Types of speedometers and cruise control (Page 1/1)
Lonzo DEC 14, 02:34 PM
I have a 86 Fiero with a 4.9 conversion. I have had the car a few months and going through it all. When I got it the speedometer was about 9 miles an hour slow, Tach didn't work and fuel gauge was intermittent. the gauges were also white from some kind of kit or something and I didn't like them, so I bought a complete set of used gauges and swapped them all out. Speedometer worked perfectly and after a new tach board for the V8 the tach works as well. All gauges work now but I have no cruise control. I have the service manual and began researching the cruise system and the service manual says there are 3 types of speedometers.

1. Mechanical: I have never heard anyone ever talk about a mechanical one and have not seen one on eBay.
2. Digital: The manual says that this is the type that cruise control cars have.
3. Quartz Electric: Uses a quartz crystal and electric circuitry.

My question is, are there really three different types used and if so, how do you know which type you have. I don't want to start trouble shooting the cruise control if I have the wrong speedometer. I took both apart and they are different boards I assume from a different year, but they appear to have the same components just in a different location. I'm thinking the little silver component is a crystal so are these the Quartz Electronic and if so, do they work with cruise control cars. The reason i am a little confused is since I am not sure the ones that were in the car are original, they may have been replaced with the wrong kind and that is why the cruise does not work. Or the manual may have bad information and all the speedometers are the same and I move to the next component for testing.

Sorry to make this long but just trying to provide as much info as I can. I hope its not too confusing. I uploaded the pages in the service manual that describe the speedometers above.

Thanks
Lonny
theogre DEC 15, 12:05 AM
In very short version...

Fiero "Speedo Board" has 4 function...
Runs VSS thru electronics to give square wave signal to the rest.
Speedo Meter and driver to run that.
Odo and Trip counters and driver to run that.
Divider to send speed to ECM and Cruise "box" of whatever type.

In other GM cars... the VSS goes thru separate box for first and forth functions.

Vac and some others need above to work.
Some needs another connection as feed back to see when opens throttle.

OE Fiero has Vac Cruise and control box under dash and needs Manifold Vac and other things to get vac all the time.
Even then Vac Cruise can have bad switches, valves w/ crap blocking or bad, and more stopping it to work.
That's even if you wired it to work w/ new engine.

Go to wire section and look at cruise diagram(s).
for S-column stalk also See my Cave, Cruise Control

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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

sanderson231 DEC 15, 12:20 PM
The 4.9L PCM can control a cruise control servo. My Fiero did not have cruise control so I had to add the brake switches and also an on/off and a momentary for the set function and a momentary for the resume function. The on/off and momentary switches were installed in the console. The advantage of PCM control vs the stock Fiero system is that a scan tool can be used to verify the function of the switches making troubleshooting easier.


I also have a 3800 S/C the was done by someone else. This Fiero did have cruise but the person that did the swap rewired the switches for PCM based control.

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formerly known as sanderson
1984 Quad 4
1886 SE 2.8L
1988 4.9L Cadillac
1988 3800 Supercharged

theogre DEC 15, 04:32 PM
Many later ECM and PCM has cruise but needs correct speed data.
That's even engines computers w/o having cruise built into them.

OE Fiero ECM and Cruise "Box" gets 2000pulse/mile from Speedo.

Others need different speed input and some connect VSS direct and often expect different VSS not the VSS "generator" used in Fiero.
So If swap is not wired right to get speed data then will often set "codes" and builtin cruise logic will never work.
GM "OBD1" like Fiero uses "randomly" sets code 24.
OBD2 sets 1 or more 4 digit codes w/o speed data.

that's not counting ECM/PCM that have 2 or more ways to input speed data like 87+ "Dukes" that changes speed input by PROM programing for use with more engine/trans setups for different model lines. This ECM also controls Cruise but older Fiero Dukes and all V6 use same cruise box and vac servo.

GM also had "Digital" cruise separate from ECM/PCM. This is an "upgrade" to replace Vac Cruise because no valves to plug/break, vac lines leaking, etc. Search archives here.

⚠️ Warning: OE Fiero has an Electric Dump Valve. This is wired to brake pedal switches and must work for all Vac Cruise Servos.
Other front engine cars have a large vac line to the pedal for same thing and don't show in electric section of whatever "book."
1. Big Port dumps vac in the servo touching the brake.
In normal use part of canceling cruise and way faster to release vac vs. tiny valves control by the "box."
If the system jams On for any reason, this maybe the only way to stop the car safe w/o killing power to the engine that in turn kills vacuum to run PB and in other cars kills hydro pump for PS and PB in some models.
sanderson231 DEC 15, 08:03 PM
The 4.9L and 3800 S/C engines I referenced both use vacuum to actuate the cruise control servo. The vacuum is controlled by solenoid valves actuated by the PCM.

I think that the cruise control servo and solenoids are the same for the Fiero system - the difference is what controls the solenoids.

If the cruise control doesn't work there are several possibilities:
-broken or disconnected vacuum lines
-wiring opens or grounds
-solenoid problems
-brake switches (IIRC there are two)
-electrical problems with stalk on steering column
- cruise control module

Easiest thing to check are the vacuum lines. After that it's all electrical. Need to consult the wiring diagrams and methodically check the possibilities

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formerly known as sanderson
1984 Quad 4
1886 SE 2.8L
1988 4.9L Cadillac
1988 3800 Supercharged

Lonzo DEC 16, 12:52 AM
Since nobody has mentioned different speedometers like mentioned in the service manual, I am going to assume that they all used the same one and mine is correct.
I will now start trouble shooting the system. I also have the factory testing kit that is used to help test the cruise functions.

Like you said, I will start with the vacuum lines and work from there.

If anyone has a link to a good explanation of all that is done with cruise on a 4.9 swap, i would appreciate any input.

Thanks

Lonny
sanderson231 DEC 16, 10:07 AM
We are travelingg until after New Years so I don't have access to my Fiero and 4.9L service manuals. From what I remember the Fiero and 4.9L systems are essentially the same. On the 4.9 the PCM performs the same functions as the cruise module in the Fiero. IIRC there are two brake switches. One is a permissive. The PCM has to see that switch closed in order for the cruise control to engage. The second brake switch operates the vacuum dump solenoid valve to quickly disegange the cruise control. The PCM also receives input from cruise on/off switch, set switch and resume switch. These are all in the stalk on the steering column.

After you look at the vacuum lines, I would next check the brake switches. These may be OK but just mis-adjusted.

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formerly known as sanderson
1984 Quad 4
1886 SE 2.8L
1988 4.9L Cadillac
1988 3800 Supercharged

theogre DEC 16, 08:32 PM
85-87 Speedo is all work the same except boards are "Program" for 85 or 120 mph display.
84 Speedo board has Cruise control logic om top of all others above.

85-87 Boards may look different as versions of them changes.
Spadesluck DEC 20, 07:52 AM
For the cruise control there is a reference signal from the speedometer board sent to the cruise control board (located near the center tunnel of car). You may have replaced your board with a non cruise setup (not sure if thet is possible though).

The pathway for all the speed signals in a fiero is ridiculous.

I have a 4.9 in mine as well. I converted to a digital cruise and got rid of all the vacuum related items. But, the circuitry is all the same basically.
theogre DEC 20, 09:23 AM

quote
Originally posted by Spadesluck:
For the cruise control there is a reference signal from the speedometer board sent to the cruise control board (located near the center tunnel of car). You may have replaced your board with a non cruise setup (not sure if thet is possible though).

No.
Cruise get Same Signal from Same Output of Speedo ("Pin" U of C1 "board edge" connector.) as the ECM.



quote
Originally posted by Spadesluck: The pathway for all the speed signals in a fiero is ridiculous.

All others have something doing 1. and 4. functions above.
Means unless the Speedo does that, there is a separate "box" somewhere and hidden way under the dash in P-side or anywhere else that's often hard to find or access.
Only later ECM/PCM runs whatever VSS type direct and may have output for Speedo or VSS shares "data" w/ all things needing the info.

Vehicles w/ OBD2 the VSS goes to PCM or BCM that turns "data" into CANBUS data or VSS is or part of a CANBUS device.