This subject comes up once or twice a year. It'd be nice to be able to adjust an 85 MPH speedo and stick on a 120 face, woudn't it?

I did some research into the Fiero speedometer; how it's made, how it works. It's a surprisingly complex piece of engineering - and it's crammed into two custom IC chips. Those and two interface chips make up the speedometer circuit.

Rather than go into a long involved discussion, I'll hit the high points in a FAQ:

Q: Can I convert my 85 MPH speedo into a 120?
A: No.

Q: Isn't the only difference the crystal?
A: No. There's a custom chip with an internal PROM that determines the speed range.

Q: What's the correct crystal frequency for X MPH?
A: The crystal is part of a reference oscillator and has no direct bearing on the speed reading.

Q: Why is the crystal different on 85 and 120 units?
A: The crystal frequency is a whole number multiple of an internal reference frequency and this frequency is a whole number multiple of the full scale frequency input.

Q: But I want to play with the crystal!
A: Leave the crystal alone; it and the custom programmed digital chip are a set. Altering the crystal frequency will only have a small effect on the full scale reading of the instrument.

Q: Where can I get the 120 MPH chip?
A: They're only found in 120 MPH Fiero speedometers. This part is not available from any other source.

Q: What about the 170 MPH conversion?
A: It's possible to cause the reference oscillator to run at it's second harmonic; this will cause the scale of the instrument to be doubled. No other "alternate" scale is possible; the reference oscillator isn't capable of running fast enough to make an 85 MPH speedo into a 255 MPH unit.

Q: But I've seen a formula that you use to determine the right crystal
A: Back to the crystal again, eh? You're wasting your time - the calibration of the speedo is NOT determined by the crystal.

Q: OK smarty, then how do I get a 120 MPH speedometer for my Fiero?
A: Look in junkyards, online sales, etc. to find a 120 MPH unit.

And finally - are you really sure you want that 120 MPH speedo? The Fiero speedo face isn't very big, so when you divide it up into 120 the divisions are kinda small - so small that the needle itself is about 2 MPH wide. It's much easier to read the 85 MPH speedo accurately - and you (almost) never exceed 85 anyway....

I would give you a bag of plusses for that insightful look into the Fiero speedometer. I learned something.

A +++`s all the way around ---good write up... Will come in very handy..

Aw shucks (blushing) - I was really hoping to find a way to extend / expand the supply of 120 MPH Fiero speedometers. Searching the forum turned up much talk about crystals and the right frequencies, formulas, etc.

But when I placed the crystal from a 120 MPH speedo on an 85 MPH speedo board and ended up with an 85 MPH speedo - well, this called for some research. I discovered a few things; one is that the GM engineers that designed this circuit back in the '70s were some very smart people. Another is that the chip set used in the speedo was custom made for GM and that these parts were never sold individually to anyone - there's only one replaceable part in a Fiero speedo, that's the whole instrument. I found out about the two big chips - the digital chip and the analog chip - through an online review of GM patents. When I discovered the PROM in the digital chip I was first excited; this circuitry was undoubtedly used in many makes and models because it would be easy to tune to any VSS / tire size. Then I realized that this meant that the only source of digital chips for Fiero speedometers was from salvaging Fiero speedos. That's no help at all; you could strip parts (the digital IC and the crystal) from a 120 speedo and put them on an 85 board and turn it into a 120. But if you've got the 120 to strip - why not just install it?

So this doesn't help with the scarcity of Fiero 120 MPH speedometers. Hopefully it'll help others avoid buying crystals for their 85 MPH speedos with the intent of making it into a 120; that's not going to happen. There is NO crystal that you can install in an 85 MPH unit to make it into a 120.

quote Originally posted by Whuffo: Q: OK smarty, then how do I get a 120 MPH speedometer for my Fiero? A: Look in junkyards, online sales, etc. to find a 120 MPH unit.

ok, got the cluster - do I need to use the whole cluster, or can I just move the speedo itself? being I have a 85 cluster, my oil pressure guage is on the tach, and I want to keep that. I think the aux guages are ugly, and dont want to use them. and, can I adjust the odometer on the 120 to match my current 85's miles?

Yes.
Yes. (tho I don't know the exact procedure, when I had mine out I noticed it looked fairly simple to remove the odometer wheelset, so I quess you have the option of swapping your exisitng one in.)

You have to be very careful playing with the odometer. The gears are very sensitive. You can end up with an odometer that doesn't work.

quote Originally posted by Whuffo: So this doesn't help with the scarcity of Fiero 120 MPH speedometers. Hopefully it'll help others avoid buying crystals for their 85 MPH speedos with the intent of making it into a 120; that's not going to happen. There is NO crystal that you can install in an 85 MPH unit to make it into a 120.

somehow i doubt that since i was able to find the right frequency crystal to convert my 85mph speedo into a 150mph speedo.. should be no problem finding the right crystal to make it do 120..

however i do believe you when you said that putting a crystal from a 120 into an 85 didn't work.. i thought i remembered the 120 speedo being different..

That is an interesting read. Especially after, like you, reading all the crystal swapping threads. I even have a new crystal to swap, just haven't done it. Glad I didn't layout new gauge faces on the puter yet.

So, where's all the speedos that have swapped crystals to read 150 or 180, and how are they reading?

i've only done 85mph to 150 - so have only done the conversion for that - it was accurate when paced with a 95 dodge pickup..

I've changed many a Fiero odometer and they are very easy to set to any mileage you want. So don't let that stop you from switching.

They built over 370,000 Fieros and nobody knows how many are on the road. In other words the supply of Fiero Speedometers is not about to dry up...

How do I know that? I am building Fiero to Firebird plug and play harnesses.

https://www.fiero.nl/forum/Forum4/HTML/031175.html

That process forces me to canabalize a Fiero 120 MPH speedometer. I really didn't want to do it that way but I found no alternative.

I put a "wanted to buy" ad in the mall for clusters and my email started ringing off the hook.

https://www.fiero.nl/forum/Forum4/HTML/030918.html

I have enough clusters to last me into the next decade.

I converted my 88 2m4 to GT Gauges with 120mph speedo and GT tach. The tach was much more difficult. I had to epoxy the coil from the 4cyl tach onto the back of the face of the GT tach. No amount of swapping resistors would make it work right otherwise. (Perhaps I swapped the wrong one.)

quote Originally posted by Mike Bucala: The tach was much more difficult.

All you had to do was swap the circuit boards over between the tachs.

swapping tach boards is definitely easy, and you had to be swapping the wrong resistor if it did not change the calibration.

Ok from this bit of detail it sounds like all the "brains" are up in the tach. I have the original gage cluster and stock trany (not sure which set of gauges) can I get the speedo working without the ECU? Have not dug into the wireing yet. Just wanted to get your take on this since it seems you have dug into this already.


Later,
Wire

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Electric Fiero conversion in progress.
http://www.austinev.org/evalbum/778

The Fiero speedometer does not rely on the ECM to operate - it's fed directly from the VSS sender on the transaxle. The speedometer does contain circuitry to divide the VSS signal by two for the ECM's use - so actually, the ECM depends on the speedometer to operate.

Great! Thats what I was hopping to hear. One less thing to have to rebuild in the conversion. Does the Tach operate in a similar manner?

Thanks,
Wire

------------------
Electric Fiero conversion in progress.
http://www.austinev.org/evalbum/778

The tach gets it's signal directly from the motor, so you don't need an ECM to get a tach signal either.

Thanks for the info. I think I am all set for gages (other than the traction pack voltage / current gauges I need to add)

later,
Wire

What if you were to add a potentiometer somewhere before the needle? Ive not looked into my cluster yet so i have no idea what the circuitry looks like yet but just an idea.

quote Originally posted by Chris Hodson: What if you were to add a potentiometer somewhere before the needle? Ive not looked into my cluster yet so i have no idea what the circuitry looks like yet but just an idea.

Unfortunately, no this would not work, as the VSS (Vehicle Speed Sensor) on the transmission does not generate power like an alternator or generator, it actually sends a pulse to the Speedometer...

The vehicle speed is measured in PPM (Pulses Per Mile)...
The faster you go, the more pulses are sent...
If I remember correctly, the measurement for the speedo is something like 4,000 PPM...
So, every mile consists of exactly 4,000 Pulses... No matter how fast or slow you go, that 4,000 remains a constant...
The slower you go, the slower the pulses, hence the slower the speedo... The faster you go, the faster the pulses, the higher the speedo registers...