My speedometer stopped working so I took the cluster apart to see what was happening. First I had to trace out the circuit so I could see where the signals are supposed to go. I made this schematic. I put "1986" in the title block because the reference designators don't match the ones in the cluster schematic I downloaded from here: http://www.batee.com/corvette/dcrg/s...cpuschematics/, which appears to be for an '84 -- hard to tell, the lower right corner is missing, which is where the title block is located. I am missing one piece of the puzzle -- I can't find the connection to the microprocessor. [ EDIT ] I did find the connection, and through the magic of Internet links the schematic has now updated itself to show that connection, shown as VSS2. [ /EDIT ]

The SPEED_WARNING output is interesting. It's labelled "SPEED SIGNAL FOR OVERSPEED WARNING" on the batee.com schematic. No C4 Corvette I know of had this feature, which makes a noise if you go over the set speed.

http://www.misterpeachy.com/VettePic..._Schematic.pdf

Then I did some testing. The sine wave input comes from the VSS at 4000 pulses per mile and a square wave goes out to the ECM and cruise control at half the frequency (2000 pulses per mile). In my case the sine wave is going in but the output pulses are not there, as measured on the speedometer IC pins. The 'scope trace shows the sine wave on the bottom and it's just a straight line on the top trace:



The one place I connected with sent me the wrong part and then they told me they only had one used part. I'll go with that (not much choice), and supposedly the right part is on its way. I don't want to buy a whole cluster just to get one part. I did get the bad IC unsoldered from the PC board. It was pretty difficult to get out (very large ground connections that really soak up the heat from the soldering iron). I also found that the conformal coating glued the chip to the PC board. It's always scary to pry on a chip and have it not come out -- usually that means damage will occur with too much force. 50 years of soldering experience pays off -- no copper on the pins and no plated through holes ripped out or pads lifted. The speedometer IC has silver paint on it (16 pins). The incorrect chip has 14 pins -- obviously the wrong part.

 

You DO have an "UPSHIFT" light though I believe. An '86 MY first I believe. There were no GM's in the mid 80's with "SPEED ALERT".

 

It forever amazes me what you EE's are willing to do for our community. Thanks for posting up tour findings. Cliff!

 

The UPSHIFT light is controlled by the ECM. It turns on if I go over 40 MPH in Drive for more than some time that I don't remember. I think it's interesting that the same parameter is used to turn on TCC. The two things are not related, so why did they use the same parameter?

There is no connection to the speed warning output. The FSM shows that pin as "NOT USED".

I happen to know about speed alert because a friend of mine showed me how it worked on his car. That was a long time ago and I don't even remember who it was or what car it was.

 

I'm just trying to fix my speedometer. And 'm tired of seeing SERVICE ENGINE SOON with error code 24. I got the "new" speedometer IC today. We'll see how that works.

I really don't want to buy a whole cluster just to fix a problem with one IC. I saw a cluster with a broken LCD on eBay at something like $17. I bid it up to over $100 and gave up there. At least with the auto bids you know right away if somebody else has bid higher so you can keep bumping up the bid until you reach your pain threshold.

 

Cliff, If the new chip doesn't work, PM me. I'm pretty sure I have known good one in my bench stock.

 

All fixed. The cluster is back in the car and working fine.

I did discover a few interesting things along the way. First is that the VSS to ECM signal comes from an open collector (Q1) so it needs a pullup resistor to see the signal (the pullup is inside the ECM).

I was trying to find a data sheet for the speedometer chip (I was not able to), but I did see that many of the speedometer ICs have a stepper motor output (4 wires), which is used to run an analog type speedometer. There are 4 unused pins on the speedometer chip, so maybe that's what they are for. I forgot to probe them, so I don't know if that's true.

The odometer stepper motor has a label on it that says HSI (Haydon Switch & Instrument) and number 33460. I think that number might be an abbreviated GM part number. Many of the GM ICs use only 5 digits out of 8. The speedometer IC is marked 23688 and I believe the actual part number is 16023688.

I also calculated the VSS frequency for a couple of speeds using (4000 pulses/mile) divided by (3600 seconds/hour) = 1.111 Hz. So 60 MPH comes out to 66 Hz and 150 MPH is 166 Hz. I was thinking that it would have been easier to calculate MPH from the VSS signal if the output was 3600 pulses/mile. Then I remembered that the ECM (and I presume the dash also) calculates MPH by measuring the period (time between pulses).

Here's my dash doing it's thing with a 66 Hz input on the VSS pin. I set the signal generator to 66 Hz and plugged in the dash. It came up right on 60 MPH. The 'scope is showing the sine wave VSS input signal on the top trace and the VSS_TO_ECM signal on the bottom trace (squared up and divided by 2):

 

..... Cliff , I have an '87 and I switched to a 4L60E trans and am using a TCI trans controller ... the controller is plugged into the VSS on the trans , it has a one-wire hook-up for a speedo with a 4000 ppm output ... the original chassis plug is a two wire ... my question is , which of the original two wires will drive the speedo or do I need to connect directly to the cluster ? ... In reading the instructions for the TCU I notice that there is some tuneability for speedo's ... I have to power up the unit to see what other PPM's it lets me select .....

 

What's TCC? Does that have to do with the torque converter? What you say above may have changed in later years...separating functions.

I think the upshift light is pretty short...only about 1 second for display (though I can't remember). It also is based on throttle opening too.

I raised mine high enough that I'll never see it. I don't need no stinking puter telling me when to shift!

 

The VSS input on the cluster goes into the yellow wire. The other VSS input is a purple wire that is grounded inside the cluster, so you don't really need it as long as your TCI box is grounded.

Usually you can select different outputs depending on your rear axle ratio.

 

TCC = Torque Converter Clutch.

My upshift light comes on at 40 MPH. I think it stays on forever as long as I maintain my speed above 40 MPH. I don't know if the ECM gives up after a while and says the heck with it, he's not shifting so I'm turning it off. It's more than just the speed. It also considers TPS, LV8 and RPM.

[ EDIT ] I did a test today and found that the SHIFT light stayed on as long as I stayed above 40 MPH. It took a fair amount of time before it came on. I was cruising at about 50 MPH for roughly a minute when the SHIFT light came on. It will go off if you stomp on it (that's where the LV8 factor comes in -- it's the ratio of the TPS reading to the MAF air flow). When LV8 is high the ECM assumes you want to stay in a lower gear for more acceleration. [ /EDIT ]

The upshift light also comes on if you hit the redline setting. That's 5200 RPM on my car.

 

........ If I read this right , the VSS feeds into the cluster NOT the ECM ... the ECM gets its speed feed from the cluster ? ....... and , the OEM output from the VSS is 4000 pulses per mile ? ... The TCI box has settings for actual gear ratio and programmable outputs to work with the various OEM and aftermarket Speedo's ...... Thanks ... Bob ..........

 

On '84 to '89 C4s the VSS signal goes to the cluster first at 4000 pulses per mile and then goes to the ECM at 2000 pulses per mile. In later years the VSS signal went to the ECM first and then to the cluster. I don't know what year that changed. In 1990 the whole interior of the car changed, so it might have been then.

 

Cliff - nice work on the dash console. Mine has what I believe to be a ground problem. The left half is bright and the right half very dim. After a period of time the right half matches the left half. Sometimes the dimmer switch will dim the display (when the headlights are on). I thought (and found after much looking) only one ground location applied to my 89 Coupe - under the car, above and slightly left of the left exhaust pipe, in front of the driver's compartment. Is there another location I should be looking for? I keep reading about a ground on the driver side pillar. Is it under the dash on the left side?

I considered splicing into the 3 grounds on the console electrical harness but if I don't have to do this I prefer not to.

Thanks for all you do -

 

If you look at the picture in your post you can see the 4 back lights shining around the reflectors. It sounds like one or more of yours are not working. You can find out by removing the dash bezel and looking for those 4 lights.

The dimmer control on the headlight switch is almost completely over-ridden by the photocell in the dash. Daylight is MUCH brighter than darkness (obviously) and the dash must be very bright to be seen. That's why you can't really see a difference in brightness by turning the headlight **** in the daylight.

The dash has two grounds. One is for the various sensors in the engine compartment. That ground is either on the side of the block above the oil filter or on a bell housing bolt. I have recently seen that '84s have a ground stud on the back of the driver's side head.

The back lighting for the cluster is grounded at G201, which is on the driver's side door hinge pillar. It's at the bottom of the dash brace above where it bolts to the hinge pillar, hidden behind the carpet.

 

Thanks Cliff! now I'll dig around in the interior to find the ground..I hadn't thought about the 4 bulbs. Just assumed they were working because I can see the LEDs. Good tip (again).

John

 

Photos of Ground

 

ive resoldered and repaired all conections, the speedometer is working and
the lighting is functioning.
however the tach display is not working can some one tell me where the chip is located on the board? and how to repair.
thanks steve

 

The only way you could repair it would be to take a chip off another dash PC board and put it on yours.

Bryan posted 3/4 of the schematic on his www.batee.com web site. He left off the title block and the circuitry in that area (which had the speedometer parts, among others. I don't remember if the tach circuit was also in that area). I'm not at home so I can't be more specific.

 

First thing, you should start a new post for your question. Your question has nothing to do with the original post.

Second, what year 'vette? Do we have twin cars?

Third, is your tach display totally dark ("display not working"), or does it read zero RPM? Are you sure your tach filter is good? Your tach filter is located near your distributor, and may be disconnected from the dist, or have a broken wire. That's easier than replacing an IC!

Cliff is good, but he's not psychic!