There's no vacuum output from the transmission, it has no way to generate vacuum.
Very true, the C4 has no way to generate vacuum, but it could switch (based on some operating criteria) the vacuum it received from the engine as an input on one line back on the other as an output. I’m not saying the C4 does that – in fact I doubt very much it does that – but that’s a conceivable function it could have in theory at least.
I think your theory is more likely, that the C4 vacuum modulator for this vehicle has two inputs. That would make sense as it would provide more shifting information than just the intake manifold vacuum for the transmission to decide the shifting points.
@PvtPublic … interesting diagram, my – presumably older than yours – 4AFE Toyota Corolla engine has a considerably more complicated vacuum hose configuration, at least as complicated, probably more so than my Ford truck’s. A lot of the complications in the 4AFE vacuum design are due to the EGR function. There’s 3 vacuum ports just from the throttle body corresponding to vacuum signals used by the EGR, the signals originating from slightly different positions around the throttle butterfly valve. Fortunately I’ve never had to debug that, as when I read that functional description in the FSM it’s so complicated that I can’t tell what it is talking about.
Ok, fyi for those interested, I did a little more research on my attempt to understand the vacuum line configuration for my truck. It seems in regards to my question about the transmission vacuum connections, the answer is a bit of a weird one.
The Ford C4 automatic transmissions installed in 302 v8/ 1973 models – apparently this was only done in 1973 models – do in fact have 2 vacuum line inputs to the transmission. Both go to a special-for-1973 2 input transmission vacuum modulator. One comes directly from the intake manifold, and one from the carb base via a valve in the heater hose port. It is claimed in the link below to be possible to switch back to the normal 1 input vacuum modulator if you want to, just a simple parts swap. A little more progress in understanding how it all works I guess.
A couple more questions to the older carb-engine experts here, I’m thinking I may have the advance/retard notation in the OP diagram reversed. By this I mean where it says “advance”, it should actually say “reverse”, and visa versa. What do you think?
The distributor vacuum advance wouldn’t normally be directly connected to the intake manifold vacuum would it? If it was that way, it would be advancing all the time. I’d guess you’d want to connect it to the base of the carb, so you get an extra hit of advance as you step on the gas. On Ford’s dual-vacuum distributors , where one distributor vacuum port is for advance and another for retard, which is which? There’s two, one port sort of in-line and furthest from the distributor, and one port butts right up next to the distributor.
Also, I need a little clarification on something that confuses me. On the vacuum line diagrams I’ve seen for the Ford dual port distributors, one is labeled “S” and one is labeled “P”. I can’t figure out why they’d use those letters? It makes no sense. Wouldn’t the letters “A” and “R” make more sense, meaning advance and retard? Anybody know what the P and S stands for?
The advance vacuum nipple that points up should be the retard part of it. The horizontal linear one should be the advance part.
I do not remember what the letter designations are for but the S is probably for straight manifold vacuum and the P is for ported manifold vacuum.
Straight usually goes to the retard feature on the advance and ported to the advance feature.
The S would be vacuum directly from the intake and below the throttle plate.
The ported vacuum is from a nipple on the carburetor and has little vacuum while the engine is idling.
A carburetor can also have a straight manifold vacuum nipple on the carburetor. That varies by make and carburetor with some carbs having both. It’s easily sorted out by placing a fingertip on the nipple after the hose is unplugged and with the engine at idle. If it sucks then it’s straight manifold vacuum and if not then it’s ported. Hope that ancient history helps.
I learned this stuff back when we had an abundant supply of these running round on the roads and had my share of fun with them I can assure you. But hey…if you don’t know the fundamental foundations of how these things work…you start to think magic is involved with modern engines…and that doesn’t help your troubleshooting skills none too much. The importance of a sound foundation cannot be overlooked Grasshopper.
Excellent info @ok4450 and @“Honda Blackbird” . OK’s description seems to fit what Ford used at the time for the vacuum advance/retard design on my 1973 truck engine. And the experiments I did today agree with OK’s explanation of which port is which. Indeed, as OK says, the vacuum nipple that butts up against the distributor and points up is for “retard”. And the horizontal one furthest from the distributor is for “advance”.
On my truck full intake manifold vacuum is always applied to the retard side. That measures about 20 inches on my truck at idle. And a ported vacuum signal is applied to the advance side. There’s very little measured there at idle, but it temporarily increases to 12-15 inches or so, when the throttle is quickly bumped away from idle.
By “ported” going to the advance side, this appears to be about the vacuum switch that switches between applying the carb-base vacuum or the full intake manifold, depending on coolant temp. What my repair manual says, in normal operation the carb is used as the source, but if the engine heats up, such as extended idling, the full intake manifold is applied to advance the timing and speed up the idle rpm, facilitating improved engine cooling since the water pump is pumping more and the engine fan is spinning faster.
So that all means indeed the diagram I posted above has the reverse and advance denoted incorrectly. I shall endeavor to correct it and repost.
If you read the “Vacuum timing 101” tutorial posted by Honda and compare it to OK’s post, the two seem to contradict each other. 101 says vacuum advance is connected to full intake manifold vacuum. I think that may have applied for cars before the strict EPA emissions rules took effect, pre - 1970 in other words. In the early 70’s the car designers must have been having fits meeting the emission standards, and by the year to year changes seen back then, some almost opposite to the year before, they seem to have been grasping at straws. And it shows up in the horsepower figures. In the mid to late 1960’s the 302 produced over 200 HP, but by 1973 it was down to 140 HP. Some of this HP reduction was due presumably to decreased compression by changes to the basic engine parts, and some to the advance/retard ignition design, and probably the EGR and PCV systems took some toll.
This is all probably not of much interest to owners of modern electronic fuel injected cars, but hopefully some here will find it useful, or at least historically interesting.
George you do have a point…I always thought we needed more advance at higher rpm because of piston speeds and the time it takes for the electricity to reach the plug and light things off…all of this takes time… Timing is everything they say.
I will need to read that thing in full again and ponder it… but I definitely slowed down when I first read about the timing at Idle. Perhaps he is mistaken…it makes sense on one hand and then seems contradictory…at the same time.
....I always thought we needed more advance at higher rpm because of piston speeds
As I understand it, the vacuum advance/retard is sort of supplementary to the main method, the centrifugal weights inside the distributor. The distributor’s centrifugal design is what causes the timing to advance as the rpm speeds up. The vacuum system modifies that curve for certain conditions.
Yes I would have to agree with you George. The only thing that had me thinking was about the advance at idle situation…that confused me slightly but I suppose I didn’t consider the mixture ratio at that engine speed…it makes sense but I didn’t think it would be full advance. You know?
The vacuum only has a sweep of degree change of about maybe 15 or so…so I need to focus on that…it isnt truly full advance…it is just the end of the advance ability of the vacuum dashpot. So…makes sense. The rest of the advance is handled by the weights and centrifugal force. See? It is making sense now… just had to “DWELL” on it a bit. Pun intended…
With vacuum advance distributors (and especially cars with EEC ignition timing) it’s critical to make sure the timing is set correctly. Some specify disconnection of the vacuum lines, others state to leave them alone, and some require test plugs to be jumped or grounded.
I remember one time back in the late 80s a gentleman who was on the road a lot taking his almost new Subaru to one of those fast-tune facilities. You know; the 40 bucks (at the time) flat fee for any 4 cylinder.
They replaced the plugs and checked the ignition timing without reading the underhood sticker about disconnecting and plugging the vacuum advance line. Of course the timing was way off when they checked it due to not following procedure so they bumped it up to where it was “correct”.
That added about 12 degrees into the timing advance and the following week when the car owner hit the highway to head to Wyoming he made it about 50 miles from OK City before the catastrophic bang of several disentegrating pistons happened. He coasted to a stop with a blown engine. All because someone did not unplug a lowly vacuum hose…
The plus side though was that he decided not to fix the car or go after the tune facility and traded it in. The car was only a few years old, had very low miles, was showroom clean and I snatched it up for 700 bucks wholesale from the Used Car Dept. A week of lunch hours, a few hundred bucks, and it became my daily driver for a long, long time.
Some years back a friend spent the winter building a custom Triumph motorcycle. When done it was gorgeous with everything chromed, painted in Candy Apple Red, etc.
On a warm Sunday afternoon he took it out for a shakedown run to a small town about 70 miles away. The engine had 12.5 to 1 pistons in it and too much advance in the timing.
About 20 miles into the return trip home it made a loud BANG and that was the end of that engine. Both pistons shredded and the wrist pins chewed up the cylinders along with bending both rods and damaging the crankshaft.
Pre-Ignition is a pain… i would have to say that each of these characters would have known there was an issue…if they…well…knew what to look for. My seat O pants Timing device works pretty good. I bet yours does too OK44. But yes…this is real stuff people.
These days we have become unbelievably spoiled with engines and timing and or the lack thereof…this stuff caused more garage visits than it didn’t at one “point in time”. We truly are in the Golden Age of Engines…its cool that I got to see it all happen. I started on old Maytag, Clinton, Briggs, Tecumseh…and all the way on up to now…the no computer, Pre-Computer and Heavy Computer ages…Pre-cell phones to now… I think people our age-ish will have seen the most dramatic changes in Life Cars and Tech than pretty much any other generation.
I know I feel this way. We are living in interesting times for sure…just dont forget where it all started and you should be OK. Imagine what kids will do with being born into the tech we have now…it can go either way…real bad or really good…Jury is still out and we wont know till Im a Geezer…we have plenty of time
Good essay on modern times @“Honda Blackbird” ; however I’m seeing a disturbing trend of the newest technology being used to boss folks around. Like car computers that tell the insurance company or whoever has authority to know the speeds you drive, cell phones that must be kept turned on and with an employee 24 hours a day, 7 days a week, and every call from the employer responded to immediately; and can also tell your employer where you go, what you do. I think you and I – Honda – may have indeed seen it all happen, even participated a little, and that’s pretty cool. But I’m not so sure the result is all that good for the younger folk in America.
I wonder if anyone here remembers vacuum wipers @ brakes on some of the older trucks It made for some interesting rides on mountain roads before interstates.
Yes, I recall those. I think early VW air cooled Beetles had vacuum powered WW’s too. I seem to recall going skiing as a passenger in one of those cars and when we were going uphill the WW’s would slow way down. It wasn’t a problem, the driver kept a small towel within reach and he’d open the window and wipe the windscreen off while driving.
I dont know why they would have ever tried to use vacuum on those cars back in the day. Either for wipers…Headlight pop ups…Headlight hiding doors…or what not. Wouldnt an electric motor have done the job with aplomb? Smaller in packaging…not reliant on rubber hoses cracking or breaking as they were surely prone to do?
I wonder who or what was the driving force behind any of these devices…Its a novel idea IF you lacked 12VDC for motors…or the electric system was weak or suspect…but none of that was true…and none of these devices would work without the engine running.
The new Stop/Start no idle/energy conserving vehicle designers would have fainted if the engine had to be idling to uncover the headlights or use the wipers etc…
Bla had ford or dodge ckbird you made a comment abot 3 miles of vavum lines. Ihave aiways driven ford or dodge pickups @ if I had any issues I would cap off everything but the disturbter @ brake lines with no problem
