I upgraded from carb to TBI with a 7747 system. I have no VSS. I'm not seeing any problems, should I be worried that I will sometime in the future? What does VSS do in these systems? I have no emissions stuff...

I really did search on this. It looks like maybe idle is adjusted by speed somehow, so if I let off at high speed, something bad might happen?

TIA

 

Yeah the car/truck has to be driven 35 mph for the IAC valve to reset. At 35 mph it will completely close and the ECM knows where it is. No VSS means this can't happens. Not to mention you are penalizing yourself MPG wise by not having a functional TCC or DFCO.

The worst I can think of without having a VSS is a surge at low throttle positions and maybe some stalling while coasting to a stop.

 

Thanks for the reply. I'm afraid I don't know what TCC and DFCO even is.

I'm not sure I understand the idea behind closing the IAC at 35 either. I thought the IAC was parked when the car was shut off, so the computer would know the position from that. Are you saying that it loses track of IAC position over time or something?

 

TCC is the torque converter clutch and DFCO is Deceleration Fuel Cut-Off (The ECM shuts off fuel while coasting to lower emissions and increase fuel mileage).

The reason behind parking the IAC at 35 mph cruise is because the engine needs the IAC open at slower speeds to keep the RPM constant to lower emissions. My IAC is wide open at lower vehicle speeds with my foot barely on the gas. It will also idle at 1,000 rpm coasting along until I put my foot on the brake and slow below about 20 mph.

The ECM does lose track of the IAC over time, mainly due to the fact that carbon will sometimes keep it from moving to the desired position.

You can also reset the IAC to 0 counts by shorting the ALDL connectors that flash the check engine light and turning the key to run but not starting the engine.

Finally the IAC park position atleast in the two ECM bin files I have been working with is 150-160. That allows the engine to start and idle high when first started.

 

I,m Back at TBI.I'm doing tbi conversion on 1970 chevy pu.The ecm is 7747,350 (5.7),w/auto 350TH.I'm having problem with getting it work real good.It runs ,starts
easy,idles unstable at times.F/P 12 psi.
Question is not having VSS.Is that the reason for unstable.
Also don't have fuel pump relay system hooked up.Does that matter ? Rex

 

Painless wiring 60115, without the prom programmed to disable the code 24 it might be in limp home mode.without a vss this can cause you to have surging idle when coming to a stop and poor fuel economy.

 

Its not in limp mode.no ses light,actually it starts open loop then goes in closed loop.
it would never go in closed loop if in limp mode.

 

At a minimum make sure there is system voltage on ECM pin B2. That is the fuel pump power monitor input. It is also used for injector voltage compensation.

Note that using a SBC '7747 PROM, there may not be enough SA if the '70 PU engine has old school heads on it.

RBob.