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GM Guy 
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Joined: 31 Jul 2012
Location: NW KS / S.C. ID
Points: 1985
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 Topic: Transistor Circuit "Black Box" inside 7040 dash
Posted: 27 May 2019 at 11:18pm |
Hey all, Trying to diagnose a problem on the 7040. I thought I had everything working fine after wiring repair, circuit breaker replacement, and a new Alternator. Last year the system bled off a little "magic smoke" so I shut the key off and kept running. Found chassis harness short, and me and a buddy fixed it (he can solder better than I) thought it had to be related, as it didnt do it again. Fast forward to today, took it for a test drive to try the freshly repaired electrical out, and to warm up oil for a change. On the road the tach drops to nothing, and a faint whisp of smoke comes up. after shutting off, hyd. restriction and trans temp lights dont do their bulb check with starter button pressed. all others working fine. So, I tear the dash apart and cant see much, and so I have it narrowed down to two things with the assistance of the wiring diagram: 12V+ input on bottom of tach could possibly have a chafed wire grounding to the tach body, same wire also feeds idiot lights. The black box, aka transistor circuit. There is related wires as well. Problem is, I have no clue what it does! :) Can anyone explain that part of the system to me? Thanks!
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jaybmiller
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Joined: 12 Sep 2009
Location: Greensville,Ont
Points: 24418
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Posted: 28 May 2019 at 5:45am |
Hopefully someone will be able to psot a wiring diagram ! That should have the 'black box' shown and maybe a part number or description. Assuming the tractors a diesel, 'somehow' they have to get a signal to the tach..gassers use the 'points' as the signal...
I like electrical challenges but would need the diagram to help further.
edit.. found this... I assume the 7060 is cousin to 7040 , it show the 'transistor circuit', upper left under tach/hour meter.
Jay
Edited by jaybmiller - 28 May 2019 at 6:00am
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3 D-14s,A-C forklift, B-112
Kubota BX23S lil' TOOT( The Other Orange Tractor)
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GM Guy
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Location: NW KS / S.C. ID
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Posted: 28 May 2019 at 8:38am |
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that is it. So it works with the alternator to develop a tach signal for the tach?
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Gleaner: the properly engineered and built combine.
If you need parts for your Gleaner, we are parting out A's through L2's, so we may be able to help.
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LB0442
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Joined: 27 Dec 2014
Location: Boise ID
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Posted: 28 May 2019 at 9:14am |
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The transistor box just completes the ground when the start button is pushed for the bulbs that don't light with engine off, key on. Oil press, gen and I think trans pressure light up normally, hyd filter, trans filter and trans temp/oil temp are the ones that I can remember that it lights up. It is just for bulb check.
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LB0442
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Posted: 28 May 2019 at 9:22am |
 GM Guy wrote:
that is it. So it works with the alternator to develop a tach signal for the tach?
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Yes the tach uses the stator pulse of the alternator for it's control. The alternator has to have power and be charging for the tach to get a signal.
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jaybmiller
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Joined: 12 Sep 2009
Location: Greensville,Ont
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Posted: 28 May 2019 at 10:19am |
I looked at the wiring.... magic black box needs to be grounded. Wire goes from it to tach/hr meter to mtg post of tach/hr meter. It might be 'marginal'......
Jay
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3 D-14s,A-C forklift, B-112
Kubota BX23S lil' TOOT( The Other Orange Tractor)
Never burn your bridges, unless you can walk on water
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GM Guy
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Joined: 31 Jul 2012
Location: NW KS / S.C. ID
Points: 1985
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Posted: 28 May 2019 at 11:38am |
OK so the transistor will only get power when in the start position then? So my issue with tach dying after a few minutes of run time will not be related to the transistor? So basically the only thing related between the tach and the transistor box is the ground? If that is the case, I will polish up the connections and possibly run a dedicated ground down to the transmission case.
I do have a spare transistor box, but it sounds like it wont be needed.
Edited by GM Guy - 28 May 2019 at 11:50am
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Gleaner: the properly engineered and built combine.
If you need parts for your Gleaner, we are parting out A's through L2's, so we may be able to help.
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GM Guy
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Posted: 30 May 2019 at 12:24pm |
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OK, added a dedicated 10 guage ground wire from the tach mount stud to the chassis, no change.
So am I looking at a bad tach, or a bad transistor, or something else?
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Gleaner: the properly engineered and built combine.
If you need parts for your Gleaner, we are parting out A's through L2's, so we may be able to help.
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LB0442
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Posted: 30 May 2019 at 12:57pm |
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The transistor box really has nothing to do with the tach. I would definitely check the tach wire down to the alternator, it goes through at least 1 connector. Most of those connectors I have seen are pretty suspect at best. You could even try an extra wire looped over the hood just for a check. If your alternator is putting out a good output and the wire checks good then I would try another tach.
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Ryanlark
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Joined: 20 Sep 2020
Location: Virginia
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Posted: 29 Nov 2020 at 11:30am |
Ohhh, I actually thought that the transistor box is actually doing something else. However, taking into the consideration the problem that the author has described, I guess it has to be the tach wire. I just do not see anything wrong with all of the other parts. It has to be the wire. At the beginning I thought that it was the transistor box, however, in case it does not really have anything to do with the tach, it just has to be the wire. However I would actually call a circuit card assembly specialist, so he will be able to see which is the real issue.
Edited by Ryanlark - 02 Dec 2020 at 1:45pm
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Joe(TX)
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Posted: 01 Dec 2020 at 11:30am |
Ryanlark wrote:
Ohhh, I actually thought that the transistor box is actually doing something else.
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It does do something, just not for the tach.
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1970 190XT, 1973 200, 1962 D-19 Diesel, 1979 7010, 1957 WD45, 1950 WD, 1961 D17, Speed Patrol, D14, All crop 66 big bin, 180 diesel, 1970 170 diesel, FP80 forklift. Gleaner A
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