Clean all battery terminal, and fuse connections, in the fuse block and under hood, same with all relays.

Then, make dadgum sure all your ground wires are secured with proper grounding at areas.

Clean these ground contact areas, and re-secure.

 

Then .......

....decide if electronic components need replacement.

sb66 said: will do - does the Hayne's have a decent diagram of these - do you know? 

thanks
Sean 

Some come with supplement wiring diagram manuals, some are all in one.

 

Look at Ebay for your year and make.

 

The C4 has an analog interior lighting controller with the adjuster lever located behind the headlight knob.  That feeds a signal to an analog regulator amplifier which looks like a large shinny heatsink located below the steering column. That analog regulator source feeds ALL the interior lights and control backlights everywhere.  However there is also a digital PWM (pulse width modulator) that is triggered from that control system, that is located inside the central body control module (located deep behind the radio).  That CCM supplies ALL the "illumination voltage" to everything in the Instrument Cluster, AC Controller, and original Radio (but not the actual tach circuitry itself).  The PWM from the C4 CCM is usually very stable,  but I have seen situations that cause the PWM to be unstable and cause the instrument panel light to flicker.  This can usually be heard as an unusual noise on the AM radio band.  The noise will be in sync with the flicker.  This seems to be always cause by an intermittent ground connection, of which there are several.  This is where the OEM factory manual really shines.  It shows you in detail where every ground connection is made and what connectors are involved and where they are all located.     

 

On my own C4, I had lots of problems with moisture corrosion on the back of the instrument cluster that caused all kinds of no-light problems.  I disassembled the cluster,  cleaned all the circuit board and connections,  and replaced EVERY light with LED lamps I got from www.SuperBrightLEDs,com .  I tried using the 161 socket adapters they sold to mount these new LED lamps, but still has some loose connections, they didn't fit the GM circuit board correctly.  So I finally discarded the sockets and soldered the new LED lamps to the circuit board directly. (I thought that LEDS should never fail and they will never need replacing again).  It came out beautiful and the LEDS are about 50% brighter than the stock 161 incandescent bulbs.  I now have no flicker of any kind and outstanding illumination everywhere.  I have since proceeded to replace a lot of other bulbs with LED replacements too. 

 

As for the unstable tach, again I recommend checking for a poor ground. The tach circuit is very simple and all on the PC board in the cluster.  There is no special filter shown but there is a white ceramic calibration resistor chip (examine that for a rare crack in the black carbon).  All of the gauges including the tach share a common ground wire circuit and "connection pin" on the back of the cluster and this connection is not common to any of the lights.  My pins were corroded.  Watch the battery voltage gauge.  This is connected to the same gauge circuit and will flicker if that voltage is not constant.  If the voltage gauge flickers this is a clear indication of a faulty ground.

 

For a summary, stay away for Chilton's and Hayne's and invest in the original GM factory manual.  They are still available as several places on the internet.  Here is a good example of a great investment:    https://smile.amazon.com/Corvette-Electrical-Diagnosis-Service-Supplement/dp/B00435L32C/ref=sr_1_6?ie=UTF8&qid=1498439756&sr=8-6&keywords=1987+corvette+service+manual

 

Good Luck

 

 

 

    

I could push (carefully) on the display and also the metal housing and would get various indications such as dash light flickering, fuel display changes, tach would rise and fall,etc. 

The pins are soldered onto one board and the second board sits on these pins. I noticed that some of the pins seemed loose and took it apart and re-soldered the pins. Once I took off the second board it was an obvious problem. It was most likely caused due to vibration and high mileage. 

Cleaned and re-soldered all the pins and reassembled.  Problem solved and still working fine after about 5-6 years. 

Hope this helps.

Fred in Alabama

Is your low voltage at idle with accessories on? ie. A/C and interior fan, electric fan at radiator, radio, etc.... ???

 

When engine is running:

13 to 13.5 avg.

14 to 14.5 at charging.

Note that Idling voltage will be somewhat different due to load(or lack of) on the system, but voltage should increase as soon as RPMs increase.