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| 85 Speedo, 4000 PPM output possible? (Page 1/1) |
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TGYK256
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AUG 24, 06:55 PM
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I started acquiring bits and bobs to add a digital cruise control module to my 1985 4cyl Fiero, but have run into the predictable snag of acquiring a suitable 4000 PPM signal for the cruise module. I understand that the VSS outputs a variable-voltage sine-wave 4000 PPM signal, which goes to the speedometer control board for a divide by 2 and square-wave buffered into 2000 PPM for the ECM and old vacuum cruise servo control module. I have found references to people modifying the 86-87 boards to add a buffered 4000 PPM output, but haven't found anything on the 1985-older boards.
It appears that the circuit design is similar, but clearly different; The board (P/N 1546477C) layout itself is clearly hand-drawn, and connections between the two DIP-16 packages seem similar to the 86+ boards. I unfortunately can't find any information on the two ICs on the board, labeled "AMI 8449HH - 6393 R - C04306" and "6394AD - 274 H 401". The AMI chip has a 4.1943 MHz? kHz? Crystal feeding into it, and appears to drive the 2000 PPM output through an NPN transistor to ground and a FET? to one of the 6494 pins. The VSS input is to the 6494 IC, and there are what seems to be 5 signal interconnections between the ICs, only one of which isn't short pin-to-adjascent-pin. The 6494 chip also outputs the signals for the speedo gauge as well as the trip/odo motors.
If anyone has any info on these chips, or any info about this generation of speedo control boards, I would greatly appreciate you sharing!
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Dennis LaGrua
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AUG 24, 10:48 PM
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The VSS sign wave signal is sent to a buffer that is located on the speedometer board for the 85-86 Fieros. In short the buffer is on the speedometer board and it should provide a reference signal for the speedometer itself and the CC. What is it about the output signal that you now have that that cannot be used? ------------------
" THE BLACK PARALYZER" -87GT 3800SC Series III engine, custom ZZP /Frozen Boost Intercooler setup, 3.4" Pulley, Northstar TB, LS1 MAF, 3" Spintech/Hedman Exhaust, P-log Manifold, Autolite 104's, MSD wires, Custom CAI, 4T65eHD w. custom axles, Champion Radiator, S10 Brake Booster, HP Tuners VCM Suite.
"THE COLUSSUS"
87GT - ALL OUT 3.4L Turbocharged engine, Garrett Hybrid Turbo, MSD ign., modified TH125H
" ON THE LOOSE WITHOUT THE JUICE "
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TGYK256
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AUG 24, 11:22 PM
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I assume that there is a buffered 4000 ppm signal somewhere on-board before it gets divided, but reversing-engineering what is happening without any clue what these ICs are is difficult.. The digital cruise module I have expects to see 4000 ppm, the stock speedo board outputs 2000. I know I could wire in a DRAC to buffer the 4k signal from the vss before the speedo for the cruise module, but I'd rather not modify the harness to add one, nor spend the money to purchase one.
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TGYK256
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AUG 25, 01:17 AM
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I went ahead and put together a schematic for the board off of my speedo.. I think it is accurate, but am unsure on a couple of the components, namely the ceramic/tantalum capacitor values, and the zener diode- It is small glass package without a polarization marking- And finally the lower transistor, which may be a FET of some kind.. Searching the part number didn't yield anything.
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olejoedad
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AUG 25, 08:36 AM
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Take the signal directly off of the VSS sending unit and build a conversion circuit found on gmtumers.com (Sinister Performance - Tech Section - Wiring Diagrams)
A couple of resistors, a capacitor, a diode and you've got the 4000 ppm squarewave signal you need.
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Spadesluck
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AUG 25, 08:47 AM
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Take a look at this article about adding a digital cruise to the Fiero. It tells you exactly where the 4000ppm signal is at on the speedometer.
Digital Crusie[This message has been edited by Spadesluck (edited 08-25-2025).]
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Raydar
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AUG 25, 09:11 AM
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| quote | Originally posted by Spadesluck:
Take a look at this article about adding a digital cruise to the Fiero. It tells you exactly where the 4000ppm signal is at on the speedometer.
Digital Crusie
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"Installing a digital cruise in the Fiero"
by Oliver Scholz
Perfect. Oliver Scholz is the name I was also thinking of. He has done all sorts of Fiero electronics.
I think he is also in several of the Farcebook Fiero groups, if you want to contact him directly.
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TGYK256
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AUG 25, 09:42 AM
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| quote | Originally posted by olejoedad:
Take the signal directly off of the VSS sending unit and build a conversion circuit found on gmtumers.com (Sinister Performance - Tech Section - Wiring Diagrams)
A couple of resistors, a capacitor, a diode and you've got the 4000 ppm squarewave signal you need. |
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Is there any way you can direct-link the article you are referencing? I am unable to visit the site you mention (I tried gmtuners, in addition to gmtumers, no site found.)
| quote | Originally posted by Spadesluck:
Take a look at this article about adding a digital cruise to the Fiero. It tells you exactly where the 4000ppm signal is at on the speedometer.
Digital Crusie
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This was my original plan, but this article only applies to 86+ boards. Mine is different in that it doesn't seem to have the "CS8524" chip onboard. See pics below
| quote | Originally posted by Raydar:
"Installing a digital cruise in the Fiero"
by Oliver Scholz
Perfect. Oliver Scholz is the name I was also thinking of. He has done all sorts of Fiero electronics.
I think he is also in several of the Farcebook Fiero groups, if you want to contact him directly. |
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Any chance he has contact info outside of that site? Deleted my profile a few years back and won't touch the place anymore.
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Spadesluck
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AUG 25, 12:08 PM
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Spadesluck
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AUG 25, 12:10 PM
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