MFU Coupler Switch: Required?

[Rick.K]

I'm building a Knight Hauler. I have the MFU01 and the motorized leg supports kit. The 5th wheel for the motorized support leg kit does not make any provision I can find for installing the coupler switch (J12) that came with the MFU01.

I seem to recall having read somewhere that the trucks electric motor output is reduced until a trailer is hooked up. True or false?

Is this the reason for the coupler switch, and if so why doesn't the motorized support leg 5th wheel incorporate the switch?

[Supermario]

Your correct Rick, the coupler switch does change motor power when not hauling a trailer. It is also used to trigger the coupling/ uncoupling sound from the MFU.
In my personal experience that switch was nothing but trouble. It was hard to keep it working properly and was constantly jamming. It made it more difficult to connect the trailer and other times would stay stuck in and not go to the uncouple position.
Eventually I took it out, and placed a jumper wire on the MFU to keep motor at full power. This all happened before I upgraded to the motorized legs so I'm not sure how to solve your assembly issue?


Mario

[Animal]

The original release of the MFU-01, came with a new 5th wheel. Did you buy the complete MFU, or the newer striped down version that doesn't come with all the plastic trees? It shouldn't be a problem, I downloaded a copy of the Knight Hauler manual, if you look at your manual, section (35), it indicates that the truck does come with the correct 5th wheel and how to install the swich, unless there was a revision done at some point and you somehow got a truck with the old one like the King Hauler, or Globe Liner did. As far as the function of the switch, I have had no problems like some people did. Just make sure there are no burs on the parts that could affect the movement of the switch or the hook, don't tighten the screws to much, and use a dab of ceramic grease on the hook. And in regards to the MFU cutting power to the motor until the switch is closed, I haven't checked it out with a meter, and I Don't really see the purpose of doing that, But I have heard it mentioned here and there on the forum.

Hope this helps,
Pete

[Rick.K]

I purchased the complete motorized support leg kit. it included a 5th wheel so I used it. it includes Part B3 which slides under the passenger side of the 5th wheel and acts as the actuator for the support legs. Step 35 of the Knight Hauler manual does include the coupler switch, but the directions for the 5th wheel in the MSL kit does not.

I connected the coupler switch to the MFU and tried 1st gear with and without engaging the coupler switch. With switch closed the transmission begins forward with slight throttle. There is a noticeable lag in transmission engagement with the coupler switch open. Switch closed the truck appears to accelerate more quickly as well.

I'm going to find a way to mount the coupler switch just in front of the leading edge of the coupler, and on the frame, so that when the 5th wheel tips forward as the trailer slides on to the 5th wheel the coupler switch will be engaged. This way I retain the sliding bar under the 5th wheel the actuates the trailer landing gear.

As a first build buying the truck kit plus motorized support legs and the MFU has been a real challenge. Three sets of instructions open at once, parts and hardware everywhere. in addition, trying to figure out where to install the electronics so that the front seats can be installed and the MFU and wires hidden behind a partition between cab and sleeper. This is a blast and a brain bender at the same time!

Don't like the vibration unit. A shudder at start-up, and frame twist when starting under a load. no truck shakes like the vibration unit makes it. Unless the truck is built by Harley.

[ricm]

The original release of the MFU-01, came with a new 5th wheel. Did you buy the complete MFU, or the newer striped down version that doesn't come with all the plastic trees? It shouldn't be a problem, I downloaded a copy of the Knight Hauler manual, if you look at your manual, section (35), it indicates that the truck does come with the correct 5th wheel and how to install the swich, unless there was a revision done at some point and you somehow got a truck with the old one like the King Hauler, or Globe Liner did. As far as the function of the switch, I have had no problems like some people did. Just make sure there are no burs on the parts that could affect the movement of the switch or the hook, don't tighten the screws to much, and use a dab of ceramic grease on the hook. And in regards to the MFU cutting power to the motor until the switch is closed, I haven't checked it out with a meter, and I Don't really see the purpose of doing that, But I have heard it mentioned here and there on the forum.

Hope this helps,
Pete

I agree with Pete....the switch works fine if you use the ceramic grease (other types of grease can be a bit 'sticky' and will only add to the problem) and don't overtighten the screws. The way I put it together was to tighten everything then back the screws off by 1/4 or 1/2 turn till everything worked reliably. If you think the screws might come undone, use threadlock but make sure you use some of Tamiya's general purpose blue threadlock where there are plastic parts. The red jelly stuff can damage plastic and should only be used on metal parts.

My personal recommendation is to use the switch - whilst it does change the power, I find it most useful because of the coupling sound effect so you know when you have coupled/ uncoupled correctly when you're not standing over the truck. It can be difficult to tell visually if you are standing a few meters away....not good to drive away and find your trailer drops off...!

The general rule with Tamiya manuals is that the white parts come from the kit that the manual belongs to, the parts coloured light grey come from somewhere else. Eg, when you look through the MSL manual, you will notice some parts in grey which you need for the assembly....these parts come from the original tractor truck or trailer kit, not the MSL kit. So for the coupler, find the manual where the 5th wheel with the switch is shown as a white part, then you know to look in that kit for it. I didn't know Tamiya changed the kit contents, so check both the MFU and Knight Hauler manuals to see where you should find it...it WILL be there somewhere, Tamiya are not known to miss parts in their kits...

I agree to ditch the vibration unit, I did the same. It drains the battery and the effect is not realistic!

[Supermario]

As a first build buying the truck kit plus motorized support legs and the MFU has been a real challenge. Three sets of instructions open at once, parts and hardware everywhere. in addition, trying to figure out where to install the electronics so that the front seats can be installed and the MFU and wires hidden behind a partition between cab and sleeper. This is a blast and a brain bender at the same time!

Ahhh, the good old days. Reminds me of my first build.:). Nothing like being frustrated and yet loving every minute! :D:lol:

Great idea to relocate the switch and act on the plate pivot! I'd love to see what you come up with! Take pics and keep us posted :)

Mario

[Supermario]

My personal recommendation is to use the switch - whilst it does change the power, I find it most useful because of the coupling sound effect so you know when you have coupled/ uncoupled correctly when you're not standing over the truck. It can be difficult to tell visually if you are standing a few meters away....not good to drive away and find your trailer drops off...!


Very good point and something I never considered. Ever since I installed a wireless kit I know if my trailer is coupled properly or not because the legs won't run if it isn't.
To be honest, I guess I do miss the sound a little too:o

Mario

[ricm]

I'm going to find a way to mount the coupler switch just in front of the leading edge of the coupler, and on the frame, so that when the 5th wheel tips forward as the trailer slides on to the 5th wheel the coupler switch will be engaged. This way I retain the sliding bar under the 5th wheel the actuates the trailer landing gear.

I would suggest you stick with the original Tamiya design - it works fine - if you mount the switch as you suggest you'll get constant coupling/ uncoupling sounds if you drive over an uneven road surface, as the 5th wheel moves relative to the tractor truck chassis...

[Rick.K]

The point where I'm stuck with the coupler switch is the motorized support leg kits 5th wheel doesn't make a provision for installation of the switch. At the same time it included the bar under the 5th wheel that slides back and forth opening the lock on the 5th wheel and I assume acting as the mechanical actuator for the support legs. If I use the 5th wheel from the original kit I can install the coupler switch but then how are the support legs actuated?

The directions with the mechanical support legs have a copyright of 1994. They call for removal of the original 5th wheel from the King Hauler or a Globe Liner. The instructions do not reference the Knight Hauler or the Scania or MAN tractors.

[ricm]

OK...

Over the years Tamiya have released different versions of the coupler:

1) basic coupler, for use without MSL or MFU (released with early tractor truck kits such as King Hauler and Globe Liner)

2) coupler for MSL (released with the MSL, to swap out with the basic coupler above - hence your reference dates go way back)

3) coupler for MSL and MFU (originally released with the MFU kit, now comes as standard in some of the newer tractor truck kits)

I've looked through the Knight Hauler instructions.....the 5th wheel as supplied as standard in the Knight Hauler kit is type 3), and therefore compatible with both the MFU coupler switch AND the MSL. You can ignore the 5th wheel provided in with the MSL....you are correct that the 5th wheel in the MSL does not make provision for the coupler switch so DON'T USE IT! You can tell the designs of the 5th wheel apart because of the shape - the one you want is oval in shape, the one with the MSL is round.

Step 35 (page 19) in the manual shows how to assemble the 5th wheel, as provided with the basic Knight Hauler, with the coupler switch (shown as optional extra).

Then BEFORE you assemble the 5th wheel to the chassis, go to page 26. The bottom of the page shows you the extra/ alternative parts you need to assemble (taken from the MSL kit) to then make your 5th wheel also work with MSL. You can see that you need to swap out a few parts you have assembled in step 35 (a ball joint BE5 and nut BD3) for alternative parts from MSL, so you can add the sliding bar. The assembly of the same parts is shown a little more clearly in the MSL instructions, make sure you look closely to check you have the right washers, nuts, etc. from MSL for the sliding bar to work properly. Note that in addition to building the 5th wheel correctly, you need to use the plastic plate (that goes between the 5th wheel and the chassis) from MSL kit, NOT the one shown as standard assembly (part G8 in step 35).

My advice for the bar again is don't tighten things too much - the bar needs to easily slide WITHOUT activating the coupler until it reaches the extreme of movement in one direction. So the coupler should easily close under the force of the spring without moving the bar, if the bar is in a middle position. You'll get the idea when you put it together..

Hope this helps!

[Rick.K]

RICM:

Thanks for the additional insight. I will print off your instructions and give this a try this evening. I noted the oval 5th wheel you referenced, since it is the one with the kit. The insutructions you highlighted on page 26 make it clear how the sliding bar is to be installed to make the MSL work along with integrating the coupler switch (step #35) in the instructions on page 19 of the Knight Hauler instructions). The base of the 5th wheel will remain the one from the MSL kit so that the control servo can be retained with proper clearances.

[dabears]

If you don't want the sound of the coupling/uncoupling do what everyone else does. cut the coupler wire and twist together. run everything else as normal. even run your servo for your support legs kit. you do not need the switch to be functional for the legs to work. I use that feature to run a servo somewhere else on the truck in mine....

[ricm]

RICM:

Thanks for the additional insight. I will print off your instructions and give this a try this evening. I noted the oval 5th wheel you referenced, since it is the one with the kit. The insutructions you highlighted on page 26 make it clear how the sliding bar is to be installed to make the MSL work along with integrating the coupler switch (step #35) in the instructions on page 19 of the Knight Hauler instructions). The base of the 5th wheel will remain the one from the MSL kit so that the control servo can be retained with proper clearances.

Yep - that should sort you out!