Bunch of electrical questions

[johnunit]

I'm finally elbows-deep in the carb swap, and am trying to figure out more about the wiring. Looking at the diagrams and the (now half-gutted) wiring on the car, I have a few things I'm trying to figure out

Car is a 5.8L with the stock variable venturi system, being swapped to a holley but retaining all emissions functions.

Note when I say "computer' I mean the MCU for the variable venturi system, which is being removed entirely


What do the wires to the duraspark ignition module and distributor do? I know one is from the computer to the ignition module, and that goes. But what all does the rest do? There's three wires to the distributor, can I remove the one that isn't just power or ground now that there's no computer to adjust timing?

It looks like the Cannister Purge Solenoid and Carb Float Bowl Vent solenoid just need power in run, no computer or vacuum related stuff. Does that sound right?

There are two wires from the ignition switch to the computer, should they just be power and ground to the computer, or is this something that'll actually have an effect on the ignition switch?

Will the factory o2 (single wire, seems to be fairly standard for the era) work with an aftermarket gauge?

The diagram shows an "oil diode" going from the thermactor dump timer. What is that and what does it do?

It sounds like the Thermactor Dump Timer gets a ground signal from the Thermactor Control Relay, and switches function 90 seconds after this ground is removed. Does that make sense?

Related, it seems like the only function of the thermactor control relay is to send a ground signal to the Thermactor Dump Timer, and that it sends that signal when the car is starting. Therefore, the dump timer starts it's timer after the ignition is turned away from 'start', and 90 seconds later the timer finishes and the dump timer switches it's function. Does that all make sense?

The Thermactor Air Diverter Solenoid is shown as purely being connected to the computer, no other inputs. Would this be diverting the air pump air to fresh air (as opposed to anywhere in the exhaust) or between the exhaust piping and the ports on the back of the heads?

Same basic question regarding the Thermactor Air Bypass Solenoid.

[gadget73]

There are 3 plugs for the stock Duraspark. One is output to the coil, one is input from the distributor pickup, and the third is timing retard from the MCU. If the MCU goes away, you have to jump the third plug (think its the yellow one) out to set the timing normally. Or just swap it for a 2 plug Duraspark II module.

The can purge and bowl vent are probably run off the MCU. They won't and can't be open constantly or the engine would have huge vacuum leaks.

single wire O2 sensor will run a gauge, but its narrow band. It won't tell you a whole bunch other than its rich or lean of stoich.


thermactor diverter switches from air to the converters to air to the heads. air only runs to the heads for a few minutes after startup, then it goes to the cat for a few minutes more to fully warm it up.

thermactor bypass switches the air on and off. after air moves to the cats for a while, thi is basically shut after the motor is warmed up.



Honestly if you're keeping the emissions stuff, I expect you'll need to retain the MCU. It obviously won't connect to the carb to monkey with the fuel mix but it runs most of the electrical smog stuff. Without it, it probably won't ever activate. I presume you need this crap for emissions testing ?

[GM_Guy]

Canister purge is controlled by the mcu. Just hide the end of the wire incase we ever get visual inspections. There is kinda-sorta vaccuum involved with the canister purge. Canister purge comes via hose from the canisister, and a "vacuum" line from the carb sucks the fuel vapours out of the cannister when the computer calls for it. If you want a pseudo functional solenoid, you can hook it up to switched voltage and run the end of the tube to an inline fuel filter hidden somewhere and let it breathe.

Carb vent you won't need with the holley (or is it a VV replacement holley? In which case it probably has its own to plug into, and yes, its switched 12 volts to the vent solenoid).

02... no. Except for an autometer 5775 type guage which is just a bunch of dancing LEDs going between rich and lean.

I'm guessing the diode on the dump timer is to prevent backfeeding the timer so it turns off when commanded to by the computer.

[johnunit]

There are 3 plugs for the stock Duraspark. One is output to the coil, one is input from the distributor pickup, and the third is timing retard from the MCU. If the MCU goes away, you have to jump the third plug (think its the yellow one) out to set the timing normally. Or just swap it for a 2 plug Duraspark II module.

Nice, I already have the blue plug Duraspark module ready to go in. So all the MCU stuff is doing is timing retard, and the original distributor should work plug-and-play by just removing the MCU and swapping the duraspark?

The can purge and bowl vent are probably run off the MCU. They won't and can't be open constantly or the engine would have huge vacuum leaks.

What would the normal activation be? Would having it only open up on decel (there's a vacuum switch that's closed above 21 in hg I could activate with) be ok?

Looking at the diagrams, the float bowl vent solenoid appears to just have a wire to "hot in start or run" and a ground wire. The cannister purge, however, seems to run through the MCU. Is it possible that it's designed so that the float bowl only sees vacuum from the cannister when the cannister purge is activated? Or am I looking at this totally wrongheaded?

thermactor diverter switches from air to the converters to air to the heads. air only runs to the heads for a few minutes after startup, then it goes to the cat for a few minutes more to fully warm it up.

thermactor bypass switches the air on and off. after air moves to the cats for a while, thi is basically shut after the motor is warmed up.

The thermactor dump (seems to be synonymous with bypass in Fordspeak) seems to be activated both with the 90 second timer and the MCU, with the 90 second timer overriding. Would there be any potential harm in just removing any bypassing/dump stuff for the system? I know in theory it'll overheat the cats, but I don't remember ever hearing the sound of it bypassing other than when the 90 second timer would have come into play. I've idled in drive for as much as an hour (not that I'd recommend that...) and not had any signs of cat damage. It seems like there'd be no performance detriment either, since the smog pump will be pumping either way.

Or maybe I could keep the thermactor dump timer, and then replace the wire to the MCU with one to the aformentioned 21" vacuum switch, thus bypassing the system during decel? I seem to recall that with the VV carb I pulled 22-23" of vacuum at idle though...

[johnunit]

Canister purge is controlled by the mcu. Just hide the end of the wire incase we ever get visual inspections. There is kinda-sorta vaccuum involved with the canister purge. Canister purge comes via hose from the canisister, and a "vacuum" line from the carb sucks the fuel vapours out of the cannister when the computer calls for it. If you want a pseudo functional solenoid, you can hook it up to switched voltage and run the end of the tube to an inline fuel filter hidden somewhere and let it breathe.

Carb vent you won't need with the holley (or is it a VV replacement holley? In which case it probably has its own to plug into, and yes, its switched 12 volts to the vent solenoid).

02... no. Except for an autometer 5775 type guage which is just a bunch of dancing LEDs going between rich and lean.

I'm guessing the diode on the dump timer is to prevent backfeeding the timer so it turns off when commanded to by the computer.

Interesting. Thanks.

It's a normal holley 4-barrel, but I already have fuel bowls from a ford truck with the vents.

Can you elaborate on the inline fuel filter idea?

[gadget73]

Nice, I already have the blue plug Duraspark module ready to go in. So all the MCU stuff is doing is timing retard, and the original distributor should work plug-and-play by just removing the MCU and swapping the duraspark?

yep. The only difference between DII and DIII is that timing retard connection. No problem doing a direct swap to eliminate that stuff.

What would the normal activation be? Would having it only open up on decel (there's a vacuum switch that's closed above 21 in hg I could activate with) be ok?

Not totally sure the logic on the MCU rigs, but on the SEFI cars its open on decel. If you have that vacuum switch and know that it opens at some amount higher than normal idle, I'd say you're fine running it from that.

Looking at the diagrams, the float bowl vent solenoid appears to just have a wire to "hot in start or run" and a ground wire. The cannister purge, however, seems to run through the MCU. Is it possible that it's designed so that the float bowl only sees vacuum from the cannister when the cannister purge is activated? Or am I looking at this totally wrongheaded?

Its possible that the bowl vent is open the entire time the engine is running. It may only shut to keep fuel from evaporating when the car sits. If thats how it works, and I don't know that it is, just powering it from key-on power *should* let it work. I'm not sure how the vacuum plumbs from that, but if it ties into the canp circuit on the charcoal can side of it, then it would only actually get vacuum when the canp circuit is powered. If you ran that off the vac switch, it should only run on decel.

The thermactor dump (seems to be synonymous with bypass in Fordspeak) seems to be activated both with the 90 second timer and the MCU, with the 90 second timer overriding. Would there be any potential harm in just removing any bypassing/dump stuff for the system? I know in theory it'll overheat the cats, but I don't remember ever hearing the sound of it bypassing other than when the 90 second timer would have come into play. I've idled in drive for as much as an hour (not that I'd recommend that...) and not had any signs of cat damage. It seems like there'd be no performance detriment either, since the smog pump will be pumping either way.

If anything I'd let it do it's thing and bypass after 90 seconds. I wouldn't run it full time for fear of melting the converters down.



Mind you that much of this is guesswork and inference on my part. I've never fooled with the VV/MCU rig to any extent. I've seen maybe 2 of them in person.

[johnunit]

yep. The only difference between DII and DIII is that timing retard connection. No problem doing a direct swap to eliminate that stuff.




Not totally sure the logic on the MCU rigs, but on the SEFI cars its open on decel. If you have that vacuum switch and know that it opens at some amount higher than normal idle, I'd say you're fine running it from that.

Interesting. I'm not sure 21inhg will be high enough for that, but I'll have to see once I get the carb on and tuned. I would think if I were to get a custom cam in there, it'd be enough of a spread between idle and vacuum switch actuation to prevent hunting. That'd be down the road but I could run without that function or with no belt on the smog pump until then. My underdrive crank pulley means, theoretically, I'm only getting about half the smog pump flow anyway.

Its possible that the bowl vent is open the entire time the engine is running. It may only shut to keep fuel from evaporating when the car sits. If thats how it works, and I don't know that it is, just powering it from key-on power *should* let it work. I'm not sure how the vacuum plumbs from that, but if it ties into the canp circuit on the charcoal can side of it, then it would only actually get vacuum when the canp circuit is powered. If you ran that off the vac switch, it should only run on decel.

Hope so, that'd be a clean solution. Just run both the evap solenoids off the pre-existing vacuum switch.

If anything I'd let it do it's thing and bypass after 90 seconds. I wouldn't run it full time for fear of melting the converters down.

Looking at the wiring more closely, it looks like it's only controls are from the relay for it (which just grounds when the neutral safety switch indicates it's not in neutral or park) and the oil pressure switch, ie. ground when you're <6psi oil pressure. So I guess I can keep it.

Mind you that much of this is guesswork and inference on my part. I've never fooled with the VV/MCU rig to any extent. I've seen maybe 2 of them in person.

Thanks. I know you're sort of working from guesswork, but everyone up to and including MitchellOnDemand seems to have even less idea how the systems work, so you and GM_Guy have been super helpful.