Speedo goes backwards.... (Page 1/1)
fpgt72 MAR 23, 07:58 AM
It is the oddest thing, everything is just fine up to 45mph. After that the speedo will start to move down....slower. It feels like it is relational to increasing speed. For example 50mph will shot 40, 55 will show 35......

Just started yesterday, they car got driven as the normal daily was in the shop. Amazing the people that say "nice fiero you don't see them much anymore"

I did get a short moment of the speedo needle going crazy and bouncing all over the place, but it calmed down in under 30 sec. It is now doing this.

Suggestions?
82-T/A [At Work] MAR 23, 08:50 AM

quote
Originally posted by fpgt72:

It is the oddest thing, everything is just fine up to 45mph. After that the speedo will start to move down....slower. It feels like it is relational to increasing speed. For example 50mph will shot 40, 55 will show 35......

Just started yesterday, they car got driven as the normal daily was in the shop. Amazing the people that say "nice fiero you don't see them much anymore"

I did get a short moment of the speedo needle going crazy and bouncing all over the place, but it calmed down in under 30 sec. It is now doing this.

Suggestions?




The Fiero's speedometer is driven by something called the Vehicle Speed Sensor, which is located on the transmission. This speed sensor feeds directly to the speedometer, completely bypassing the engine ECM. So it's not a problem with your ECM or anything like that.

What kind of Fiero do you have? Do you have a V6 or an L4, and do you have a manual transmission or an automatic?


My guess (without knowing more details) is that possibly the VSS is failing, or that the gear on it might be going bad... which is causing the VSS gear to slip as you go faster. It depends on what transmission you have, but I suspect that COULD be the problem.

The other thing is that the speedometer is actually driven by a motor... and not cable-driven like older cars. The Fiero uses little motors for both the tach and the speedometer that look like this:




Note, "M" stands for "Miles" and "K" stands for "Kilometers."

I apologize in advance, my hands were dirty. The "metric" gauges were used in Canada, the only other place the Fiero was sold outside of the U.S.. I lived in South Florida at the time, and a lot of Canadians have homes down here, so I routinely found Canadian vehicles in the junkyard. These gauges are relatively rare though... as I've never even seen one on eBay since (granted, I haven't looked in a long time). I converted my Fiero to the Canadian gauges so I could get the super-cool 180 kph factory speedometer... haha. But in reality... the speedometer now shows less since I think 180kph converts to like 115mph, haha... but larger numbers look cool.




Anyway, let us know a bit more information about your car... but I suspect it could be one of those two things.

Question... does the odometer work properly? Meaning... when you drive a mile, does it actually correctly log a mile, or is that off too? (Note: you may not immediately notice it).


The reason I ask is because the dash actually has TWO motors behind the dash... one for the odometer, and one for the speedometer. They are the exact same motors. In my case, I kept the odometer motor for MILES, but swapped out the speedometer motor for KILOMETERS. So, if you know that your ODOMETER is 100% tracking correctly down a straight road, then you know the VSS is good (and that also the motor for the odometer is good). Which would be a quick indication that the speedometer motor is bad.

My guess though, it's probably something going on with the VSS.


Anyone know where you can get a new one?

[This message has been edited by 82-T/A [At Work] (edited 03-23-2023).]

J Gunsett MAR 23, 08:59 AM
I would first remove the cover under the steering column and than remover the connector going to the speedo and then reseat the connector back on the gauge. If that does not fix it I would look for someone that has a spare speedo and plug it into you connector. You did not give out your location, I do have a few spares.

Jack
theogre MAR 23, 09:59 AM
Speed display and Odo is 2 separate functions of the speedo board and speed wrong is often not = to odo problems.
3rd job is divider/driver for cruise and ECM speed data.
Any 1 to all 3 can die and seems the speed meter driver is "dead" but rest still works.

Speedo reading wrong or backward likely isn't the VSS on the trans.
If you use a ECM scanner, will proof isn't a VSS problem.

Speedo "Meter" works close if not exact same as Tach meter that goes bad from dead capacitor(s) etc on the board.
When the Speedo Board is "bad" for any reason... need a new speedo setup for metric or miles.
Odo Motors only change K or M. "Meter" needs correct board to display K or M and max range for the meter IE 85 or 120mph meter.

Can try unplug/replug the board edge connector a few times as may "clean up" the joint but likely isn't your problem.

Can take the speedo out by the front trim off.
If lucky, Could be Speed Meter is bad or loose connection with the board. Loosen and tighten the nuts a few times to "clean" the connections. To really clean use deoxit or other contact cleaner for electronics. Warning: the meter nuts do not need you to screw down hard. Over torque will damage the meter, board, nuts or all even will those "weak" stamped nuts.
If the meter is bad can replace w/ any used meter. Does Not need to come for exact same speedo assembly as meter only tells you what the board says.

------------------
Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should.
(Jurassic Park)

The Ogre's Fiero Cave

fpgt72 MAR 23, 10:36 AM
Sorry I should have given the car specs.

1986 Fiero SE V6 with a 4speed.

Car is sitting at 70k miles.

I will check connections, when I got the car out of storage this last time evidence of mice was found. I had not thought about that as a possible issue. Even if not chewing on wires their "waste" is not good for electric parts.