1974 GT750 Gear Indicator Question

ilbikes · Post by **ilbikes** » Sun Oct 25, 2009 6:40 pm

Guys, I have a question - on the 1974~1977 gear indicator, which unit controls the LED numerals? I ask this question because I installed a NOS unit and wiring to the shift drum and had two very nice units for the top that both tested good. With 89 miles now on the GT, the 3rd gear indicator looks like a backwards C. The middle horizontal line does not light up sometimes - about half the time yet the same row of lights works perfectly for the 2 and the 5. My assumption was thet each gear would be connected to the same LED and activated as needed to display the corresponding numeral. This is early 1970's tech so it has to be a mechanical connection. The question in my mind is which of the units is to blame?

Inputs appreciated.

Gordon

Suzukidave · Post by **Suzukidave** » Sun Oct 25, 2009 7:16 pm

Gordon my guess would be a fault in the gear indicator light control circuit . If you have a spare dash gear readout it would be a simple test to just plug it into the harness to see if it shows correct numbers .

ilbikes · Post by **ilbikes** » Sun Oct 25, 2009 7:26 pm

Dave, I sent my spare down to Allen Wood because he had the same problem - my spare did not help his 1st gear lighting return. I am suspecting it's the bottom unit, just hoping for some insight on someone else's research.

Thanks,
Gordon

tz375 · Post by **tz375** » Sun Oct 25, 2009 7:28 pm

Gordon,

There are two possibilities. First is the rotary switch on the shift drum. I'd check that for continuity first. Try each gear in turn and move the shifter around and see if it holds a solid connection at the connector block before the headlight (under the tank).

If that checks out as solid, the net possibility is in the LED unit itself or its connections.

When you check out the rotary switch, keep a record of which colors are connected in each gear. Us that knowledge to apply power to the led unit to test each gear circuit.

Suzukidave · Post by **Suzukidave** » Sun Oct 25, 2009 7:53 pm

If just some gears didnt read out at all i might suspect the contacts at the motor end but being the indicator is showing the gear but screwed up i still think its in the electronic circuit controlling the readout .

pjmcburney · Post by **pjmcburney** » Sun Oct 25, 2009 8:18 pm

ilbikes wrote:Guys, I have a question - on the 1974~1977 gear indicator, which unit controls the LED numerals? I ask this question because I installed a NOS unit and wiring to the shift drum and had two very nice units for the top that both tested good. With 89 miles now on the GT, the 3rd gear indicator looks like a backwards C. The middle horizontal line does not light up sometimes - about half the time yet the same row of lights works perfectly for the 2 and the 5. My assumption was thet each gear would be connected to the same LED and activated as needed to display the corresponding numeral. This is early 1970's tech so it has to be a mechanical connection. The question in my mind is which of the units is to blame?

Inputs appreciated.

Gordon

Hi Gordon,

I can categorically state that it's NOT your sending unit on the end of the shift drum.

I've had a similar problem to what you describe, and it happened to me when reusing original (old) components when I put my bike back on the road.

I decided to diagnose the sender first. After a fair amount of checking continuity etc, I came to the conclusion it was working perfectly.

The sender unit is just a rotary earth switch, nothing else. There are no 'smarts' in it whatsoever. It will either work, or it won't - it can't half-work because you would have no display at all for one or more gears otherwise.

All that happens is, when you cycle through the gears (turning the selector drum in the process) the sender turns with it and completes a circuit from a common earth to each of the individual wires in turn.
These wires are connected (via the harness) to the LED display circuitry. Any weird reading you get on the display is to do with that circuitry. This is of course a real PITA as the circuitry is sealed in epoxy...
If you can remove the epoxy and gain access to the PCB beneath, then you may be able to repair it - good luck!

Otherwise, the only fix I know of is replacement of the display, and that's what did it for me.
I bought a (cheap) damaged set of instruments from eBay that had a perfectly good LED display within - voila!

BTW... there are differences in the LED displays as well.

The early ('74, '75) displays have very fine LED elements compared to the later much coarser elements - it's very obvious when you compare the two next to each other. Plus the early displays have an almost greenish viewing window, whilst the later versions sit beneath a purple window.

Good luck in your search
Cheers
Paul

Coyote · Post by **Coyote** » Sun Oct 25, 2009 8:35 pm

Don't know what gear you're in? I am ashamed of all you very accomplished riders.
Ride by the R's. You can't see the damn thing in the daylight anyhow. My GT550 is the first bike that ever had a gear indicator. It doesn't work correctly (some of the LEd's are burned out). If you have ANY question to what gear you're in, go back and start over at square one.
Really people, the gear indicator was just no more than a gimmick.
In you're case Ibikes, I realize you are doing a total pure resto. I do have a spare plastic one that you are welcome to try if you want. I had another metal one that I gave to another board member. I have no idea if the one I have works as there is no way to bench test these babies.
But you are welcome to it Gordon. Just PM me and we can work it out. I am talking about the rotary switch that attaches to the gear selector.

Post by **Suzsmokeyallan** » Mon Oct 26, 2009 7:38 am

Its more of a "having it there" and wanting it working as it should. To add to Pauls post, the first generation LED units for 74-76 are housed in a red plastic piece when you look at the unit out of the instrument housing, the later 77 ones are black with a white coloured '8' display section.

tz375 · Post by **tz375** » Mon Oct 26, 2009 9:51 am

Gordon,

The most likely problem is the LED display itself, but the PCB that triggers the LEDs is somewhat sensitive to resistance, so that's why I suggested that you check the wiring and rotary switch first.

If they are solid, and that is most likely, then testing the display is in order. Fortunately, it is easy.

The display is connected through a power (orange) and ground (Black/white) plus the six pin connector.

Disconnect the 6 pin either under the tank or in the front of the left frame side cover. Turn on bike power and nothing should show on the display with the connector disconnected. Use a test meter probe to ground each terminal in turn, and the appropriate segments should light up.

If any individual element fails to light, the problem is most likely that light array, but if it switches on in some positions and not in others, it is the controller board or in line resistors that are at fault.

Yes. there's a circuit board embedded in that epoxy in the display unit. It is prone to damage from moisture ingress - so I'm told.

For the seventies it's quite advanced - almost as good as the Sinclair pocket calculator I bought in 73

It is possible to re-engineer that display but unless that turns you on, I'd replace it.

If you need further clarification, call or PM me.

TLRam1 · Post by **TLRam1** » Mon Oct 26, 2009 11:03 am

On my 74 I need to get into my issue, one leg stops working after a short ride, otherwise it works until I ride it.

Sounded like a loose connection I thought I could resolder, sounds like I can't get to the connections from what am reading.

tz375 · Post by **tz375** » Mon Oct 26, 2009 11:25 am

Terry,

That sounds like a bad connection to one segment of the display. There's not much space in there and there's thirty year old epoxy obscuring the view, but it's worth stripping it out and checking.

In theory, it would be simple to redesign the display to use modern LED displays in the same 7x5 dot matrix format. Of course they need a driver and voltage has to be around 1.5v, but that's not too complicated.

With a 7 digit LED, you have to drive 7 segments, with the dot matrix it's necessary to drive 7 segments each containing 5 individual LEDs and that appears to the the way the spec sheets show from nightbrightusa.

You probably want to stay away from simple 7 segment LED like this one from Radio Shack http://www.radioshack.com/product/index ... Id=2062557 .

ilbikes · Post by **ilbikes** » Mon Oct 26, 2009 5:00 pm

Paul, Richard, Dave - all thanks for the inputs. I'll get me a new display. It makes sense that it's a contact related issue - it's working sometimes, and not about half the time.

Coyote, you nailed it - I want it to be working just because. Not one of my other bikes has this and I think it should be standard on every bike. I ride different bikes every week and it's just impossible for me to memorize the rpm in every gear or the sound. The guages are the face of every bike - they are the part that looks back at you no matter when you look. I'd prefer to see that light to be working even if I'm in 3rd gear for no more than a minute total each time I ride.

Thanks again guys -

Gordon

rngdng · Post by **rngdng** » Mon Oct 26, 2009 8:12 pm

Gordon,

Other than the gear indicator, how's the bike running?

Lane

pjmcburney · Post by **pjmcburney** » Mon Oct 26, 2009 9:10 pm

Gordon,

Yes, it's more about it being 'there' and working properly than whether or not you use it.
I've seen the pics of your restos and I understand completely the need to get it all 'right'.
It depends on your degree of anal-ity(?).

I have to confess I wanted mine to work properly, but I almost never actually look at it.
Many bikes of the era had similar indicators, and it's my guess they didn't get much of a look at by their owners/riders either.

The bigger and torque-ier the engine, certainly the less of a need for one.

Cheers
Paul

TLRam1 · Post by **TLRam1** » Tue Oct 27, 2009 12:00 am

tz375 wrote:Terry,

That sounds like a bad connection to one segment of the display. There's not much space in there and there's thirty year old epoxy obscuring the view, but it's worth stripping it out and checking.

In theory, it would be simple to redesign the display to use modern LED displays in the same 7x5 dot matrix format. Of course they need a driver and voltage has to be around 1.5v, but that's not too complicated.

With a 7 digit LED, you have to drive 7 segments, with the dot matrix it's necessary to drive 7 segments each containing 5 individual LEDs and that appears to the the way the spec sheets show from nightbrightusa.

You probably want to stay away from simple 7 segment LED like this one from Radio Shack http://www.radioshack.com/product/index ... Id=2062557 .

I will try to get in there and attempt getting the epoxy out. Those were my thoughts also, resolder connections and that may do it, the problem sounds like getting to them.

I, as said before, like all to work as it should, don't need it, part of the character of the bike.

1974 GT750 Gear Indicator Question

1974 GT750 Gear Indicator Question

Re: 1974 GT750 Gear Indicator Question