The following procedure is a fix for EDM and JDM type clocks, a similar fix for USDM clocks is described later in the article

I have found that a common failure in 99% of all defective/non-working clocks turns out to be a continuity failure between
2 passthrough joints indicated by red circles in the picture below (these joints connect top to bottom layer of the circuitboard),

In a normal working clock these two points should be connected and measure zero resistance (a short), but in many broken clocks this
connection has failed, probably due to age and/or heat stress caused by the two 181 resistors (these heat up a lot when the clock's display is on)

clock1

A very simple fix is to restore the connection by soldering a wire between these two points.

clock2

resulting in a working clock:

fixed clock

 

 

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Procedure for USDM type clocks


Although there are design/hardware differences between the EDM/JDM (single circuitboard) and USDM (dual circuitboard) clocks,
the USDM clocks have a very similar issue.
In this case the display circuitboard may have continuity issues between several passthrough joints (see picture below).
All of these points (each color set) should be interconnected and measure zero resistance (between yellow & yellow, blue & blue, red & red)

NB: it's not always the case that all of the points have lost continuity.
Eg. if the clock still works but display is dim, usually only one or two of the passthrough joints have issues.

To be able to check/fix the passthrough joints, you will need to seperate the 2 circuitboards.
More detailed information on how to do this: http://twinturbo.net/nissan/300zx/tech/clock/clock.html
(this article also describes how to replace the two 181 resistors, which is a fix for some failed USDM clocks)

clock_us


Once the 2 boards are seperated, you can do a continuity test on the board that holds the display.
With a multimeter check for zero resistance between each different color set.
(so you should measure zero resistance between red & red, between blue & blue and between yellow & yellow.)

You can only restore the connection on the ones that have failed, but it's probably best to be safe and restore all and solder in 3 wires (see picture below)
NB: the wires do not bypass any components, but only restore lost/failed connections between top & bottom layer of the circuit board.

clock_us_fix

 

 

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UNIVERSAL CLOCK FIX (for if nothing worked)

As a last resort, if none of the above fixes worked, you can purchase a simple and cheap universal DC 12V clock and make it fit inside the original housing.
In the example below I used a cheap clock / thermo & voltmeter combo from Aliexpress (LINK)

china_clock

For those that have a 3D printer, you can use this STL to fit this chinese clock's circuitboard inside the original 300ZX clock housing

reclock2

reclock1

reclock3

You will have to modify (shorten) the switches on the board a little.
That way it'll fit nicely and you will still be able to use the H/M buttons of the original clock to adjust the new clock

You can re-use the original connector of the clock as well if you want.
From the original clock's connector use the black wire for ground and blue wire for (switched) 12V.
(These little clocks have a built-in battery, so there's no need for a constant live 12V )

END RESULT:

time
(CURRENT TIME)

 

volt
(BATTERY VOLTAGE)

 

temp
(TEMPERATURE)

...this particular clock has 2 temperature sensors that you can use for several purposes, eg. outside & cabin temperature or coolant & oil temperature