P J Moran

As I understand it, the ECM monitors the vehicle speed at the tailshaft of the tranny. Either at the mechanical speedo or with a sensor in the tranny. In either case, the ECM uses "counts per mile" or something like that to determine the vehicle speed, right?

When changing from '87 code to '89 code, couldn't that parameter need to be changed? Aren't the chips installed at the factory specifically calibrated for the rearend gear ratio, tire size, etc. for that particular car? If so, I can foresee the need to make adjustments when "borrowing" chips (or .bins) from other cars. Comments?

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'87 IROC 5.7l TPI - original owner!

87IROC350

The bin or chip you use has nothing to do with your gear ratio. If you have the correct speedo gear on the tail of your transmission it shouldn't make a difference. I changed from the my stock 87 bin to the 89 6E ARAP without changing anything and had no problems. When I scanned it registered the speed perfectly.

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87 Iroc 5.7, AFR190, SLP Cam(218/224), SLP 1 5/8 headers & cat back, Random CAT, SLP Runners, 48mm TB, custom chip(by me), Vigilante & Probuilt trans.

afgun

My understanding is that the '730 ECM uses a 4000 PPM input and the '165 uses a 2000 PPM input. The pulses are adjusted like the old speedometers, with gears in the transmission tailshaft.

The newer PCM-controlled cars have a fixed number of pulses per tailshaft rotation and it's tuned inside the PCM to get a proper MPH reading.

Along this thread, I'm going to be swapping to a '730 ECM soon. Anybody know if the later '730 style pulse generator will fit in place of the earlier style one in my '89 GTA?

--andrew

P J Moran

Yeah, I see what you mean. The speedo drive/driven gearset should be set up to generate the correct number of pulses per mile fully considering the rearend, tires, etc. Those gears are what acutally change with the options, not the PROM. I should have figured that out! Thanks.

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'87 IROC 5.7l TPI - original owner!