Jump to content
Sign in to follow this  
wheeledhorseman

Indicator lite circuit board repair

Recommended Posts

wheeledhorseman

With the exception of the low oil pressure lite, the others weren’t working properly on my tractor and the previous owner had been told that the circuit board needed to be replaced. As the tractor ran fine without he hadn’t bothered, cost probably being the main consideration. There’s an excellent thread by Chuck (Save Old Iron) which ran way back in 2009 in which he proceeded to reveal the secrets of the board and resurrected a very badly corroded (pretty much worst case) board from a 416-5.

If you’re new to this topic and looking to fix the status lites on your tractor then reading the original thread is a pre-requisite. I would normally have added this content to the original thread but as a key image, the circuit schematic, appears to have been lost in the move across from the old RS site I thought it better to start a fresh thread on the topic and reinstate the missing diagram.

I examined the board on my 227-5 during the summer and found that the sealant had cracked open in one or two places so, assuming that corrosion would be the likely problem, removed the rest of the rubbery compound only to reveal a pristine board. Clearly, the components on the board would need examination and testing, a job which could wait till winter when the tractor would be out of use. The situation was that the oil warning lite worked fine, the parking brake lite was on all the time, the PTO and clutch lamps glowed at various brightnesses when they should be off, and the seat switch never lit up at all except when the test switch was used.

The parking brake lite issue was simple to fix though it took a while to track down and was unrelated to the board.

lites1.jpg

A standard micro-switch was used here which sounded really good as it operated with a clean click but the issue was that someone in the past had put the cables in the wrong place. These are three terminal two-way switches with normally open and normally closed contacts. The common terminal is on the far side of the switch and out of view so I could see how the mistake had been made. Moving the yellow lead to the side terminal fixed the problem, well partly.

I now had three lites which lit a full brightness when they should do but glowed at various other levels when they should be off and the brightness of the glow depended on which of the others were on, and there was still a lamp that never lit when it should. Oh well, the board has to come out to be fixed. It may not be the case with all tractors fitted with the indicator system but removal and reinstallation of the board on my tractor was made much easier by first removing the bottom two lite lenses otherwise there is a danger of damaging the little lamps.

lites2.jpg

The board is pretty much identical to the one Chuck dealt with in the original thread though in this case it included the additional components round a 555 timer chip (bottom left) to make the oil lite flash.

lites3.jpg

Chuck produced the following schematic for the board, though obviously the 555 circuit was not included.

lites4.gif

In tackling the non-functional seat lite first, I removed the relevant transistor to establish if it was the transistor or chip that was faulty. It turned out that the logic gate in the 54LS86 chip was non-functional in that the output remained low even when the input conditions should make it go high and turn the transistor and hence the lamp on. Now the 54LS86 was a military spec chip, it is still available from specialist suppliers, but read on before attempting to order one. Chuck pointed out that a 74LS86 has the same logic function and is much more readily obtainable but more about making this substitution in a moment.

The puzzling thing was that the three other logic gates in the chip were still working though struggling to turn the transistors fully off. The chip had obviously been fried in some way but not a total wipe out. Chips cannot be repaired but in this case it was worth experimenting and trying to give it a helping hand so I added a 1k resistor between the base of each transistor and ground to help pull the outputs from the chip low. The residual glowing in the off state was much improved but did not entirely go away.

lites5.jpg

Determined now to force it into submission I took a few voltage measurements and discovered that by reducing the supply down to about 10 volts the three logic gates worked perfectly. Further investigation of the voltage at the supply pin (14) on the chip revealed two things. Firstly, unlike most 54 and 74 series chips which are designed to operate with a a nominal supply of 5 volts, this particular 54 series chip appears to have an internal feature allowing it to operate from the 14 volt tractor supply provided the supply is via a 470 ohm resistor. I think the clue to this might be in the suffix. The chip used on the board is actually and very specifically a 54LS86A/BCAJC.

Much searching on the net hasn’t turned up much by way of an answer other than the BCAJC version costs considerably more than the plain old 54LS86A though as far as I can make out all 54 LS chips have a supply voltage range of 4.5 – 5.5 volts. All that I can suppose is that the version used here includes some kind of internal shunt circuit designed to deal with spikes on the supply line as part of its MIL SPEC and that this has been exploited in the circuit to act as a regulator. My voltmeter indicated that it isn’t a steady regulation and that by the time the supply to the board reaches 14 volts the supply to the chip at pin 14 had reached pretty much the 5.5 volt limit. Easy to see then how the chip could get fried.

lites6.jpg

My solution was to add a zener diode to clamp the supply voltage to the chip. I actually used a 4.7 volt zener though a 5.1 volt zener would have done just as well and…….. the lites on the board now worked as they should. I tried removing the pull down resistors but the glow returned. I wasn’t surprised as the chip has been internally damaged and actually needs replacing. What I’ve really done is worked round it and learned from doing so that if you are going to replace the chip with anything other than the original 54LS86A/BCAJC then I’d add a zener to be sure. It’s an absolute must, I believe, if you use a 74LS86. Note the polarity from the photo – the end marked with a band (black in this case) goes to the track leading to pin 14 of the chip.

When I’m next in town I’ll pick up a standard 74LS86. They’re about 70 pence here in the UK so I’ll guess about 70 cents in the US. When time permits I’ll use it to replace the damaged chip on my board but for now it works again so I’ve put it back.

lites7.jpg

Ok, so I still don’t have a seat switch lite that works but I haven’t yet reached the age or state just yet when I need a visual reminder that I’m not actually sitting on the seat. I’ve not been that impressed by the lite system up to now but now the others are working properly, I think they’re actually quite useful.

For reference I made the cables out to be as follows (left to right):

BLACK – Earth

ORANGE – 12 volt when ignition is on

BROWN – 12 volt during start only

WHITEish (perhaps once a pale pink) – Clutch switch

PINKish RED – PTO switch

DARK GREEN – earth feed to one of the switches

LITE GREEN – seat switch

WHITE – parking brake switch

YELLOW – test switch

ORANGE – 12 volt when ignition on

RED – 12 volt ever present

BLUE – Oil pressure switch

The colours used for the cables are I think different on other models of tractor but the function should be the same.

Comments welcome and feel free to chip in if you have more to add or you've spotted a mistake.

Andy

Share this post


Link to post
Share on other sites
 
meadowfield

Good explanation Andy !

milspec 20-30 years ago = standard spec now (ok only 0-105 dec C usually)

...so almost any 74x86 variant should work (as long as it's not LVC...)

Are you saying it relies on the input clamping the voltage to +5V ? I can't see a 5V rail on the diagram?

I agree it needs a 5V1 zener, but 6K8 shunt sounds a bit high...

mark

Share this post


Link to post
Share on other sites
 
Save Old Iron

Very timely post guys - I'm currently investigating a test / repair fixture at SOI University. In the future, we may be able to offer reasonably priced repair services of the older 5486 based indicator boards.

projects001_tn.jpg

I have also sourced a supply of useable edge connectors, Its a far east source and necessary to buy at least 10 pieces per order. Extras are on hand in the event someone needs a replacement connector.

spareconnectors.jpg

repairedpcbwconn.jpg

Great work guys, thanks for adding to the knowledge base on the electrics.

p.s.. I believe there is also a 3rd version of this board which is total diode based logic instead of the 5486. The 555 flashes the oil pressure low light.

Parts are on order to covert the grain o wheat bulbs to longer lasting and brighter LED lamps.

and please don't let the lads in R&D know I let this one slip out

digitaldash.jpg

  • Like 1

Share this post


Link to post
Share on other sites
 
can whlvr

i have 2 boards that need work,good to hear from ya again chuck

Share this post


Link to post
Share on other sites
 
73-18 automatic

I love the look of the new board that you can read how much ????????

Share this post


Link to post
Share on other sites
 
wheeledhorseman

p.s.. I believe there is also a 3rd version of this board which is total diode based logic instead of the 5486. The 555 flashes the oil pressure low light.

You included a comparison photo of this in the original thread - so here it is again for completeness in this thread.

diplaypcnnewversion.jpg

Andy

Share this post


Link to post
Share on other sites
 

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
Sign in to follow this  

  • Similar Content

    • BrianKoch
      By BrianKoch
      Hello gentleman, 
       
      Problem: will not crank on its own, will crank with jump box—but still not fire
       
      Recent things I did prior to this issue: cleaned fuel filter, replaced battery cables, replaced cable from ignition to starter
       
      I have my test light out and am probing around. 
       
      There is no power at ignition/magneto wire or the ignition/starter wire
       
      There is power everywhere else






    • Jeff-C175
      By Jeff-C175
      Detailed wiring from the Demystification Guide #492-4509
      4 pages 1.4MB Color
      p7-67 to p7-70
       
      Onan powered
       
      1989 416-8 model 31-16O801
    • trident
      By trident
      Need some help. My 800 Special has an electric PTO. It is not working. Checked Ohms - nothing. My questions are 1) how does the PTO come off and 2) can they be rebuilt? I took the bolt out that I thought holds it on but no luck. Must be something more?
    • grasscuts
      By grasscuts
      Hi, here's what happened. I changed out 9 pin moles plug...had issues tracter quit running and changed the plugs while I was at it....got plugs from auto store cross reference to ngk they said it was the same heat range and I check electrod  length with calipers ,same...tracter started right up and ran great for about 10 min. And started back firing and quit.....going to check it out tonite anybody have an idea?    
    • jakester
      By jakester
      hello all I'm a new owner of a 312-8 tractor, have starting problems, came across a very small, what I believe is a relay between the battery and the dash,its about 2/3rd size of a cube of sugar silver on the outside blue on top with a hole bout the size of a pea, theres 6 pins with wires connected to it on the bottom do not know what this is any thoughts? I cannot determine the exact year of this machine for there is no tag anywhere only tag on engine kohler magnum md #m12s   ser# 170370459  spec#  471527  I figure its mid to late 80s to early 90s. of all the wiring diagrams ive looked at it does not show any thing like what I just described also can that little circuit board be removed that's like a test point for idiot lights? any help would be appreciated  jeff
×
×
  • Create New...