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Charge Air Cooler Bypass


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On my 1994 RD with EM7 there is this gadgetry. I am wondering if it’s really necessary. I have never really seen it on any other trucks newer or older and quite frankly I have never used it even when cold starting and the truck starts fine without using it. The thing is after sitting for any length of time the truck is out of air and can’t cycle the air cylinders anyway. Also, if you were to use it the cylinder on the bypass duct crossing over top of the engine won’t return to closed when it’s switched to “off”. You have to  tap it closed after you turn it off.  I just see it as a more leak paths for boost air to escape from or leak past the bypass valve. Can I eliminate the whole system? Should I? What’s everyone else’s experience with these? I’m just looking for any information out there on this setup…..879BE446-0564-46D0-8E0A-1822CCCAE0F8.thumb.jpeg.a75457991dbef8999779cc52fd46ff02.jpeg4A088CF2-E895-4906-BAB0-DA14A6D12F8A.thumb.jpeg.3845bc142ec13b897c5bf26aa4f58fa2.jpeg

Edited by 67RModel
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so does it pull hot air off the exhaust manifold, for warming up the engine?  I would like to think the valve is in the bypass position when cold, and then when air pressure is up, you can switch to 'normal'  I dont know, just curious..  jojo

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I have seen cement mixers with this on them and i am thinking it just to by/pass the cooler when not under power to keep the intake air warm.It does not take warm air off of the exhaust but only keeps the cooler from cooling the intake air when not needed .I see no problem with removing it.

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glenn akers

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Not necessary !This was developed for  real cool climate warmup believe I have wired a few of these closed !It by passed the after cooler for quicker warm up  rather usless to the average guy!

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I too have seen these on mixers. Maybe in really cold climate (like you'd be pouring concrete there in the first place) a smaller HP engine might benefit from it on a cold start cause there's less fuel at full fuel throttle position ???   It'd have to be pretty cold to need it. A Mack engine in anything much above fair condition usually starts easy in the cold.

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2 hours ago, Mark T said:

a smaller HP engine might benefit from it on a cold start cause there's less fuel at full fuel throttle position ??? 

There could be some validity to this because my E7 is factory 250hp. I don’t think there were any ratings lower than that for the E7. 
 

Either way I am going to do away with the whole setup while I’m installing a boost gauge….

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you may want to take it off, with care and box it up..  someone may want it and be willing to shed a few dollars..  plus you are not throwing hard to find parts in the trash as I still see guys do..  just a thought... jojo

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12 hours ago, Joey Mack said:

you may want to take it off, with care and box it up..  someone may want it and be willing to shed a few dollars..  plus you are not throwing hard to find parts in the trash as I still see guys do..  just a thought... jojo

Yup I got a shop full of junk that MAY have a use some day😡 ! at some point something has to go! I have an old vintage lathe I no longer use I want rid of Can't sell or give it away! Has  Dozens  of accessory's!   It may have to go to the scrap guys!

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I too have a bunch of parts, its getting smaller.. I had a bunch of old Ford car engine parts that I rescued from a bulldozer,  I was able to sell a lot of it at a major Swap meet event near me,  I sold them cheap, but they went to good homes, and not back to China in little pieces, only to be resold to us as new junk... :)  I wish I could find a place to bring my 'core' parts..  I hate to trash them, especially because finding the replacement part was an effort..  jojo

Edited by Joey Mack
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  • 6 months later...

Ok I try to always follow up on threads I start. Anyway I got to working on removing this system from my truck. I started with removing the switch, indicator lamp and associated air plumbing in the cab. What I’m a little stumped on is what the valve in the first picture is? I will preface that question with how the switch was plumbed behind the dash. There is a large maybe 5/8” airline coming from I assume the air tank into a tee. Out of one branch of the tee goes to the air gauge. The other branch went to the switch. If the switch was turned “on” air would be sent to the valve in the first picture. The line went into a tee that was on the back of this valve and out of the other branch of that tee a line went to the cylinders on the bay pass duct. You can see there is a 3/8” plug in the back where the tee used to be. There is also a line going from this mystery valve to what I think is the aneroid (spelling?) valve on the intake manifold for the puff limiter in the second picture. The left and right sides of the valve in question are just dumping to atmosphere. Is it a quick release valve for when you flip the switch to “off” and/or when the manifold pressure gets high enough to dump the air on the puff limiter? 

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10 hours ago, Onyx610 said:

Start at 10:00 

Thanks. Ill have to dig into it a little more to try and understand how/why that CAC bypass is plumbed into that valve. I assume the valve I am asking about it the LDA valve? Thanks.

Edited by 67RModel
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Doesn't one of the air lines go from the intake manifold to the front of the injection pump?  If so, I believe it actes like a reversing relay, and high boost pressure, operates the Number valve to push the rack back..  if I remember correctly..  it helps to keep from over boosting the engine..  Jojo 

Edited by Joey Mack
Pore cell fone sperring
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1 minute ago, Joey Mack said:

Doesn't one of the air lines go to the front of the injection pump?  If so, I believe it actes like a reversing relay, and high boost pressure, operates the Number valve to push the rack back..  if I remember correctly..  it helps to keep from over boosting the engine..  Jojo 

Joey,

Yes. In the second picture the airline painted in gray that goes out the left side of the picture from the manifold goes to the injection pump to limit rack travel before manifold pressure gets high enough to burn all the fuel from fully open rack. The line coming in the bottom of the second picture to the manifold I think comes from the transmission to keep the rack from opening fully until you switch into transmission high range (I have not confirmed this yet though). I'm just wondering now if I should remove that plug I put in and put tank air to it or tank air via the parking brake valve like the old puff limiters were set up. I'm just not sure schematically what the valve in the first picture does or what its "looking for".....

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16 minutes ago, Joey Mack said:

So the tranny line is a torque limiter?

That is my understanding of it. Well at least how the older style puff limiters worked on the E6s. You got full rack on start up (to aid with cold starting). As soon as you released your spring brakes air was sent to the puff limiter until manifold pressure (boost) was sufficient for full rack. Also, If you had a Mack transmission with lolo gears you wouldn't get full rack until the transmission was in forward high range. I'm not sure about this LDA system and why that valve is mounted up off the manifold. The older style puff limiter valve was tapped directly into the manifold.

Edited by 67RModel
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OK I got this figured out now thanks to the video @Onyx610 posted and actually having time to sit down and watch it. The video along with some other sources got me all straightened out. Mechanically governed E7s use the LDA system and not the puff limiter system. For those that don't know and want to know here goes: In a cold start scenario the puff limiter system (think E6 and possibly older) is full rack by default. Rack travel becomes limited once you release the spring brakes and air is sent to the injector pump. Once boost pressure is sufficient for full rack the boost pressure overcomes the air pressure supplied by the parking brake valve via the reversing valve that is tapped directly into the intake manifold. Boost pressure and chassis air pressure are always hydraulically "battling" each other in the reversing valve for control of the air cylinder in the injection pump. In a cold start scenario the LDA system is limited rack by default. There is no reversing valve at the intake like the puff limiter system. The LDA system just has a line tapped into the intake and constantly adjusts rack travel based on boost pressure alone. No compressed air is involved. In a cold start scenario limited rack is not desirable so a workaround is needed. That is the purpose of the valve in the first picture a few posts back. It is a Bendix SV-1 Syncro Valve. When you flip the dash switch to "on" for the CAC bypass, tank air (assuming it hasn't all leaked out) is sent to the SV-1 valve as well as to the two cylinders in the bypass duct plumbing. The cylinders simultaneously close the CAC outlet and open the bypass duct allowing warm compressed air to directly enter the intake manifold. The air sent to the SV-1 valve is regulated down to whatever maximum boost pressure is and passed down and connected to the the tee fitting in the intake manifold where the LDA is tapped into. Thus, the LDA is "fooled" into thinking boost is max and full rack is available before you start cranking the starter. Once the truck warms up a little the CAC bypass can be switched off, the air cylinders cycle back to their normal positions, boost air begins passing through the CAC normally, the air going to the SV-1 valve is exhausted, and the LDA goes back to sensing whatever the actual manifold pressure is. Of course it doesn't stop there. Both systems utilize a torque limiting valve if the truck has a Mack transmission. Once running, if the transmission is put into reverse and/or the low forward gear(s) the torque limiting valve is activated. It is piped from its location on the transmission up to the reversing valve (puff limiter) or to the tee where the LDA is tapped into the manifold. Once its activated it dumps the boost pressure to atmosphere that would normally build up in the system and allow full rack travel. By doing this rack travel is limited in both systems to protect the drivetrain in the low and reverse gears. Once the shift rails are moved to select a higher gear the torque limiting valve closes and allows boost pressure to build up and allow for full rack travel in the injection pump. Below is a picture taken out of my E7 book explaining that all fully mechanical E7s made after 1991 have the CAC bypass setup installed by default. I assume the VMAC trucks can electrically advance the timing during cold startup making this system not necessary for them. I will say that I have consistently been able to start my truck down into the low 30s high 20s temperature range without using this system and it fires up very quickly.

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I wonder if the intercooler bypass is for running not starting 

There must be temp when the air is to cold and gets to cold to be efficient, we will never see this in Australia but places like Canada and parts of north America and Europe and Vlads part of the world and up into Siberia this would be a issue

I wonder if the 6x6 Macks Australia sent to Antarctica had the bypass ?

A few were for sale recently, should of bought one just find out, didnt know anything about this then

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Sorry side tracked this thread, but the brain is running at 100 MPH now lol

Is there anywhere the R model hasnt been, they must be the most versatile truck ever built

Sorry I'll take myselfoff to the naughty corner lol

It doesn't look like these are coolpower trucks

Have  never seen a chassis mounted intercooled Flinstone Mack

Paul 

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