Sootah,

I received the speedo today. I'll take a look at it this weekend and try to trace the circuit so I can figure out how it works.

In the meantime, does anyone have the connector diagram for the unit? This will help me determine the IGN, GND and VSS inputs.

-L

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Cool beans. If you can't make that one work for you I may have another available.

Keep me posted.

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^{The Black Beauty (85 GT) - Bustedato (86 SE, Parts car)
Nobody likes a loser, be a cheater.
RAPE ME!! Er, wait, no.. That's not quite right.. RATE ME!}

I may be wrong.... I thought the only dif for the speedo M to K was the markings on the face and the Odometer/Trip is running off a different divider.

I'll dig around. I thought I had at least some of this printed some place but I haven't seen it in a long time. If I still have it I'll post whatever I find here.

I seem to remember Oliver's site has info about the speedos that might be useful. (FieroLuke I think here.) I don't remember how much detail he had posted.

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11-Sept-01, The day the world as we knew it ended.

You might try fierohoho as well. I sent Steve a speedo in KPH and he was playing with something in the electronics. There were a couple of very detailed threads on crystals, etc for a while. (then mysterious silence)

Gary

There is some info on Oliver's site that I will be reading very carefully.

Ogre, I agree, the only difference should be the markings on the face. At some speed, the needle will be pointing somewhere. If you are metric, you put a metric number on that point. Otherwise, you put an English number there.

The idea is to be able to change the position of the needle so that you can actually read MPH on a KPH scale. If this is possible, it would be like changing the scale on a voltmeter. You could cover the KPH label on the dial face, and use that for the "extended range".

-L

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What we are trying to do is make it so the 180 KPH speedo is a 180 MPH one. (or 140, since its more feasible and there are way more of those guages available)

quote Originally posted by lawrence: The idea is to be able to change the position of the needle so that you can actually read MPH on a KPH scale. If this is possible, it would be like changing the scale on a voltmeter. You could cover the KPH label on the dial face, and use that for the "extended range". -L

I followed that for a while but never heard of any positive results. I am very interested in this as I wanted to try it some time ago.

I feel certain one of the electronic guru's on here could figure this one out.

Ignore any of my posts (as terryk) that you can rescale the speedo with a pot. I did that on several other cars because they use the National L1819 Air Core Meter driver just like the tachs. Unfortunately the Fiero speedo doesn't and I haven't been able to figure out what chips the AMI and TI parts are. I think one is a divider and the TI part might be a Delco custom part with a fixed full scale setting.

I hope I didn't mislead anyone into oblivion.

The crystal eliminator circuit does work but the odo and trip will be off unless we can get an odd divider. That's how I changed the first speedos.

A new circuit could be designed around the LM1819 (since the speedo is just a tach anyway), but the motor drivers for the odo and trip and a cruise output divider would be needed. I'm not sure it's worth doing.

Someone out there has the schematic to the Fiero speedo.

[This message has been edited by TK (edited 11-15-2002).]

If anyone has a schematic for the speedo, I would love it.

I looked at those archived postings. The speedo board I have looks nothing like the one pictured here:

https://www.fiero.nl/forum/Archives/Archive-000001/HTML/20010706-2-006673.html

The board I have has about half the number of parts and no resistor packs (the light-blue integrated circuit looking things).

The crystal is 4.1943, the same as some of the other posters.

If you go to this thread:

https://www.fiero.nl/forum/Archives/Archive-000001/HTML/20020208-2-013619.html

Fierohoho tested several different crystals. It looks like the correct one is somewhere between 7.372 and 6.144. If I multiply 4.1943 by 1.6, I get 6.71, which is somewhere in between. So, If I have a KPH face, I can put the 6.71 in the unit and read those numbers in MPH. This is what we want, correct?

-L

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Bingo.

I found the correct connections for IGN, GND and VSS. I connected a 60Hz source to the VSS input and the speedo pointed to about 54 MPH, which is correct (I'm not going to post any math -- you can do it if you are interested).

While tracing the circuit, I found an 8-pin chip that drives the odometer motors. There is a trace that goes from this chip to the divider chip that has the crystal attached to it. This is important, because I don't know how the odo motors are driven. Do they run off the VSS or off the divider?

I ran the speedo at 54MPH for about 23 minuntes, and the trip odometer ticked up 20 miles. The actual calculation would yield 20.7 miles. The 85MPH speedometer I received didn't have a needle on it, so I had to improvise with a piece of wire. This could be one source of error, but it is close enough for my test.

Then, I removed the 4.19MHz crystal and installed an 8MHz crystal. I connected the 60Hz signal to the VSS and the speedo indicated 28MPH, which is about right based on the ratio of the two crystals.

I then ran the speedo for 23 minutes, this time with the 8MHz crystal attached and the trip odo again accumulated 20 miles. THIS IS GOOD because it means I can change the crystal to adjust the speedometer, and that won't effect the odometer.

So now I need to find a 6.75MHz crystal. This is the value that I calculate will allow us to read MPH on a KPH scale. Anybody have one?

However, there is still a potential problem. To install a remote switch that will select between crystals, we will need 3 or 6 wires to run from the divider chip to the switch. The wires will add capacitance to the circuit that might slow down the oscillation, or maybe even kill it. So even if I can find the correct crystal, it might not work.

I'll keep you posted.

-L

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quote Originally posted by lawrence: However, there is still a potential problem. To install a remote switch that will select between crystals, we will need 3 or 6 wires to run from the divider chip to the switch. The wires will add capacitance to the circuit that might slow down the oscillation, or maybe even kill it. So even if I can find the correct crystal, it might not work.

Hey - awesome work!

What about putting a tiny DPDT relay right on or beside the board (ctr pair to the board, outer pairs to each crystal), thus keeping the lead length to near nil. Then you just have to run the power wire for the relay to a remote switch.

Could even wire it up to a momentary push button so that the next time you turn off the ignition the relay would revert back to it's natural KM/H state. I've already figured that part out in a previous project, so all you need to do is find that crystal!

watts,

Yes, this is one way to do it, but running those frequencies through a relay has it's own problems. Ideally, you would want a frequency synthesizer placed on the board. Then the control would set the frequency of the synthesizer.

-L

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quote Originally posted by lawrence: So now I need to find a 6.75MHz crystal. This is the value that I calculate will allow us to read MPH on a KPH scale. Anybody have one?

There are several VCO chips out there that will run in that range. I think the 74LS628 is out of production. It will run at 20MHz. I used one for my Ciera KPM speedo originally. Still, any crystal company should have a 6.75MHz part.

TK

Very interesting guys.

I never did get it figured out but I did get close, please keep us updated on the test if you find the 6.71 crystal.

I may try and located one myself to check against my radar unit.

Good to see this thread revived and hopefully solved as I still have a 140 KPH dash pod around here somewhere.

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Me & Mrs. Fierohoho

I see said the flee there's a horsey on me.... Makes sense now... Somewhere along the line this thread got fractured.

If you can do this with a VCO, then you may find it easier to do the range switching thing. (I have no Idea if the VCO would actually work... I have no schematic data at all to look at. I'm just expanding on stuff in this thread.)

A thought comes to mind... if you use a VCO, 2 transistors (with needed control voltage on each) and a flip flop... You could range switch with a simple debounced push button and have it default to the "normal" range on power up. The idea is to switch the control voltage what is simple DC voltage. Maybe the 2 transistors I'm thinking of could be one leg of a voltage divider??? Or control one resistor in a voltage divider???

I think you can do this whole thing with the VCO, a 7400 family Quad NAND, and a few other discrete parts.

I remember you can make both the flip flop and the debounce from a single Q Nand... But then I'm half senile... There are also dual JK FF chips around.

Don't know how much I can help... been years since I built much of anything. If I can randomly ponder anything else that may be remotely useful let me know. (Pinky, Are you pondering what I'm pondering?)

Thank You all, man you guys are wicked cool. I can't belive that I missed this thread. Yahoo. I had no idea that we were having a problem locating a good 140hph gauge cluster. Send me your address and I will send another speedo tommorow.

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David hotrodfiero@shaw.ca
85GT 2.9 4spd

www.david.westcoastfieros.com

A VCO, 2 pots and switch. Done.

If there was some place to post the VCO schemetic I would. 74LS628's can still be found. Otherwise find another one or use a PLL's VCO. Worked fine in my Ciera GT for the 140 speedo until I fingered out the other method.

Anyone check out www.digikey.com for the crystal?

Epson 6.757MHz.
Digi-Key Part Number SE3411-ND Quantity
$1.17
Manufacturer Part Number CA-301 6.757M-C
Description 6.757 MHZ CYL XTAL CA-301 TYPE

[This message has been edited by TK (edited 11-18-2002).]

quote Originally posted by Capt Fiero: I had no idea that we were having a problem locating a good 140hph gauge cluster. Send me your address and I will send another speedo tommorow.

You talking to me? I won't need a different speedometer until I get something to work on the one I have. Then, it would be good to get an actual dual scale speedometer to test.

quote Originally posted by TK: A VCO, 2 pots and switch. Done. If there was some place to post the VCO schemetic I would.

If you think this will work, then knock yourself out. No flame intended, but I'm not sure that a VCO is the best way, or even a good way to do this. Unless you drive your car at the exact same temperature at all times, there will be quite a bit of drift in the components. Plus, you will have to use a precision voltage reference in order to set the voltage you want for each frequency.

This would be a good time to ask what kind of accuracy people want. Temperature or design accuracy might not really be an issue. The last time I got stopped for speeding, it was for ~25 over -- not the couple percent error we might get with the conversion.

If you do have a schematic, send it to me.

quote Originally posted by theogre: I think you can do this whole thing with the VCO, a 7400 family Quad NAND, and a few other discrete parts. I remember you can make both the flip flop and the debounce from a single Q Nand... But then I'm half senile... There are also dual JK FF chips around.

The other thing I don't want to do is add a bunch of 74LS parts. These require a power supply to work (5V supply) and now we're talking a printed circuit board and complexities that I'm not interested in pursuing now. Well, maybe one -- just to say that I did it.

The first thing I'm going to try is long wires and see if the thing still runs. If it does, then the entire project might turn out to be a simple switch.

-L

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Think about it. The long term (even the short term for that matter) stability of that VCO at that frequency far exceeds the error in the speedo itself. An additional 7805 and a couple of R's and C. 10 parts max. 30 minutes to solder up. Simple. No PCB is needed unless you were going to mass produce it.

I had a handful of the 628's and needed a solution rather than searching for the right crystal for the next month.

I was just providing an option. I'm sure you folks can find a 6.75MHz crystal.

You wouldn't want the schematic to an inferior circuit now would you?

[This message has been edited by TK (edited 11-18-2002).]

quote Originally posted by TK: You wouldn't want the schematic to an inferior circuit now would you?

After failing for the 200th time trying to find the correct material for the incandescent lamp, Edison maintained that he learned something -- and that was the he now new 200 materials that wouldn't work.

I'd still like to see it.

-L

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I thought it was one of his assistance that failed for the 200th time.

I'll dig it out.

Terry

[This message has been edited by TK (edited 11-18-2002).]

So, since I don't want to have mine be switchable, could I just go out now and buy a 6.75 Mhz crystal and solder it into my speedo? I'd then modify Aus's guages to say MPH instead of KPH and be done. Right?

Also, is the 6.75 crystal for the 140 kph pod or the 180?

From reading above, it looks like it's for the 140 (which I prefered since it's not marked in MPH like the 180.) Just blank out the KPH and it looks stock.

[This message has been edited by TK (edited 11-18-2002).]

Sootah,

I'm not familiar with all the different speedometers. The one I have is 85MPH. However, if you install a 6.75MHz crystal where the 4.1943MHz crystal was, then whatever the speed was previously, is now 62% of that value. So if you have a KPH scale and you were going 100KPH, it would now read 62 on that dial.

I don't know if all the speedometers work that way, but it's not that difficult to try, other than removing the speedo from the dash.

I did try adding 3' of wire and a switch to see if it would switch between the two crystals I was using, and that didn't work. Sometimes it would osc. at the lower frequencey, sometimes it didn't work at all.

-L

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