When he describes testing the speedometer on the batee site, he says to hook the negative to C15 and the positive to D11, and describes the signal as 1 volt peak (2 volts peak to peak) square or sine wave. You should be able to do a continuity test to figure out which is which, and if you are not wanting to pull the cluster, you could pull the pass. side hush panel and use the connector there. The diagram does make it seem like it does not matter, though.

 

I may be over thinking this. Im going to test the two wires coming from the connector and see if ones connected to ground. Then just connect the sender output to the other one. I hope its that simple.

 

I would not think that would be a conclusive test, since both wires connect to circuitry within the cluster.

 

Im going back to what Cliff HNarris said about one wire being connected to ground in the cluster. The schematic on Batee for the later year cluster shows that. Maybe the 84 doesnt connect one to ground in the cluster, but I should be able to tell with a dmm I would think.

It seems odd that they show the conector pinout and one says,vss iput use the other for ground, then the other says vss input use the other for ground. I wish they had a better schematic of the 84 cluster part of it is totally missing.

 

In the hand drawn schematics of the cluster on batee, C15 is labeled as speed sensor low, and D11 is labeled as speed sensor high, which lines up with the testing procedure polarity.

 

This is correct to go by only for 84, in 86 everything is quite different as far as wire location, only the colors are the same. Using this and batee, D11 (at the cluster) will be yellow, changing to tan with a white stripe, and C15 (at the cluster) will be purple, changing to pink with black stripe.

Btw, this is the single row connector under the ecm, or somewhere near it.

 

What Im trying to figure out now is, my gps sender has one lead out for the sender signal. Its referenced to ground. If one side of the vss input isnt ground or cant be safely grounded, I have to figure out some way of connecting it then. Thats why I wish Batee had the whole schematic for the cluster. Its missing at least one page, the one that shows where the vss connects to of course (not the pins bu the circuitry, so I could decide if its safe to ground d11 or c15)

 

The FSM is screwed up. It shows the same thing in my '86 manual.

The lower right corner of batee's schematic is missing, which just happens to be where the VSS circuit is located (well, half of it is there). I traced the circuitry on the PC board and drew my own version when I was having speedometer problems. The reference designators changed between '84 and '86, but I'm pretty sure the circuitry is the same:

http://forums.corvetteforum.com/c4-t...84-1989-a.html

You can check the connection from the VSS to the cluster by disconnecting the top cluster connector and measuring the resistance between D11 and C15 (I got 435 ohms):

 

I get what you are saying, but unless the guy at batee used two completely separate power sources for the cluster test power and the signal generator power (maybe he did? the vss creates its signal independently), they will end up sharing a ground which should be the "low" he indicated on pin C15. I understand your hesitation to just hook it up and see. This is when it sucks to have one year only hardware, but like cliff said, I think all the earlier circuitry was the same, although it did have some exterior differences. Tell you what - my car is on jackstands, so I will run it and see if I can detect a polarity on the vss circuit. hopefully that will help, I would like to know about this, too.

 

Okay, checked it out and found what I expected - no discernible polarity on the output of the vss, it's an ac signal. I also could not tell anything from checking the wires going to the cluster, no apparent single connection to ground. I would still be inclined to use batee's instructions on testing the speedo as to whether or not it is safe to hook a ground to one of the pins -

Testing the speedometer: Warning: This step will cause the odometer to increase. It's a good way to test the odometer, but if you don't want to do that, disconnect the odometer wire before performing this test. You'll need a signal generator (or a simple 555 timer circuit). Set it to square wave or sine wave, 1Vpeak (2Vpk-pk) amplitude, and 20Hz. Connect the (-) terminal of the sig gen to the C15 terminal of the cluster, and the (+) terminal to the D11 terminal of the cluster. The speedometer should show 18MPH. Increase the frequency to 200Hz, when the speedometer should show approximately 178MPH. Don't increase the frequency beyond 200Hz.

 

Well this is what I did. I measured both wires to ground with an ohmmeter. One wire showed less than half an ohm to ground so Im sure that one is connected to ground inside the dash probably. I hooked the output from the gps sender to the other wire that used to go to vss connector at the transmission (the connector was melted, the wires are now coming up inside the car)

I wasnt sure if the gps sender was connecting to the satellites, it says in the instructions from Intellitronix, the red light means its not connected, the green light is power, and the blue light comes on to show it is connected. I have a red and a yellow on, and after a while the green one came on. I tried the last 2 days calling they take your number no one has called back. Dont expect a lot of support.

So, I got the frequency reading dmm out and hooked up, 10 20 30 40 50 as the car speed increased. I think on mine the yellow may be power, and the green one comes on when it connects. I kno wIm not color blind, so I think the documentation is about as helpful as their tech support.

But, it seems like it is working. Next thing I guess is pull the dash out again and send a signal to c11 d15. I have a feeling it wont show anything on the speedometer, but if the odometer runs at least I know the rest could be ok (odometer and output to ecm I hope) and I could build a small digital circuit to show the speed.

My last option of I give up, is to buy a programmable speedometer and hook it up I dont know where I would mount it yet. My dash is nearly perfect and I wont use any tape or screws on it. I was thinking about closing the air duct on the left side of the steering while and mount it in there but its kind of far to look over.

Maybe I will have time tomorrow to pull the dash and go from there.

One other thing is Im not sure how dmv or my insurance company will feel about me using an after market odometer.

 

I don't believe you would have any trouble with the DMV, in most states I believe a licensed inspection station can issue a speedometer replaced sticker that gets affixed to the door jamb of the car and that should satisfy both state safety inspections and since the car is over 10 years old I doubt that DMV would even record it. The inspection station would certify the odometer from the car presently and date it.

You might try calling your local State Police - in most states they're the controlling agency for state safety inspections and such.

The not working issue could be that it's transmitting something other than the Sine wave the cluster wants to see. The AutoMeter that does similar to what I believe you bought doesn't generate a Sine wave.

Good luck!

 

I checked my speedo out again and wrote down the readings so you could compare to yours and hopefully be able to tell if the cluster is at least receiving your signal. I did not do any key off tests before.

Between the two vss wires, I got a reading of 395 k ohms with the key off. With the key on, the reading began to drop to about 352 k within about a minute, then kept dropping even more slowly(I did not keep watching past 346 k, which took a minute or two more) When I turned the key back off, the resistance began to climb back up.

Checking pin D11 (yellow wire at connector by ecm) to ground, the readings were the same as between D11 and C15.

Checking pin C15 (purple wire at connector by ecm) to ground, I got less than 1/2 ohm key off, and got a reading of -19 ohms key on, so I swapped leads, and got a reading of 950 ohms with the positive lead on C15, and the negative lead to ground, so I checked for voltage and got 26 mv key on, and 0 key off.

After that I checked for voltage between the two wires, and got .5 volts key on with the positive lead on D11 (yellow wire at connector by ecm) and negative lead on C15 or ground.

Guess I should have been more thorough the first time, as it appears that C15 is basically a ground. I did every reading I could think of, hope this helps. BTW - I took all of these readings with the vss disconnected. When I did a frequency test of the vss. I got 60 hz at 52 mph. Using that reading and batee, you can multiply the vss signal hz by somewhere between .86 and .9 to get mph.

 

Thats about what I calculated too and saw at batee and on Cliff Harris thread.

And Im sure the input shouldnt really have to be a sine wave, since they say you can bench test with a 555 circui. They pretty much produce a square wave (not exactly square if it isnt 50% duty cycle). Most digital circuits if theyre fussy have more trouble with a sine input than a square wave. I cant see that as the problem, but ill see if mine reads when bench tested as per Batee. Or Cliff Harris.

Thats not to say the level isnt incompatible, now I can see trying to get my oscilloscope to work in the car

 

A 'scope is one thing I wish I had. I'm no electronics expert, but I really like seeing how things work and figuring out issues, (and more tools is never a bad thing, either) Do you get the voltage readings I did from your cluster? I was hoping that would help to see if there was a problem with your cluster without removing it.

 

I havent gone back out tonight it was almost 60 today bit now we hav freezing rain

And the dash is already half way out, I hadnt put the screws back in yet. Its the maneuvering that gets me, I might try to drop the steering column to get it out.

I appreciate you doing that research Im still trying to piece together in my brain a way to implement the data you gathered. Im expecting that not only is the sender screwed up in this car but the dash itself too (it was not taken care of well at all ) That might explain why I bought it for 2200. Im lucky it drove home from Michigan to St Louis/ I went against a lot of good advice to not even try it ( I felt adventuresome at the time )

 

I've had my cluster out quite a few times, but always with the dash pad loose. A few wiggles and jiggles at the right angles and it comes out pretty easy.

 

The speedometer chip in the cluster is a custom GM part, so no help on that one. If there was a data sheet available, that's ancient history and I was not able to find one.

I believe the input to the speedometer chip is a comparator, so you should be able to put a square wave into it without any trouble. The chip just squares up the VSS signal and puts out a 5 volt square wave.

 

It finally dawned on me why the fsm is like that - it is not screwed up. This particular diagram is only meant for use with the connector unplugged from the cluster, and to check inputs.(The view is looking at the side of the connector that plugs into the cluster) Since the vss is a separated device creating it's own signal, you HAVE to use the other wire to get the resistance measurement of 350 to 500 ohms that they specify. Since you are only checking resistance, polarity does not matter.

 

I did all my tests at the connector by the ecm, but it will be the same to do them at the vss plug you brought into the car. That will identify any opens or shorts in the wiring, as well as identify which wire is which. If you get similar readings to mine, it should indicate your cluster is probably ok.