Grim Reaper

Now that I am doing my analysis of the 730 ecm with the 8D tdf (I'm using TunerCat), I notice that one of the constants I can adjust is the Road Speed Constant.

Does this mean I can just change this constant to adjust for my pending rear gear swap??? My current constant is 4007 Pulses/mi... so if I adjust it by 4007*3.73/3.23 will this cause my ecm to automatically compensate without having to replace my speedo gears? Also, does this mean I can fine tune for different tires?

If so, I am REALLY going to enjoy this PROM burning. And I have some speedo gears (drive and driven) available to some person who is too lazy to learn how to burn a PROM.

PLEASE SAY YES GRUMPY!

Blade

I'm not grumpy... but YES! lol

Grim Reaper

Anyone want a set of VSS drive and driven speedo gears for a 3.73 rear axle?

Grumpy

Sorry, but no.
If yours or anyone's car has a speedo cable then that has to be right.
That setting allows you to go from optical to magnet sensors. The 730 and it's derivatives are used in literaly hundreds of applications, so some of stuff looks wierd

Grim Reaper

I understand that a mechanical speedo would need to have this remain constant, but I have an electronic speedo so I am assuming that it might from your response. Guess I will give it a try...on correcting a speedo error on my car for the smaller snow tires, when I get the actual burner for Christmas.

Got nothing to loose...but a little time.

Blade

As far as I know there's a "switch" for optical or magnetic sensors in the bin. BUT Glenn is talking about the pulse setting which can vary greatly. I dont know the formula for setting that value, but I guess looking at a bin from a car with your kind of gears should tell ya.

Grim Reaper

Blade, the way I was going to adjust the Road Speed Constant (initial) was to multiply the constant (Currently 4007) by the amount of error that I currently have (3.5% with my snow tires on) and see what happens. I will know within 10 kilometers whether this worked or not. Just have to wait until Christmas.

Grim Reaper

Well, I made the adjustment to the Pulses/mile and it half worked. It DID recalculate my speed display on my Diacom+ by the amount of correction, but the electronic speedometer still diplayed the same. So, unless you are prepared to drive around with Diacom+ running, this adjustment does not work.

Good thing I didn't sell those speedo gears yet.

TRAXION

Glenn,

Great info. Thanks for the post!

Tim

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TRAXION's 1990 IROC-Z
Best Time = 12.244 @ 112.51mph (1.778 60' / 7.819@88.32mph in the 1/8)
All Natural. No Force. No Drugs. Stock Bottom End. Stock Body Panels.
Gunning for NA 11's after installing Hooker LT Headers and reducing weight.
-=ICON Motorsports=-
Moderator: PROM board at thirdgen.org

Blade

There is a little circuit board with an couple ICs on it insider the instrument cluster right below the speedomenter. Maybe there's some sort of resitor or dip switches (or something) you could do to change the pulse width to the speedo drive circuit. I dunno.. just an idea.

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92 Z28 L98 350
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Ported stock heads, XR270HR-10 cam, TES headers, Crane AFPR, Flowmaster catback, MSD ignition components.
Best ET 13.5@105 mph
"Take that auto, drop it in first, hold the brakes, stomp the gas and grin from ear to ear! :-)
Click here to see my car at Truspeed.org

Check out TruSpeed at http://www.truspeed.org
Check out the Bin Archive at http://www.truspeed.org/bins.html

Grim Reaper

I will review the schematics for the circuitry. But my hunch is that the wiring to the actual speedo comes from the a common input to the ecm, not an output from the ecm.

For this change to the PROM to work, the ecm would have to drive the speedo AFTER it receives the signal. It obviously doesn't or it would have already worked.

If there were any DIP switches, they would be at the ecm's side, not the speedo, as you would need two inputs to the speedo if any DIP switches were at the speedo side. I highly doubt GM would incur that additional expense. They are too money conscious.

[This message has been edited by Glenn91L98GTA (edited January 03, 2001).]

Blade

Are you sure the speedo is ECM driven ? I dont know how to wiring goes, but I was the under the impression the ECM gets its signal from the little circuit board inside the cluster as well.

Grim Reaper

I think you are misunderstanding what I am saying...the speedo is NOT ecm driven. Thus any adjustment to the Pulses/Mile constant has no effect on the speedo itself.

InTech

I have never played with one for anything but a truck, but I think you are needing info on the drac module which should put both the ecm and speedo in line. I think there is a breakdown of the drac on the diy/incoming site. hth's
-Carl