I am amazed that it would work without caps.Even with caps on the i.c. power pins you still have a risky arrangement. Especially in an electrically noisy automobile.Note that a cheap TM1638 or MAX7219 module would give you an intelligent 4-8 digit display.With the ability to alter the brightness too.As a kludge you could just put a low pass filter on the CLK line e.g. 100R + 10nF. (RC=1us)Likewise, you could apply treacle to every signal line. I suppose you could use longer time constants.You could also place 10k resistors across the input pins. I would guess that the display goes absolutely haywire when you remove the input cable.David.
Ok need more suggestions, I have installed the 0.1uf caps on pin 16 and 13 of the 595's and still same problem. Whatever it is I am doing by placing my finger on the clock wire I need to duplicate, it runs so good while I am touching that wire.
Do you mean you have connected 0.1uf from pin 16 to pin 8 (gnd) and another one from pin 13 to gnd ?
From the schematic pin 13 is connected to DGND and the GND header pins are connected to DGND.
If the 10k pullup and pulldown resistors work, you have/had bad wire connections.You can always add low-pass filters in the future. But if this is going to live in a real-life motor car, I do not expect a long life. (unless the car remains in an air conditioned garage)David.