VSS and EGR function?

gadget73 · #1 January 7th, 2007, 10:58 PM

I've got a couple questions regarding the VSS and how it relates to EGR operation.

First question:
Does VSS input play any role in determining EGR function on a 1988 and later Mark VII? I ask this because the 1986 Townie EVTM shows no connection to the ECM from the VSS. Mark VII I assume should be similar in 1986 because no integrated cruise control. I know in 1988 the ECM got VSS input because it has integrated cruise, but I need to know if this signal has anything at all to do with EGR function.

Second part:
If the VSS signal does play some role in determining EGR function, can anyone tell me what pin the VSS input is on a 1988 Mark VII ECM?

The reason I ask this, my EGR valve opens just off-idle when accelerating lightly from a stop light, causing a slight stumble and hesitation. I also get a constant VSS code. I know there is no VSS input to the computer, hence the code, but I'm just wondering if I can ignore that code since I do not need the integrated cruise functions, or if I need to provide VSS input so it knows how to deal with the EGR properly. I'm using the stand-alone cruise controller module and have no plans on changing it away from that since it works fine as-is.

Bluegrass · #2 January 8th, 2007, 02:29 PM

VSS connects to pins 3 and 6 on a Mark EEC. Same on a Mustang EEC.
EVR to pin 33 on both EEC.
The VSS provides a number of functions the EEC program routines use.
The speed for cruise control applications, speed for decelleration IAC and fuel control, Decell signal for EEC to update tables.
The EVR is opened after the TPS position and time are sampled vs engine speed range of about 1800 rpm to about 2400 +/- low load.
These signal conditions logic an enterence into the EGR open routine so the EGR in never opened at the wrong times.

If your EVR is being signaled under acceleration, either the EEC is faulty or wireing is incorrect.

Tony T · #3 January 8th, 2007, 03:50 PM

Any EGR codes?

gadget73 · #4 January 8th, 2007, 08:34 PM

I get no EVR if I take off hard, just if I'm lightly accelerating from a stop. Its had the VSS code (29, insufficient input from speed sensor) for a while now, and randomly the cruise hitches. I've been blaming the clockspring for the cruise issues since it happens when I turn the wheel mostly. The wiring under the car has been cooked to a crisp and patched back together, so its possible something is amiss there. The VSS wasn't replaced after I caught the car on fire. It doesn't have any obvious physical damage, but its still possible its been damaged.

The 1986 Town Car EVTM shows nothing on pins 3 or 6, however it wouldn't surprise me terribly if it is not complete. The BCE manual doesn't give me any information on the ECM's wiring. Neither manual show anything branching off the VSS wire to go off to the ECM. It shows only a connection from VSS to the cruise control module on non-electronic dash cars. On cars with digital dash, input for the cruise module comes from the the digital cluster assembly which has it's own built in speed sensor.

Tony: Not that I'm aware of. Last I checked it gave me thermactor codes and the VSS code. The thermactor I expect since there is no smog pump.

I guess I'll have to unplug the ECM connector and meter out whats on pin 3 and 6, if there are pins there. If there aren't, I guess I know where my VSS code is coming from, and why the EVR system isn't operating. Shouldn't be a big deal to add in two pins and tap them in to the VSS wiring.

Bluegrass · #5 January 8th, 2007, 08:59 PM

You won't get any EGR action at WOT because the intake vacuum is to low to operate the EGR... and the EEC does not signal the EVR under those condition unless something is faulty..
The VSS could be tested for output using an electric drill to spin it and meauire the AC output with a meter or a frequency counter.
My meter has both functions.

Tony T · #6 January 9th, 2007, 04:07 PM

The EVR foam filter may be clogged or the orifice in the EVR is (partially) clogged, this will cause vacuum to be applied to the EGR. this will set a code 34 engine running. A code 34 engine off means EGR is being held open, possibly by carbon buildup.

gadget73 · #7 January 9th, 2007, 10:00 PM

The VSS must be mostly working otherwise I would have no cruse. I wouldn't be surprised if there is a poor connection in the cobbled harness though. The connector isn't in the best of shape, and I know there are 4 crimp connectors patching the thing back together. I've been meaning to swap an undamaged harness in there, but have yet to get around to it. I'm still thinking maybe an 86 Towncar doesn't have VSS input to the computer, hence the VSS codes it sets and the abnormal EGR function. I'll check the wiring and see if I get continuity from the VSS to pins 3 and 6 on the ECM connector. If I do, I'll try subbing in another ECM to see if it behaves differently. I can get my hands on 3 or 4 of them without any trouble so it would be easy enough to do a test swap.

Tony: EVR filter is fine, and the EGR doesn't stick. I get no EGR codes. The system is essentially functional, just not at the right times. I'm just trying to figure out if the VSS code and wiring is related to the issue.

Tony T · #8 January 10th, 2007, 02:51 PM

I don't think the VSS plays a roll in EGR funtion. It's mostly engine speed, temp, TPS angle. It may be (just guessing here) the programing differential between the 86 TC and the 88 MK EEC's.