I noticed a flicker with the dash lights and well timed high pitch pulse when the radio is on - as well as a moody tach readout - the interference seems to keep close time with what sounds like the pulses of the motor - I was wondering if anyone else has come across that? I saw somewhere that the '85 has a tachometer filter - does that have anything to do with it? - sorry this is on a 1985 standard
any thoughts or opinions would be greatly appreciated
Vette(s): 1990 C4 coupe, Bright Red, L98, auto (stock)
The Corvette has a pretty complicated electrical system in the dash. I have a 1990 and have extensively studied the wiring diagrams from the original GM Factory Service Manual. ( I must say that GM has an outstanding manual, especially in the electrical areas).. Neither Chilton nor Haynes manuals will even come close. They are much too general and don't cover the necessary details.
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.
Vette(s): 1986 Gray/Blue Z-51 Coupe & 1998 Pewter Coupe
I also had similar flickering and several intermittent digital dash problems on my 86. Turned out to be the pin style connectors on the circuit board inside the display assembly.
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.
wow - thanks everyone - I found a service manual for my year(85) as all I had before was the Hayne's - it has some of the diagrams - but from the sound of it, not nearly enough. I will be honest I am not very good at this electronic gremlin finding - hope it's ok, I may have more questions to follow?
I will start checking some of the obvious grounds and work my way from there
In looking at many of the details of my car, I do not get the impression that the last couple of years before I bought it were very good to the car - kind of a get in and drive - nothing else - an earlier owner did change the wheels - but that is all I can tell that has been done.
I had ordered a powermaster 120 amp alternator for replacement and a
rebuild kit just in case- also for the sake of wanting to know how and
if I could ($13) there were a few issues with the power master -so they
are sending a new one for exchange -
the long and the short -
the lights do not flicker any longer - however, the volts now only read
11.4 - 11.7 and they have a red light next to them - I am presuming
that's not a good red light?
the volts read steady between those
numbers when driving - but I went from 14.3v before to 11.4 - ish should
I be concerned? the battery is roughly 6 months old. I am going to
put a charger on it to see what happens - but that seems like quite a
any ideas or help would be greatly appreciated
(I am still chasing grounds and cleaning when and where I can)
update - did a rebuild and clean while I am waiting on the new unit (extra $13 - thought it was worth a try for my first time) everything cleared up - no flicker and my voltage stays 13.9-14.2 driving and stationary. So, at least for now, everything is ok
The commonest fix for this is to replace the headlamp switch. Remove the dash top (easy) to access the wiring at the back of the switch.
The carbon track wears or gets stuck in its set position if you don't use it much rather like a crackly volume control on an old radio where you would wind it back and forth a few times to clean the carbon track. You can do this with the headlamp knob, but replacement is easy and a better fix.
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