Eliminating VSS input on a '427 PCM

[dave w]

I've been wondering if there is a bullet proof way to eliminate the need for a VSS input on the '427 PCM? I've been wondering if by-passing the near idle table of the '427 PCM is practical or possible. The '427 PCM does not have a neutral / park switch input like the older '7747 and similar ECM's. It seems the '427 PCM really needs the VSS input? I did some bench testing and it seemed that setting the maximum speed for near idle tables (MPH & TPS) to Zero's by-passed the near idle tables. I'm thinking the bench test is not as accurate as an actual vehicle. I've been thinking maybe setting the near idle MPH table settings to Zero but leave the TPS setting at the stock setting of about 2.7% might allow using the near idle table and eliminate the need for a VSS? Anyone try this on a vehicle yet?

dave w

[Six_Shooter]

Why would you want to eliminate the VSS?

I've kinda looked at the lack of N/P inputs and believe using either a relay or two, or possibly just using one (or as many as needed) of the inputs for the gear select, might get the same effect.

[dave w]

Why would you want to eliminate the VSS?

A board member here is trying to make a '427 PCM work for a Marine application work closed loop. I'm looking for ideas to get a '427 PCM to function correctly in a stand-alone type application. The Marine application of a '427 has many challenges!

The '427 conversions I've done were for vehicles that had '7747 or similar ECM's, so there was already a VSS in the vehicle.

dave w

[Six_Shooter]

I can see where you're going with that, but (just about) every boat I've been in had a speedo, so by mechanical or just electrical means, I'm sure you could add a VSS, to take advantage of the benefits of having a VSS feeding the PCM.

[EagleMark]

I see where your going and remember trying that but had severe idle issues? When I went back to trying to make it run off Idle tables I had no issues higher RPM... just looked at a couple last logs and BLM were good way past idle tables. Idle spark enabled was always no yet idle VE went on and off...

[dave w]

Switching back and forth in the Idle VE table would / should be Ok, as long as both near / off idle timing tables are the same.

dave w

[EagleMark]

Anyone try this on a vehicle yet?

dave w

Yup! Only a manual trans though. Still works great on the kids truck and we never got around to adding the VSS. He out towed a 5.3L Chevy with same Toyota rock crawler on same trailer up hill, actually he passed him by 1/4 mile till top without pushing it, his IH with utility bed loaded is 1500 punds more then Chevy, Chevy was floored... both had 3:73 gears. This year his gas bill was half of last with carb, really half on way up and way back! Within a gallon or so of the Chevy. The IH engine pulls hard to 5000 RPM.

Took quite a bit of fiddling but the bottom line was making idle and off idle tables identical in spark and fuel. I think it is running on idle tables only. don't remeber if I messed with TPS reading to force a change of tables. I can eMail the bin if you want?