My speedo on my 85 vette just quit,after acting crazy for last several months. Cfi-Efi and my self have ohmed the wires coming form the vss and we have confirmed no problems. When I hit a bump the speedo seem to try to work for a while and then quit again. I have two digi-dashes, and know both are in proper working order so we have eliminated the digi. With the wiring to the vss looking good we are questioning if the vss is our problem, and how do we check it. With the way the speedo malfunctions, we don't think it is the internal gear. Anybody out there have similar problems and how did you fix it.

 

To elaborate, the speedo in this car indicates, correctly, not at all (0 mph), and falsely, intermittently, when the car hits bumps. The stock wiring under the dash of this car has been hacked by an incompetent, attempting to install an aftermarket ECM and accompanying wire harness. There are a multitude of splices into the stock wiring, a good many that were unnecessary, and many that are of poor quality. Trouble shooting for other malfunctions has revealed this poor wiring and it has been repaired as it was discovered. We checked for intermittent connections, shorts, and continuity in the wiring between the VSS and the digi dash connectors on circuits 400 and 401, the yellow and the purple wires. They both seem solid. We have checked, corrected, and added, grounds. Two different digi dashes have been substituted, with identical results, to eliminate the cluster as the problem. The questions are: 1.) Has anyone ever had a VSS that was intermittent? One that worked, or didn't, as the car hit bumps in the road? 2.) If, when, the speedo drive gear starts to go (apple cores) is it ever intermittent? Can it work, or not, intermittently, as the car hits bumps? 3.) Are there any other wires that affect the speedo readout? Any other wiring that should be checked? This is a one owner 1985 that, in my opinion, was foolishly entrusted into the hands of a...

RACE ON!!!

 

just throwing this out there....

cruise control needs 25mph to engage correct?
can you see if the cruise drops out when the mph gets erratic or zero's itself?

 

I can confirm that I have seen erratic speedo operation with an apple cored gear so it is certainly worth inspecting. If the gears check good, try to get the car on a lift and run it in gear while whacking the speed sensor with a rubber mallet to see if the display drops out.
I'm not sure of the wiring on an 85 but another possibility would be the ECM or cruise modules if the speed sensor signal is sent there first.
On my 93 for example the signal is first sent to the ECM for conversion from sine wave to digital then sent to the cruise and body computers, finally the body computer or CCM sends the information to the dash for display. I don't think the 85 has a body computer but it may well send the signal to the ECM first, just an idea, good luck.

 

The 85 froom what I read goes first to the digi-dash. Is there someone who can confirm that. Also, the cruise control does act erratic also.

 

I just had this on my 86. Wouldn't really act intermittently, but after driving for approx 20 min, the speedo would start working.

I replaced the speed sensor on the trans, problem was solved.

 

Thanks toptechx6. That is the kind of info I/we were looking for. The '85 has no BCM and THIS '85 has a poorly installed Accel DFI Gen VII. On the '85, the wires from the VSS go to the cluster. The cluster houses the processor for the cruise control. In an '85 with a stock ECM, the VSS signal is sent from the cluster to the ECM. Among other things, the stock ECM controlled the TCC lock up. For a short while, I thought no complaints with the TCC indicated that a good signal was leaving the cluster. I have since been informed that the TCC is controlled by the Gen VII via rpm settings and not vehicle speed. Plus, I didn't realize, but just learned, that the problem also triggers erratic cruise control operation. Shoot!

As a reminder, the speedo acts the same with either of two different clusters in the car. Wiggling the wires and connections between the VSS and the cluster showed no variation on an ohmmeter. Therefore, we are back to either a bad output from the VSS, the unit or the gears, or some other unknown input to the cluster that we don't know enough to test.

Based on the report "I have seen erratic speedo operation with an apple cored gear", I think the next step is to follow the advice and get the car on a rack and check as advised. Some had reported that they thought, if the gears were shot, that the speedo was totally, no go. That was my first thought, too, but it occurred to me that if bumps could cause momentary contact... Hearing that it HAS been observed, means that we have to check it. Thanks again, toptechx6.

RACE ON!!!

 

As most people who have been around the early C4s know the information surrounding the "atari" dash is kind of elusive....those who "actually know" don't usually share the info....because they think it is worth ($) something.

I don't have documentation to prove it to you but the VSS actually goes to both the ECM and dash at the same time...via a splice point somewhere.

To confirm a dual feed: unplug the dash and remove it from the car then hook up a scanner to the car's ALDL connector and start and drive the car and you'll see that you can read the vehicle's actual speed from the ECM scanner.

Below is a 84 digital dash diagram which is virually the same as an 85 for all intents and purposes it clearly shows the VSS feeding directly to the cluster:

Note: don't land the VSS AC grounds with the DC grounds.

To me this sounds like you've got a wiring clusterfuke going on there which is going to take time to go through each circuit. This is kind of how it goes with buying a "pre owned" car....you never know what you got until you really get in there. Sounds like your off to a good start though and CFI-EFI seems to really know his 85's so just with that your better off than most having someone there to hold your hand through your ordeal. Take your time wiring problems can be very frustrating even when they haven't been tampered/butchered by previous owners.

If you haven't already done so take a look at the firewall junction 42 cavity plug "C100" bulk head connector. This connector has caused me some problems over the years. This plug can be trouble to get, to clean and seal but parts of the circuits your dealing with run through this connector. Ensure that the connector has no water in it and that the pins are not pushed back.

Hope this helps...have fun guys,

 

There are changes in the wiring made by a company called TAS auto, (Corvette Specilists) when they installed the Gen 7 EFI. They hacked the wiring up pretty bad as we are now finding out, so other than the 400 and 401 numbers going into the dash, we do not know if any of the original wiring is good, or what other wiring is needed to complete the circuit. Any knowledge here would be good. We are inclined at this time to test the VSS with another loaner to eliminate or confirm a good VSS

 

That is a great post, engle1147. Thank you. Just to clarify a couple of points:

Possibly stock, but if so, can you explain why my TCC and 4th gear (OD) don't work when the cluster is removed from the car? On T. Wayne Nelson's car, the Accell DFI VII has no VSS input.


I don't believe that the ALDL is active on this car any more. All read outs, codes and adjustments are done by laptop.


The whole under dash wiring is one massive "clusterfuke". This is actually a one owner car, for all intents and purposes. The many problems that have plagued T. Wayne Nelson, are the result of a highly incompetent, sloppy, or otherwise wiring challenged "person", that installed the DFI Gen VII. There are rat's nests of wires, plastic "splicers" that are failing, even where splices weren't needed and in circuits not affected by the ECM swap. The dash board has been broken and butchered as badly as the wiring.

That too is a good tip. However, I am not sure ckt 400 or 401 still pass through that plug. I did detect a butt connector on one of the wires high up in the tunnel. We are first going to have to get it up in the air where we can do a better, more thorough, inspection, and VSS and speedo gear check. Thanks again, engle1147.

RACE ON!!!

 

is the gen 7 setup going to stay in the car?

Testing the VSS is easy:
Hook a AC voltage meter up to the sensor's output conductors (with probes or clamps) and spin the input gear. Any AC voltage output should indicate a good VSS. Nothing to really "fail" inside the VSS itself.

 

The cruise contol function is built within/into the cluster. The ECM needs to see the 2002 pulses per mile to make that OD stuff work. The digital cluster is what cuts/clips the 4004 down to 2002 for the ECM on pre '90 cars. I can not expain the "clip" process ....schematic's "black box" drawing mystery. From what I've heard most cluster repair guys run like hell when they see a "cruise control/TCC failure issue" within a cluster...I've never come across a problematic one....so far.

I'm not sure what your looking to do with the car but if the car is that bad I think I'd order a "Painless" wiring kit and skip the checks. I've used these wiring kits on applications in the past and they are pain free....an you get to keep your sanity.

 

How often does a VSS go bad, cause, if that is not the problem then the other issues may be a SOB, one would be the Gear and the other is chasing more wiring......

 

Cool! Like I said, my TCC and OD don't operate when the cluster is removed. It was simply an observation while the cluster was out. Neither my TCC nor OD have problems (knock wood).

The nutzo crazy part of this whole nightmare wiring fiasco, one problem after another, is that is the DFI CAME with a new harness. The DFI was bought new and installed by people that, in my opinion, shouldn't be allowed to change a light bulb. There are WAAAY too many splices, using inferior blue quick splicers that are failing, one after another with great regularity. Circuits are spliced that never should have been touched. It appears to me that some of these splicers are to repair screw ups during the DFI installation. When the oil pressure gauge failed, he took it back and as a matter of routine, with not one second of trouble shooting, they replaced the sending unit. Amazingly, had they taken 30 seconds to check the gauge readout with the wire disconnected (it should read 80#) and then grounded (should read 0#), they would have (if they understood how it worked) realized that the wire from the sender to the gauge didn't have continuity between the two. It read 80# constantly. The replacement of the sender accomplished nothing, but to generate a repair bill, with no repair. Then, as we started to fix it our selves, we traced the break in continuity to a blue splicer. Why a splice in that circuit is beyond me.

I mentioned earlier, that possibly, there is a wire, we don't know about, that could affect the speedo operation. In your post with the schematics (and my FSM, now that I look) I see in addition to the ckt 400 yellow wire at D11 and the ckt 401 purple wire at C15, a black/white wire (speedometer ground) ckt 153 at D3 . And, while the ground (ckt153) does go through the C100 bulkhead connector, the VSS hi input (ckt 400) and the VSS lo input (ckt 401) do not. They connect through C467 (pins C and D) under the center of the IP. Thanks again, engle1147.

RACE ON!!!

 

Follow up to the speedo problem, we put the car up on a lift and pulled the vss, checked it out along with the gears, and everything was fine. BAck to the wiring again. Several years ago i had a company out of Ogden Ut (TAS AUTO) install a gen 7 stand alone. The wiring that they did is now falling apart. They had rewired the speedo (I have no idea why) and just twisted the wires together under the dash. That's it no tape, no solder, no anything. One of the wires had came undone.
Beware of shops that claim to be Corvette specialists, this one bled me dry, and will not back up thier work.

 

My 86 had the same issues when I ran it without the digi dash