Search is your friend "cleaning score reels" brings up this and another topic:

https://pinside.com/pinball/forum/topic/cleaning-score-reels

Anything that is a multiple of 5 on an EM goes through a score motor switch.

Consult the schematic and then check the score motor switch — clean and ensure it’s properly gapped.

If it scores 1000 points consistently in other cases it’s not likely to be the reel.

I think this is the most helpful video I've watched on Decagon cleaning and adjustment:

Easiest way to determine if it's the reel or the score motor:

Rapidly hit a 1000 point switch (or the 1000 point relay itself) at about the same pace as the score motor triggers. Do it 10 times and see if you advance 10,000 points. Do this several times. If you consistently get 10,000 points then your score reels are probably fine, and you can concentrate on the score motor switches. If you sometimes only get 7000 or 8000, you will know that it is missing scores even when not running through the score motor, so then you can concentrate on the score reel or 1000 point relay switches.

Here is something to look for: each score reel has a brake that keeps it from back-sliding after it steps forward. Gottlieb called it the “index flat spring” and you can see it pictured here (A-9158) in this diagram from their parts catalog. If you can turn your thousands reel backwards with your hand, it means this brake is not engaging correctly and you must adjust it so that it does.

index flat spring (resized).jpg

Agree. Watched that video before rebuilding reels. Very helpful.

just to make sure ... there's a few ways to score 5K on the game. Are you only having problems scoring 5K from a lit drop target?

lit drop targets don't score via the C relay, they use the B/500pt relay to run the score motor and the D relay to cheat and send the pulses to the L/1000pt relay instead of the M/100pt relay.

Have you tried cleaning and/or adjusting the score motor switches associated with the 5000 point scoring? Dirty or mis-adjusted score motor switch is most likely causing it to miss a pulse or two.

Do you have a schematic?
Can you read/understand a schematic when looking at it?
Do you own a switch adjusting tool?
Just trying to see what your EM skills are.
Chances are very good: score motor switches, score motor switches & the score motor switches.

I don't know if it's been suggested, but make sure to tighten the switch stacks on the score motor. Should be done before adjusting any switches.

Contact Steve at PBR and get a schematic, switch adjusting tool & flex contact cleaning tool.
By getting the schematic, you can see the whole picture of when people post pics of certain area's in this thread & if you plan on keeping the pin and/or get more of them you really need to learn
The good news is: you have a single player pin which makes things easier to see and adjust compared to a 2-4 player pin.
PBR description, part #'s and price:
Flexable file: part# TL-PLASTONE $2.86
Schematic for "king Pin" SCH Copy $18.00
”L” Contact Adjuster ADJ $4.56

Marco sells a set of adjusting tools for different angles:
Switch adjusting tool set, 3 piece Part # 77-SWK $12.99

Always try and buy all of your parts from Steve at PBR. Why...??? Because he supplies business's like Marco with parts but you can buy from Steve for not nearly the cost from others. Example: A coil from Steve- $10 exact same coil made by Steve supplying Marco- $19.99 & Steve offers a 10% discount on any orders over $100 on most everything he sells.

And remember, 99% of the time you can see the problem. EM repair/diagnosis is very visible to the eye, and have patience. Sometimes trying and doing guess work on things just creates more headaches.

5K is scored by pulsing the 1K (L) relay five times. The switch creating the 5 pulses is score motor 1A with red+white and slate+red wires.

you'll need the schematic eventually, but the typical diagram you will find on it is below. See https://pinside.com/pinball/forum/topic/score-motor-schematics-help for some info.

use the diagram to figure out where stack 1 is, then notice the A switches are on the bottom level ... so of course you care about one that is hardest to see/access It's easiest to find stack 1 by seeing where stack 3 is due to 3 being mounted diagonally.

some of the gtb score motors are mounted on a hinge so you can flip it up and see the lower switches more easily.

the normal problem is the 1A switch has arced a lot and you'll need to file the contacts to remove most of the pits and burning. Get the metal contact file for this as it's more aggressive than the flex. You pinch the blades to apply light pressure while filing both contacts at the same time to make sure the contact faces are parallel.

if the contacts are mostly gone, you get the fun of replacing the switch blades/contacts.

it could also be the switch stack is just loose or the blade needs a little tweaking to get good switch closure.

having said all that, the same 1A switch is used for scoring 500 points, so if 500 always works, the 1A switch is probably not the issue and it's more likely the L relay switch that powers the score reel solenoid is marginal. Could also be a switch in the path from 1A to the L relay coil is flaky so L occasionally misses a pulse.

suggest you watch the L relay while scoring 5K points a few times. If the L relay is always evenly pulsing closed 5 times, that points to a different circuit than if the L relay is sometimes missing a closure.

gtb_score_motor (resized).png