open the fuse could blow.
At 28 mph the governor makes a ground that energized the relay that sends power to the solenoid.

Re: 1950 Mercury Overdrive Blowing fuses
 

Fuses burn due to shorts to ground or is some cases due to a high resistance conductor but those are slower to burn than a short to ground. All the wiring from the firewall down is in a swampy environment so it tends to deteriorate if driven during or after rainfall. The folks that fabricate new harnesses do a fair job of recreating the original wire protection but it is still vulnerable at the terminal connections.

With the ignition key on, the system is in a powered state but doesn't kick the smaller relay till the governor switch closes at on speed. The 1949 through early in 1951 systems have a lock out switch in the line to the governor switch to keep the system dormant until the cable control is pushed back in to the overdrive position. Some folks bypass the lockout switch so be aware that there may be old wiring in that circuit that could short if not properly insulated and stowed. Later 1951 cars have no lock out switch since it was considered to be redundant. Personally, I like having that lock out switch but I keep spares around in case is gets fowled up.

The operating solenoid starts it's shift function as soon as the governor switch allows it at on speed but it doesn't shift all the way in until the operator lets off the throttle. After it shifts all the way into overdrive then the contactor set is closed inside the solenoid to put the unit into the holding coil mode for distance driving in overdrive.

The kick down switch is what drops the solenoid completely out of overdrive above on speed. It also performs the momentary ground to the ignition coil to allow the drop out until the unit is completely dropped out where the interrupter contacts inside the unit open and restore ignition function. The drop out happens so fast that the ignition off/on function is barely noticeable. As soon as a person lets off the throttle then the operating solenoid shifts back into overdrive as long as the forward speed is still above the on speed of the governor.

The operating solenoids can be opened up to check the condition of the holding coil contact set and the interrupter contact set. A person can check the innards for signs of shorts or funky wiring. Older units have screw terminals but later ones have wires that go directly into the unit. It's not difficult to test the function of the operating solenoid on the bench but most folks don't use a fuse in their jumper leads. If a test lead gets hot you will know it pretty quick.

Re: 1950 Mercury Overdrive Blowing fuses
 

What amp fuse do you have in the relay? If my memory serves me correctly it is a 30 amp fuse. Just a thought.

Re: 1950 Mercury Overdrive Blowing fuses
 

This is the third time I have tried to respond to this forum.


I have used all of the advise and checked out all of the possibilities that BobC, Rotorwrench, Flathead4rd, Tubman, and Kurt in NJ, have posited--a very grateful ---- THANK YOU.



I have not talked to Mac VanPlet about the solenoid disassembly or how to check the "holding" circuit or if the relay is working properly



However, I still can not fathom what breaks (grounds) the coil circuit at 28 mph for shifting into overdrive. If I could only see this in my mind I could understand.


I just bought some more fuses.

Re: 1950 Mercury Overdrive Blowing fuses
 

I think you have it backwards. When you let off the gas above
28 MPH the throttle switch under the gas pedal lets the governor ground the solenoid. When you mash the pedal to the floor it grounds the ignition and kicks out of overdrive.
In the link read #12 on page five. https://www.zampano7.com/uploads/2/5...ive-manual.pdf

Re: 1950 Mercury Overdrive Blowing fuses
 

The governor grounds the overdrive relay at about 28 mph to energize the solenoid. The solenoid has a set of points that grounds the wire to the coil to quickly interrupt the ignition to relieve the torque so the OD can drop out when the switch under the gas pedal is activated. As soon as the solenoid drops out, the grounding points are opened and ignition resumes. all this happens in a fraction of a second.

Re: 1950 Mercury Overdrive Blowing fuses
 

My advice is two different possibility's. disconnect the wire from the coil and see if the OD works . It won't and don't try to get it to kick down. If the fuse stays good you found you problem. Check number two try a different solenoid. there is a small possibility you may have your wires crossed somewhere or your relay has a dead short on the solenoid terminal. You should have continuity from that terminal to the solenoid. It should be hooked up to the wire that actuates the solenoid. If you have these two wires backwards you will always blow the fuse. Tim

Re: 1950 Mercury Overdrive Blowing fuses
 

Trying to understand;


I must manually press the pedal switch in order for the OD to engage?


I do not see any other way to ground the coil but through the foot switch--is this correct thinking?

Re: 1950 Mercury Overdrive Blowing fuses
 

The "kickdown" switch (under the gas pedal) is engaged to kick the unit OUT of overdrive.. Since this is kind of a "passing gear" the engine is under power when the "kick-down" engages, thus the necessity for a momentary cut in power. (The solenoid pulls it out of overdrive). To put it into overdrive (when you are going fast enough), just let off on the throttle (no power to the drivetrain, so no cut in engine power required), and it will pull itself in (no solenoid activity needed). The governor is there to ensure that a certain minimum speed has been obtained for the overdrive not be strained accidently. The lockout cable mechanically takes the unit out of overdrive and keeps it out as long as it is pulled out.

Re: 1950 Mercury Overdrive Blowing fuses
 

Attached is the BW Service Manual with operating principles. This should help you understand it better.

Re: 1950 Mercury Overdrive Blowing fuses
 

With the OD handle pushed in you would drive around 35 mph let up on the gas pedal and you will feel / hear a slight clunk and your in OD. You can still use all of your forward gears. If your speed drops below 21 mph and you take your foot off the gas it will kick out of OD. You would/should read a operators manual. They are a dream to drive but there are things not to do to them. Oh yes one last thing, if your decelerating with the handle in and your speed drops bellow 21 mph it will freewheel this is normal. If you gently press on the gas until you feel it engage you can pull the handle out and your back in under drive with full deceleration and will stay that way .