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TunerPro is telling me I’ve got a Code 24 VSS error.
When I replay the TunerPro data log, it tells me my test run through the neighborhood produced a lot of triple-digit speeds, with a special fondness for 254mph.
So, I guess something is going on with the speed sensor.
I’m about to crawl under there, and excavate the goop from all the oil-leaks off the transmission, to see what can be seen, in order to make progress toward repair.
I’m already getting the scent that new replacement sensors are unobtainable.
Are there specific features of the VSS, or the transmission, that I should make note of for when I go begging on the Internet for a used sensor?
My car is 1988 V6 2.8 5-speed.
Will the VSS from other GM vehicles with a 2.8 V6 and 5-speed interchange with my Camaro?
Grab yourself a copy of the service manual for your car, and look up the trouble code. Follow the flow chart to determine what is setting the error.
I've never heard of the cluster mounted VSS failing. 20+ years on the forum, not once do I remember anyone talking about one going bad, or looking for a replacement. It's solid state, if the speedometer spins and the sensor 'sees' it, it should be fine. Now the plastic circuit on the back of the cluster? That could fail. The contacts between the flexible circuit and the harness header... The speedometer cable... Speedometer gears, etc... Plenty of other things that could fail. That's where the book and the charts come in, so you don't end up randomly throwing parts at the problem.
May the luck of the Seven Pillars of Gulu be with you at all times.
I’d be curious about that speedometer cable....is the speedometer operating normally? Not jumping around as you drive? A bad speedometer cable can produce those symptoms, but it’s usually obvious when the cable is about to die.
I get a small amount of bounce of the needle at speeds below 20 mph. Steady needle at speeds above 20. New cable was installed by professional when I got the car about 5 years ago because cable was making helluva squealing racket when I first got the car.
I need to pull the gauge cluster and have a look nonetheless.
I look at stuff, but sometimes don't know what I should see.
Suggestions?
Here we have the female wiring harness connector with the VSS pin-socket marked
That is the CD connector, the smaller one is the AB connector. Note the pin numbers on both the ECM connector and the harness connector. Top row (A side), pin 10 is the VSS input from the buffer.
That is the CD connector, the smaller one is the AB connector. Note the pin numbers on both the ECM connector and the harness connector. Top row (A side), pin 10 is the VSS input from the buffer.
RBob.
I see that now on the wiring diagram.
There are TWO different brown wires feeding into the ECU through each connector-plug respectively. Each respective brown wire is located nearby a black/white-stripe wire. I got decoyed by that black/white-stripe wire. As soon as I saw that black/white-stripe wire near a brown wire, I assumed I was on the right connector.
I'm a real novice at reading wiring diagrams.
I see now there is a sort-of dual notation on the diagram that I really need to pay attention to. I square-outlined (green squares) the two connections that I confused. I see one is marked C1 and the other C2. Do the "C" references denote "conncector 1" and "connector 2?"
I really appreciate the oversight.
Heading out there now to inspect the A/B connector.
The C1 & C2 within the ECM outline are as you mentioned, connector 1 and connector 2. But that only makes the diagram more confusing.
The labels in the small box are the pin designation.
Each connector has two rows, the small connector, the top row is A. The lower row is B.
Similar to the larger connector but they are row C and row D.
The row value is molded into the connector shell (remove the combs to see it). You can see the pin number molded into the front of the shell (remove the combs to see it).
This makes pin A10, the tenth pin over on the top row of the small connector.
Note that the combs can have the row value and pin number molded into them. But don't use it as a bunch were molded incorrectly.
The C1 & C2 within the ECM outline are as you mentioned, connector 1 and connector 2. But that only makes the diagram more confusing.
The labels in the small box are the pin designation.
Each connector has two rows, the small connector, the top row is A. The lower row is B.
Similar to the larger connector but they are row C and row D.
The row value is molded into the connector shell (remove the combs to see it). You can see the pin number molded into the front of the shell (remove the combs to see it).
This makes pin A10, the tenth pin over on the top row of the small connector.
Note that the combs can have the row value and pin number molded into them. But don't use it as a bunch were molded incorrectly.
From what I’m seeing, everything looks like it is where it is supposed to be, and – with the exception of some (non-critcal???) areas of the laminated circuit board on the backside of the gauges cluster, everything looks like its in serviceable condition. I perhaps should mention that the red wire from the VSS to the speedometer-pickup has one small spot where the insulation looks like it is mashed, but not breached.
Any suggestions as to focused circuit-testing or other remedies would be greatly appreciated.
The pics that follow are pics of the smaller ECM connector (which I understand now is the connector associated with the VSS circuit), and pics of the gauge cluster and VSS unit itself.
Smaller ECM connector (wire side) Third wire fromn the top I believe that is the "brown wire" we have been talking about as being the wire that feeds from the gauges to the ECM to deliver the data stream for the VSS unit.
ECM sockets - both sockets - smaller socket brightest
ECM - both sockets - closer - smaller socket brightest
ECM wiring harness connector - smaller side
ECM harness connector - backside – blue clip/”comb” removed - smaller side Brown (third wire from top) VSS wire looks OK?
gauge cluster connector – VSS/tachometer side - pulled out to show numbers Is that the brown VSS wire (looks more gray in this pic than in real life) going in there at position #14?
wiring harness – passenger floorboard - hanging below dash Brown VSS wire maybe coming in here?
wiring harness hanging below dash – larger view
gauge cluster - front side – shows usual tach position – tach never moves
Rbob correct me if I’m wrong.ohm the brown wire from the ip to the ecm to check for a open in the wire. I think the optical vss signal from the ip to the ecm varies from 0-12vdc . You might want to backprobe a10 and take it for a drive and see if there is voltage.
Rbob correct me if I’m wrong.ohm the brown wire from the ip to the ecm to check for a open in the wire. I think the optical vss signal from the ip to the ecm varies from 0-12vdc . You might want to backprobe a10 and take it for a drive and see if there is voltage.
Assuming abbreviation "ip" means "input" as in connection point of brown wire at speedometer?
I will experiment with this.
That would be interesting to know whether the VSS is capable of feeding a full 12 volts to the ECU. In my limited reading about ECU's I formed the belief that the maximum voltage on any of the ECU circuits or sensors was 5.0 volts.
Not being much of an ECU guru, I also wonder if it is possible (or probable) that the ciruitry in the ECU might be receiving good signal from the VSS, but choking on it somehow because of a bad circuit or component in the ECU? TunerPro was reporting a constantly-fluctuating vehicle speed of 0-254mph. Mostly in the high double-digit numbers, but bouncing frequently into triple digits as I drove around the neighborhood.
Since I have a spare ECU I will swap-out the PROM and CALPAK from the ECU used for the first test, and I will see if I still get the same crazy vehicle-speed, and same two error-reports (VSS and Fuel Pump) with a different ECU running the same PROM and CALPAK.
Nothing like over thinking an issue and making it much more complicated than it has to be.
Seriously, there are flow charts for every trouble code in the GM/Helm Service Manual that walk you through the entire process, so you don't have to tear apart the car looking at wires and pins that aren't related. Beg, Borrow, or Steal a GM Service Manual. Usually can find most of them on Ebay for $30-50, sometimes you'll find them on the shelf at the local library... Or if you ask nicely someone will usually photo or scan the pertinent pages and post them for you.
Looks like you now you know where to probe ! Now that you've checked that all the terminals look OK and the wiring is still intact, it's time to start putting stuff back together so you can trace the source of the issue.
** Your Cluster printed circuit looks like it's beginning to deteriorate. ( all the green crud is the copper breaking down. ) be sure to check the connection from the VSS buffer thru the printed circuit to be sure there are no breaks in the circuit !!
Once you've got everything together you can use the flowcharts to try and isolate the bug. This pair of flowcharts is for an 86 MPFI V6. I'm GUESSING it's similar to your 88.
** BE SURE to turn off Cruise Control - if equipped .
I get a small amount of bounce of the needle at speeds below 20 mph. Steady needle at speeds above 20.
My GUESS is a bad speedo head OR bad/missing teeth on a speedo gear are causing the Code and the bouncing needle.
The optical part of it is that black thing with the 3 wires that bolts up right next to the speedo cable.
It "looks" into the speedo head, at the physical piece that the cable plugs into. That is a sort of bar-shaped thing, with the ends bent up, that's magnetic. It spins around a sort of drum that's attached to the speedo needle. The faster the cable (and bar) spins, the more it tries to pull the drum around with it. Kinda crude. Anyway the optical thing "looks" up in there at the bar through a hole, and produces a pulse every time the bar passes by it. The cable spins 1000 times per mile. (NOT 1001... pretty simple to see that it's impossible to create an odometer gear train that will indicate tenths of a mile in 100.1 revs, then advance 1 full mile for every 10.000 revs of that... it advances to the next mile on the 1001st revolution, which is to say, after 1000 have been completed... kinda like the way the centuries are divided, since there's no "year 0 AD") Then, since the bar spins 1000 times per mile, and has 2 ends, there are 2 pulses per rev, which is 2000 pulses per mile.
In any case, you should see nice square pulses on the input to the ECM, at the above rate; you could use a square wave generator to simulate it. 33.33 Hz = 60 mph.
The bouncing needle is usually caused by friction in the cable, which makes it twist and get torsional tension in it, and then release the torsion, unevenly. The end of the cable at the speedo doesn't run at a constant speed as a result. The cruise regulator is REAL bad about causing this. There are no "teeth" in the speedo chain of events, only in the odometer's, which doesn't enter into the picture in any of this besides helping expose the 1000 revs per mile arithmetic. Because of how the optical thing works, nothing about the speedo head can make it fail, unless the bar thing has become invisible to the optics. Which maybe rust or something could do, but I've never seen or heard of that happening. It's really quite reliable for being so ball-of-mud simple.
Last edited by sofakingdom; 06-16-2018 at 12:23 PM.
Thank you to all of you for taking the time to look at the stuff I posted.
I'm working on it today.
My first experiment today will be to swap-out the ECM and run the car with another known-good (or at least "known-not-completely-failed") ECM with the original 24-pin PROM and original CALPAK.
In the meantime, I want to share with you a short video. This is a video of six gauges shown on the TunerPro dashboard. Just to give you an idea of what I was talking about regarding the vehicle speed jumping all over the place.
At this point, rather than swapping parts, I'd recommend coming up with an oscilloscope and looking at the waveforms going into and coming out of the yellow box. You can hook up a drill to the far cable end, or to the cruise regulator if you have that, and spin it at some moderate speed, and see if the pulses you get match the speed you're spinning it at. A typical drill will turn in the 1000 RPM neighborhood, which is exactly perfect for 60 mph. You should see pulses at 2000/min, i.e. 33.33/sec, which means every 30 msec on the scope.
Never overlook an opportunity to buy a cool tool!!!
You could even just look up inside the speedo where the little black opto block goes, and see if there's extra stuff floating around in there besides just the bar piece; spider webs or the like might cause bizarre behavior. Never overlook the value of a good thorough visual inspection, either.
Today l swapped out the ECM #1 (modified with Moates G1 adapter, and running 28-pin copy of original PROM with original CALPAK chip) used in last-weekend's datalog test.
After letting the engine warm completely in the driveway, I took a short drive with ECM #2 running the original 24-pin PROM chip and the original CALPAK chip.
Same behavior of the reported speed. Vastly-high abnormal speed numbers reported via TunerPro.
Curiously, with ECM #2, TunerPro is not reporting the "ERROR" code messages for the VSS and fuel pump voltage as were reported with ECM #1. Maybe because I didn't drive it very far, or let it run long enough, the codes didn't have time to "set."
During today's test, I initially noticed the speedometer needle bouncing quite a bit. Remember, I pulled the gauge cluster out of the car for the first inspection. So the speedometer cable got disconnected during that process. I'm 99% sure I got the speedometer cable re-connected securely. The gauge-cluster speedometer engagement-clip "snapped" neatly and audibly over the flange of the speedometer cable when I re-installed. The needle-bouncing seemed to lessen as I drove the car today, and the wildy-wrong speed numbers settled down some - especially when I was able to hold a steady speed for any duration. But, I was still getting crazy speed numbers whenever I would slow or accelerate.
I'm fairly confident now that the issue is NOT in the ECM. Both ECM's reported crazy speed numbers.
Would a buggered speedometer drive-gear or driven-gear at the transmission cause this?
I realize after viewing the video I'm saying "computer" while showing a video-image of a laptop PC.
Just to minimize chances of confusion, when I say "computer," I'm talking about the ECM, and not suggesting that I used a different laptop computer to do the datalog today.
I drove the car about 20 miles today. Seems to me that if the ECM was able to see an "error" in the VSS or the fuel pump voltage, it would be "reported" by now. Nothing showing as an "error" so far today.
Also - and this maybe is a more general question: Why do some fields on the list, for instance "VSS Pulse" or "MAF Pulse" show nothing to indicate status?
I replaceed the VSS (along with the ENTIRE GAUGE CLUSTER - albeit a V8 cluster in my V6 car).
With the new gauge cluster, I'm no longer getting that jumping-all-over-the-place VEHICLE SPEED number on TunerPro.
I feel pretty good that the vehicle speed reported on the new-cluster speedometer is correct, or very close-to-correct. I'll do a comparison using Waze when I can get the car out on a straight stretch of road, where I'm not constantly changing speeds.
I'm pretty sure that the vehicle-speed number, and the tachometer RPM-number that I'm getting on TunerPro are higher numbers than actual.
I believe the VEHICLE SPEED SENSOR issue on my car is now solved.
Replacement of the gauge cluster (which included the VSS), cured the jumping-all-over-the-place speeds being reported to the ECU.
Because the gauge cluster I installed is from a V8 car, the dial for the speedometer and the tachometer on the dash read low. However, the information that the gauge cluster is sending to the ECU is accurate information.
can these sensors be purchased?I am running into same answer.I have 87 firebird with cable speedometer but I want to change to electric.I have cluster and yellow"buffer" and plug for sensor.
06-10-2018, 08:16 AM
