84 Datsun 720 Gauge Hackery.

[igotatruck]

Greetings Rat-Monkies!

First of all, thanks to everyone for your previous posts, they have been very helpful.

 

I recently purchased an 84 Datsun 720 King Cab in need of some TLC.

One (of the many) things that needs attention is the Panel Gauge Cluster.

The fuel gauge, water temperature, and tachometer were not functioning properly.

I do remember that the gas gauge was working when I picked it up.

 

I have been lurking the forum for the last few weeks trying to sort this out and made a little progress tonight.

I had little success finding an auto electrician who would touch it and since I do electronics for a living, this was personal.

 

So I tried every little test, fuse, relay and connection I could find before yanking the cluster module out to test it.

I hooked up a DC power supply to the Fuel and Temperature gauges to see if they worked, and they both worked fine.

In another post around here I was reading datzenmike saying the voltage regulator keep them at 8V but they worked with 12.

 

It does seem like the internal voltage regulator is busted, and though this seems to be more a matter of calibration/consistency than function, I will put a small replacement together tomorrow and post my findings.

 

Here is a link to the pic of my disassembled fuel level gauge, responding to the DC power supply. I cant seem to get the Ratsun forum to show it, sorry.

https://goo.gl/photos/R2qqggYTkdK3ux9V7

 

 

 

Other associated pics:

https://goo.gl/photos/atPymVYiPeUWstfG9

 

I cleaned/reseated all of the connections on the plastic flexy circuit board and put it back into the dash (I need it to drive to work tomorrow), only to find that the fuel gauge was now working! Hopefully it just had a loose connection in the dash and wont turn up again. I also have yet to see if the Temp gauge works.

 

More on the Tachometer later; I do know that its little resistor module measured at 2.2kOhms the other day (sounds okay to me). When I hooked my meter to the tach connections, I seemed to find: a chassis ground, +12Vdc and a third voltage around +12V that would drop (~0.5V) when I revved it. Hoping this is the correct control scheme for the tach, and I dont have to undo any wire wrap.

 

If anyone has any ideas or anything please let me know, hoping this might help someone else out there.

Thanks again!

[Lockleaf]

Temp gauge you can easily test in the vehicle. With the keys on, remove the wire from the temp sending unit and ground it to the manifold. If the temp gauge goes to max everything is wiring fine and the issue is connection to sender or the sender itself.

 

2.2k is the correct resistor for the tach.

 

Maybe this will be of help.

 

[igotatruck]

On the ride to work today I found that the Temp gauge was now working also, gonna chalk it up to loose connections to the cluster.

https://goo.gl/photos/L1tJQZnqX7rL9LAF6

 

Tonight I will try injecting some voltage into the tach meter to test if it works.

[Lockleaf]

There are many corroded connections in these harnesses at this point. Often just disconnecting and connecting can be enough to get them working again. Happens often with the fuse block in these trucks too. Did you check to see if the tach wire is connected to the coil?

[igotatruck]

I will check the Tach wiring after work tonight, it has a much more robust three prong connector on the back so I am less suspicious of that connector.

 

Spent my lunch break scraping a little replacement for the Voltage Regulator. I made it using an LD1117 variable regulator with a trim pot so I could dial in a range between 7-9Vdc and kinda tune the gauges to the sensors.

 

Picture of the replacement regulator:

https://goo.gl/photos/G3U3PLKcJmZwPHnn7

[Lockleaf]

Very cool. I'm just starting to try and learn electronics right now. I'm attempting to schematic out a bluetooth/aux input radio, controlled by arduino, tucked inside the stock radio housing. Fun times, but i seriously have no idea what I'm doing.

[igotatruck]

That sounds awesome, do you have a thread going on that? I might be able to help, I specialize in audio gear and I have done quite a few arduino projects.

[Lockleaf]

No thread yet. I would only have one post in it, it would say "here's this idea i can't do...please wait months while i begin to figure this out." :)

 

If you don't mind, i might pm you a couple questions.

[igotatruck]

Hey, its been a few weeks but I have done a bit more work on the gauges.

As of now, all gauges are working except the Tach.

I pulled the tach from the cluster and plugged it back in so I could rule out connection problems.

The wiring is all good from the ignition coil to the Tach Resistor and then to the Tach Gauge.

I have ground and +12V in the connector as specified, and I am getting an interesting signal on the final connection.

 

The signal coming out is a gnarly square/sine wave that goes up in frequency as I rev up. The voltmeter says its ~24Vp-p but my scope cant wrangle it into anything it can measure, as it 'clips out' wildly.

 

This leads me to believe that my coil is somehow messed up and putting out a signal that is too ugly,

OR

The coil is putting out the correct pulse and my gauge is hecked.

 

I wish I had a real function generator I could hit the gauge with to test it, but that will have to wait till I can get back in the shop.

 

Anyone got any thoughts?

 

UPDATE: I am measuring the spring/coil on the gauge as being an 'open circuit', meaning it's probably cooked, it should be a few ohms at least. Ordered a replacement from ebay, should be here next week. I payed $45 for the replacement if anyone was curious.

[igotatruck]

No thread yet. I would only have one post in it, it would say "here's this idea i can't do...please wait months while i begin to figure this out." :)

 

If you don't mind, i might pm you a couple questions.

Also feel free to send me any questions.

[Charlie69]

Is the 3 wire tac plug plugged into back of the gauge cluster?

[igotatruck]

Is the 3 wire tac plug plugged into back of the gauge cluster?

Yes it was, then I pulled just the Tach from the cluster and plugged it back in, by its self outside of the cluster housing. I was measuring the voltages from the front of the little gauge circuit board so I know it was getting to the board.

 

I am 90% sure that the gauge is toast, and I have a new one on order, I will post an update once it arrives.

 

Now onto replacing the stereo!

[Charlie69]

Sounds like you have it under control.  On trucks that do not have the tac installed from the factory the 3 wire tac plug is taped to the main harness.

[igotatruck]

Happy Tacho Tuesday everyone.

I am pleased to report that the new Tach works like a dream.

When I got it I measured the resistance across the spring/coils that move the needle and got a healthy 70ohms, which means it's not burned out like my old one.

So if your Tacho craps out, you can test it with the ole' DMM.

[PNW 720 97499]

Stumbled across this thread looking at tach problems. Mine reads 1500 rpm. Gunna break out the DMM this week and see. What was the replacement tach? Just some generic replacement or a specific unit? 

[datzenmike]

Maybe your engine is idling at 1,500.

[igotatruck]

On 2/18/2019 at 8:41 PM, PNW 720 97499 said:

Stumbled across this thread looking at tach problems. Mine reads 1500 rpm. Gunna break out the DMM this week and see. What was the replacement tach? Just some generic replacement or a specific unit? 

I got it on ebay, the exact replacement, it was a lucky find.