I'm great with electrical problems on paper, but in the field, my head spins.
So heres my problem: As I accelerate, the speedometer almost looks like its doing what its supposed to, but at a much faster rate. meaning by the time I'm doing about 30 mph, it says I'm doing 120. At a stop, it says zero though. After trying 4 different speedometers, I have decided that the speedometer is not at fault.
So, I have the schematic, but I cant make anything of it. I was just gonna throw some parts at it and hope wiring is not the problem, but thought I may get some input first. Thanks in advance.
The speedo gauge is soldered to a circuit board that does several things. The board receives a dirty, unrefined pulsing electrical current from the VSS. That pulsing frequency is related to the speed of the car, but it needs to be "cleaned up" and converted into two different, more refined signals. One is a 4000 pulse per mile signal that is fed to the speedo and odometers, and the other is a 2000 pulse per mile signal that gets fed to the ECM and Cruise since they have different requirements.
To isolate where your problem is, you can forget about the ECM and Cruise circuits since they only receive signals from the speedo board. That leaves the speedo board, your speedo gauge, your VSS, and the wiring as the only possible problem areas. Since you've already replaced the speedo and the board several times (they are a single integrated unit), you can eliminate them from the troubleshooting as well. That leaves only the VSS and the wiring as possible sources.
The most likely culprit and the easiest to check is the grounding point for the system which is notorious for breaking at the eyelet or being burnt from the heat of the exhaust crossover pipe. The ground point is called G504 and on the V6 models it's a pair of wires (one green and one black) that are soldered to a common eyelet found here on a cylinder head stud:
It's hard to see from above but often the wires are broken right off the eyelet.
If that's not the problem, then the next most likely culprit is the VSS: borrow a known good one from a friend and swap them out. Make sure you put your own specially colored gear on the end of the VSS to make sure it will be properly calibrated.
If that still doesn't cure the problem, then you are probably getting some stray electromagnetic interference that's changing the signal sent from the VSS to the speedo circuit board. Usually the only kind of thing that can cause that is something like a failing ignition coil, alternator, or even water pump.
So the G504 did not look broken, but to be sure, I checked resistance directly from the ground in the connector under the speedometer to the negative terminal and found almost zero resistance. I guess step 2 is vss. And since I don't know anyone with a known Good one, I'll just try my luck with a new part. I know new doesn't mean good, but even if it's bad, I'm sure it won't do the same thing that's happening now.
So the G504 did not look broken, but to be sure, I checked resistance directly from the ground in the connector under the speedometer to the negative terminal and found almost zero resistance. I guess step 2 is vss. And since I don't know anyone with a known Good one, I'll just try my luck with a new part. I know new doesn't mean good, but even if it's bad, I'm sure it won't do the same thing that's happening now.
cant remember, easy to remove right? could pull the one outa my 88 gt and let ya try it.
I think so. I think the vss is closest to the trunk, near the dipstick? And wow that'd be great! Are you free today? Gimme a call or text if you don't mind 410-877-5484 Thanks, Greg
Hey blooz, you were talking about not losing the gear off the vss, but I think mine is a magnetic. I believe I have a 5 speed getrag in my 87 2.8 and all I could find is a magnetic sensor close to the trans dipstick. It's held down with one of those little fork looking things.
If your transmission has a magnetic speed sensor, that would explain the skewed speedometer readings. The electrical signal from the magnetic speed sensor is different from the Fiero's gear-driven speed sensor. You'll need to build or buy a signal converter. I think Dakota Digital sells such a thing.
goto your car and check to see if it uses the newer style HTOB or the old style fiero slave(not sure if its possible to do that or not with the newer trans?) that thread also tells the dakota digital box needed. http://www.dakotadigital.co...duct/product_id=126/
id def make a post to see if anyone knows how to make a simple conversion circuit for it tho.. save a bit of $.
Unfortunately, the slave cylinder location isn't a "slam dunk", so to speak. It is possible for the gearbox to have a magnetic speed sensor and an external slave cylinder like the Fiero uses. Actually, my Fiero has one of these gearboxes. It came from an '88 Beretta.
That said, if the slave cylinder is bolted directly to the bellhousing (i.e. no mounting bracket) or it has no slave cylinder at all (hydraulic T/O bearing), then it's definitely not a Fiero gearbox.
just wanted to say thanks to you guys. slowly hunting speedo issues. replaced vss and gear,next was rip apart dash to inspect board and speedo,then electrical line troubleshooting. following this thread, i looked at my ground wires from above pic(bloozeberry) and while not corroded off, loose bolt was found. will strip/reconnect/heat shrink wires and tighten. Don't know if it was my issue yet, but would never have found these with this forum. thanks again.
Yeah, so as hookedonspeed said, I have the wrong transmission in my car, resulting in a magnetic pick up vss instead of the gear driven one. I skimmed a page on Rodney's website and he explained everything in full detail on the conversion. With parts, I'm looking at like $20 plus the vss. Aaaaaand taking the transmission out of the car to do so
Well I just don't like the idea of it. Are they adjustable? I also just think it's not the right solution for the car. I like things being stock..sometimes haha
Hey the swaps I was looking at were stock! To the manufacturer! The turbo set up however is not. So conflicted on how to give my poor little car some ponies. And I may be looking into the digital converter. I call Rodney about a product and the subject came up about the vss, and he highly recommended to do the converter. I need to look at some diagrams of the trans to see what all i need to take apart.
Hey the swaps I was looking at were stock! To the manufacturer! The turbo set up however is not. So conflicted on how to give my poor little car some ponies. And I may be looking into the digital converter. I call Rodney about a product and the subject came up about the vss, and he highly recommended to do the converter. I need to look at some diagrams of the trans to see what all i need to take apart.
id do the digitial... ive heard bad things about splitting a 20 year old trans.
Hmmm. Okay, but I'd still like to do some research (I have the manual, just not the time). If I just need to remove a cover, I may do that way. However with the digital, if I ever decide to change tire size (which I don't but options are nice to have ) then I'll be set.
Hmmm. Okay, but I'd still like to do some research (I have the manual, just not the time). If I just need to remove a cover, I may do that way. However with the digital, if I ever decide to change tire size (which I don't but options are nice to have ) then I'll be set.
you gotta split the case, i know that much is needed.
You would basically have to disassemble the transmission (see the link I posted above), just so you can swap on the speedometer gear. Then you get to put it all back together. Wiring in the signal converter would be MUCH easier.
You would basically have to disassemble the transmission (see the link I posted above), just so you can swap on the speedometer gear. Then you get to put it all back together. Wiring in the signal converter would be MUCH easier.
Oooohhhhh so that's the tapered bearing Rodney was talking about. Yeah, I guess if I crack it open, I'd basically be in for a rebuild.
Does anybody have any experience with the Dakota converter? It seems like a straight forward install, but I'm not sure how accurate it is.
Ps- I'm throwing a code for a vss sensor ever since I replaced the bulb in the MIL
I have used several in different Fiero swaps. It is very easy to install and you can calibrate it using you cell phone to give you speed and you just have to hold down a button on the Converter until your speedo reads correctly. The unit from Dakota Digital comes with very good instructions ( you can even download the instructions from their web site). http://www.dakotadigital.com/
Joe Sokol
------------------ 85 SE Daily driver with a 3.4 DOHC OBD II 88 Formula/GT 4.9 Allante Intake (My Baby) www.fieroking.com
Quick question about the Dakota. The ground terminal: directions say "one pin off the vss is signal, the other is ground. The ground pin will go to the GND terminal (on the converter), along with the ground wire." Now do I 'T' in a wire or do I just have the ground wire go from the converter to the speedo? I was assuming all the wires I need should be in the connector under the speedometer. Also, I'm looking at the pinout, not the actual pigtail in the car, and I see a vss low and a vss high pin. Any idea? Is there only one wire?
The VSS high (yellow wire) is the signal, and VSS low (purple wire) is the ground. It looks like they want you to connect the purple wire to the ground terminal, in addition to a ground wire.
[This message has been edited by Blacktree (edited 03-11-2014).]
I got it to work! They had directions for what sounded to be my exact situation, but I had to do application #3 (regarding replacing a GM transmission with a newer one), but in addition I had to turn all 4 switches to off, and change the output to output #3. Just for future trouble shooters I am having some electrical interference though. The directions said to twist the wires together to reduce that, so I'll try that.
I got it to work! They had directions for what sounded to be my exact situation, but I had to do application #3 (regarding replacing a GM transmission with a newer one), but in addition I had to turn all 4 switches to off, and change the output to output #3. Just for future trouble shooters I am having some electrical interference though.
I never got my Dakota digital to work cleanly. The box seems to be very susceptible to noise. One minute the speedometer is reading correctly and the next is bouncing around like a pinball machine, (which depending on your age you don't know what that is ) And from time to time it would not work at all. Overall I was never happy with the way it worked. Maybe it's a problem with my car, your mileage may vary.
I had noise problems with my first SGI-5 (model B, with a metal case) But, on my SGI-5C, with the plastic case, I've never had an issue. Maybe they worked out the issue.
Did either of you guys use a twisted lead wire to the VSS?
GM uses twisted wires for the wheel sensors and for some VSS sensors. They also use twisted for some crank sensors.
In simple terms, twisting the wires marries them providing better transmission of the signal and provides some noise immunity for higher frequency signals.
[This message has been edited by phonedawgz (edited 07-10-2016).]
) then I'll be set.
