Help! Dim LCD Speedo Dashboard C4 1990

[AV8R]

The lighting for the dashboard on my 90 vette is barely visible at night. During the day its fine (not sure if the back lighting matters during the day or not).
I know that their are basically three common problems with these dashboards:
a. PCB can have a bad or loose connector connection, but this usually results in sporadic display information.
b. The LCD itself goes bad.c. The LCD itself goes bad or gets damaged.
c. The bulbs on the PCB can burn out or go dim over time.

Mine seems to be more like the third case, except I dont believe its the bulbs. So there may be another kind of problem that causes dim lighting.

The first picture shows how dim the speedo lighting is during night time or even dusk driving


The second picture shows how bright the speedo LCD is when I first turn on the light switch (engine off but accessory mode on)


The third picture shows the state of the LCD brightness after the light switch turned on stabilizes. Note that its dimmer than boot up mode.


Because the LCD lighting is bright during boot up mode, it leads me to believe that the back lighting bulbs are OK. So possibly something else
is dimming the lighting during low light ambient conditions. Is there an ambient light sensor that controls the dashboard lighting?
If not, can someone explain to me what is going on with my vette's speedo back lighting?

 

[John Robinson]

My guess is that you have a bad or weak ground on the dash circuit.

 

[AV8R]

The back lighting does not flicker, nor does it when the car hits bumps or if I smack the dash pad or tap the dash face plate.
Which tells me the connection isn't intermittent. Note I have a new battery and strong alternator.
If there is a poor ground issue, why would the display be very bright when the light switch is first turned on?

Hmmm, could the problem be caused by the rotory switch itself?
Where are the ground points to check that affect the dash lighting?
Do 90 vettes use light sensors to adjust the dash lighting?
How does one remove the dash cluster unit to get to the PCB?

 

[John Robinson]

This may be of some help to you. If your not familiar with the Corvette wiring just remember the life blood of the Corvette system is the grounds.

Thats not to say that someone else may have a better handle on your problem from first hand experience. Fortunately I have not had any instrument problems.

 

[AV8R]

John,

Thanks for the responses and the picture...
I didnt know there was an under the hood left side fuse panel. Is this ground point picture for a C4 (1990) vette?

On my 1973 Honda SL125 motorcycle, I fixed a battery charging issue by cleaning a ground point under the seat.
THis after swapping batteries, stator (very expensive to find OEM), light bulbs, and new wire harness.
So yeah, Im with ya on the value of good ground points. I wouldnt really know where to start with a problem such as Im having.

Again, any idea if my car has a light sensor? I know the 1984's had them in the dash itself.
And, how does one remove the instrument cluster to take a look at the dash PCB to inspect the bulbs and connections?

Im all about electronics, but automotive applications to get to them is not where my skill sets are.

Update: look what Googling around found for me. Its instructions for removing the instrument cluster and replacing the bulbs:
Note too, this site also has bulb replacement kits and digital instrument repair service too.
http://content.mamotorworks.com/pdf/601022.pdf

 

[John Robinson]

It is from my 93 FSM but I think it is pretty accurate for the C4 model. I have not had to remove the instrument panel yet but I know there are others who have. They should be along shortly.

 

[2VettesMike]

Do 90 vettes use light sensors to adjust the dash lighting?
How does one remove the dash cluster unit to get to the PCB?

It does, check around on the DIC panel, upper right with the buttons, and you will see a small hole with a light sensor. It dims it at night versus day. Sometimes near sunup/sundown driving you can put your finger over it and get it to shift the light level.

Thanks,

Mike

 

[boomdriver]

Yep..look for the 1/4" square hole in the dash display somewhere. Thats the sensor/photo cell and those CAN fail or break the soldered connection in the board. Use a flashlite in the dark to see if you can get a response. Key on, aim light into it, hold for a few seconds then see if there is a change. Remove the light, cover the hole so its totally dark and again give a few seconds for a reply. If nothing that could well be the problem. If the backlighting seem to work ok, and the dimmer works, its likely the sensor. These circuit boards do go thru a lot of grief with the vibration, heat, age etc..

 

[AV8R]

Yes, thanks for reminding me where to look for this sensor that is part of the DIC (Driver Information Center).
In the picture below, its at the top and to the left of the Trip Monitor labeling.

I plan to clean it out with a Q-Tip, then try the dark and light tests as suggested; then will report back the results.



This site does reference the light sensor and its mode of operation:
http://www.suncoastcorvette.com/forum/viewtopic.php?f=23&t=111
TIP: the CCM uses an ambient light sensor input for courtesy light control. When the CCM is used to cycle the relay, the ambient light sensor input is not used. Whether there is enough ambient light available or not, the courtesy lights should cycle on and off during testing. If all else fails and the courtesy lights will not come on in full daylight, test the ambient light sensor operation. Use a piece of electrical tape and a flashlight. Place the tape over the light sensor in the DIC button panel’s upper left corner while monitoring CCM 1.2 data at 03. You should see approximately 240 counts. Shine the flashlight on the sensor the counts should drop to 0. We have never seen one of the ambient light sensors fail, but as time goes by, they will.

 

[AV8R]

OK, so tonight I tested the dash brightness after first cleaning the light sensor with a Q-Tip.
I used a bright LED micro flashlight and aimed it at the sensor.

From the below left picture, you can see that the LCD is very bright and the gauges on either side are not uniform.
From the below right picture, you can see what it is like without illuminating the sensor with the flash light. Its very dim.



So this test tells me that the ambient light sensor works - how well I dont know, but it does work.
The problem seems to be one or both of these:
1. aging dash bulbs and/or their connections to the PCB
2. aging ambient light sensor

My next action will be to buy a new set of dash bulbs and try my best to replace them without messing up my instrument cluster.
The instrument cluster bulb kit is around $20. Also found a nice 91 vette (with 56K miles) DIC unit for 50 bucks.

This link offers instructions on how to remove the radio (for $5)
Car Stereo, CDM (Tuner/Receiver) Box, Front and Rear Speaker Removal For Corvette
Reference link to an article about brightening up the instrument cluster by modding the ambient light sensor:
(I assume this is after youve replaced the bulbs, as it makes little sense to do this with old bulbs)
1984: Increasing Instrument Cluster Illumination .: Corvette Action Center How-To

 

[AV8R]

Fixed the LCD backlight not working well and giving me a dark LCD at night...
It surely was the light sensor that was going bad. The DIC module that resides under the cooling vent. Its really a two part module. The left side has the switches and the light sensor, while the right side has the information lights in a rectangular box with a foil PCB that connects the light indicators to the back connector plug. I used the left side of the Ebay replacement DIC with the right side of my original DIC. I did this because the DIC PCB had different places for bulbs and I didn't want to take chances.

Figured out how to pull the center console apart:

Replaced the original DIC with one bought on Ebay used.

Well now my LCD speedo works as is should. Problem solved. However, now I have an INF and seat belt light that stay on. Which is now my next problem to solve. It may be the process will require the codes to be cleared on the ECM/CCM, else I have new problems. I may have to put back in the original DIC (both right and left parts) to see if the INF and SBelt lamps go out or not. Obviously maintaining a 20+ year old vette takes trial and error plus a lot of patience for the novice.

 

[vigman]

Good JOB!

Congrats~

 

[AV8R]

Happy Ending...

So there is a happy ending to this mystery story...

Not only did changing out the DIC (which had a good condition SCR-light sensor) solve my poorly back lit speedo LCD; but I was also able to fix the INF REST and SeatBuckle warning lamps on the DIC. I noticed these two warning status lamps on the DIC shortly after replacing the DIC module. This led me to believe that the cause of these lights had to do with the CCM or ECM on board computers. There must have been some power on when the module was swapped, there by making the computer think something was amiss with the system. I hear that power can stay on the computer for as much as 30 minutes after the battery is disconnected. This might be what bit me. Anyways, its likely that the active computer's programming sensed something wrong with the DIC unplugged, thus resulted in its posting the INF REST and BS lamps on the DIC. Still with me?

I dug into this forum and read everything I could about SB and INF lamp causes and repairs. From crash motion sensors, seat belt sensor/switch, cabling, alarm chime module, bad grounds and lack of dielectric grease, bad DIC or CCM. What I did do was test the seat belt receptacle for any foreign objects and the connector that goes to the main cable under the center tranny console because of advise I was given. Makes sense, maybe a bad connection. How that relates to the INF REST, I don't know, but do it anyways. So I tried both open and closed circuit testing on the SB sensor/switch and cable connector. Nothing would turn off the SB light.

My next tact was to return to my hunch was that it had to do with the on board computer because my plugging in the new/used DIC to place mine was related to the DIC status lights coming on. After filtering through the suggested fixes, I did find the one where it explains how to do the reset based on code12 being active. Theres a grounding and ungrounding on code value dance that has solved this INF and BS issue for others. I took it to my trusted mechanic who has the right tools for this kind of diagnostic and sure enough the codes matched what was in the procedure. We following the prescription and now the lights are cleared and we are good to go. It was a computer related solution for my situation and it wasn't hardware or grounding failure caused. I still might do well to replace the alarm chime because I still don't get an unbuckled seat belt alarm (never have as long as I can remember), but I do get a keys in the car buzzer alarm. Probably different modules, but its not critical.

Its important to note, that if the INF REST light is on, its likely that the airbag system is disabled. This is what motivated me to fix this promptly, begin that my teenage girl now drives. Also note, that when a continuity or ohm/ammeter is used; don't use one that actively supplies power as it will either blow a fuse in the restraint circuit or might even deploy the airbag.

There is almost nothing wrong with my 90 vette at this point in time with 87K easy Southern California miles on it. Im sure it will continue to need fixing as she is in her middle life crisis years. Such is the cost of holding on to these near-antique "classic" drivable vettes. Well, new leather seats would be nice… if I had 1000 bucks to burn.

there she is...

Or otherwise known as the ...

 

[Schrade]

...

begin that my teenage girl now drives.

...

Need pics of said driver.