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| GMC Driving fundamentals question [message #293882] |
Fri, 15 January 2016 09:50  |
jknezek
Messages: 1057
Registered: December 2007
Karma: 5
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For years I have manually downshifted my GMC when necessary. I find this happens in primarily two cases: 1) when pulling a long slope and the engine drops below 40 mph (roughly) in 3rd gear and 2) in town driving when I have to slow for a stoplight or traffic and end up at slowest between 30 and 40 mph. I have a Manny Tranny, so no kickdown switch.
However, for the holidays my loving wife purchased a vacuum gauge and a tach for me to install. Rather than using the speedo, I wonder for those of you with a tachometer when you choose to manually downshift. When does it become lugging in a higher gear?
Other important information, I have a 3.55 final drive and, as far as I can tell, the original 455 that came with the coach.
Thanks,
Jeremy Knezek
1976 Glenbrook
Birmingham, AL
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| Re: [GMCnet] GMC Driving fundamentals question [message #293906 is a reply to message #293900] |
Fri, 15 January 2016 16:13  |
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Loffen
Messages: 1087
Registered: August 2013
Location: Norway
Karma: 1
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USAussie wrote on Fri, 15 January 2016 22:08Jeremy / Espen,
I've discussed this with Manny face to face and while I've forgotten the technical reasons I do remember him saying "go down a hill
in the same gear you went up it" and "brake pads are a LOT cheaper than transmissions!"
Regards,
Rob M.
The Pedantic Mechanic
-----Original Message-----
From: Espen Heitmann
When I bought Peter's used Manny transmission I asked Manny regarding the kick down and he said it is still there and is working in
the transmissions
he rebuild.
I do always downshift when I go down steep down slopes to save the brakes, going up hill the transmission decides, if you have to
down shift manually I guess you have a modulator problem, a automatic transmission should never start lugging in a higher gear it
should down shift.
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Yes we had that discussion here earlier, and yes that is true, same gear up as down, the problems with brakes is that they get really hot really fast and when really hot no brakes 
So using the transmission to take up some of that energy helps a lot. I promise you that if you did try to drive down the hills at my place using only brakes in a GMC you would not survive it
1973 23' # 1848 Sky Blue Glacier called Baby Blue and a 1973 26'-3 # 1460 Parrot green Seqouia Known as the Big Green,
And sold my 1973 26'-2 # 581 White Canyon lands under the name Dobbelt trøbbel
in Norway
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| Re: [GMCnet] GMC Driving fundamentals question [message #293932 is a reply to message #293900] |
Sat, 16 January 2016 10:03 |
Bob de Kruyff
Messages: 4260
Registered: January 2004
Location: Chandler, AZ
Karma: 1
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USAussie wrote on Fri, 15 January 2016 14:08Jeremy / Espen,
I've discussed this with Manny face to face and while I've forgotten the technical reasons I do remember him saying "go down a hill
in the same gear you went up it" and "brake pads are a LOT cheaper than transmissions!"
Regards,
Rob M.
The Pedantic Mechanic
-----Original Message-----
From: Espen Heitmann
When I bought Peter's used Manny transmission I asked Manny regarding the kick down and he said it is still there and is working in
the transmissions
he rebuild.
I do always downshift when I go down steep down slopes to save the brakes, going up hill the transmission decides, if you have to
down shift manually I guess you have a modulator problem, a automatic transmission should never start lugging in a higher gear it
should down shift.
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Most of my camping in AZ involves driving in the mountains with some grades around 10%. If I counted on brakes only on the downhill grades, I would be dead by now. It's not an issue of brake wear, but brake overheating. Although the notion of using the same gear down as you used going up sounds nice but I have yet to encounter a hill/mountain where the uphill grade is the same as the downhill grade.
Bob de Kruyff
78 Eleganza
Chandler, AZ
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| Re: GMC Driving fundamentals question [message #293963 is a reply to message #293882] |
Sun, 17 January 2016 10:33 |
gbarrow2
Messages: 765
Registered: February 2004
Location: Lake Almanor, Ca./ Red Bl...
Karma: 3
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Jeremy,
I drive a lot in the Sierra Nevada's just to get from home to civilization. I'm in the Bob deK and Espen camp regarding using the tranny/engine brake on the down hills.
I also have a tach and vac guage and on the uphills I manually down shift when the rpm falls below 2500. And manually upshift at 3000 or so when it is clear that momentum can be maintained in 3rd gear.
With my 455 and 370 final drive 2900/3000 rpm is 69/70 mph and 14/15 inches of vac on the flats at 300 ft elevation. I try to keep it between 2700/3000.
There was a thread on this topic a couple of years ago that had more information about manual shifting and lugging. I started the discussion so you may find it by searching my old posts.
I'd be interested in your vac readings on the flat at 3000 rpm with the 355 gear.
Gene Barrow
Lake Almanor, Ca.
1976 Palm Beach
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| Re: GMC Driving fundamentals question [message #293973 is a reply to message #293882] |
Sun, 17 January 2016 13:28 |
jknezek
Messages: 1057
Registered: December 2007
Karma: 5
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Senior Member |
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Thanks Gene. That was the info I was looking for. Our next trip is South to Florida. Probably won't downshift that direction but I'll let you know some flatland readings when I get back. Appreciate the help and will look for the old thread.
Thanks,
Jeremy Knezek
1976 Glenbrook
Birmingham, AL
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| Re: GMC Driving fundamentals question [message #294014 is a reply to message #294004] |
Mon, 18 January 2016 08:49 |
jknezek
Messages: 1057
Registered: December 2007
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Loffen wrote on Mon, 18 January 2016 04:36And please do not ignore what I said in the second post, if the transmission is like you say in the first post it has serious problems and I would recommend that you took a talk with Manny
Maybe I'll get a chance to talk to Manny about it if he gets to Dothan, but both Jim Bounds, who put in the tranny, and Jeff at Alex Sirum told me Manny does not hook up kickdown switches because they cause too much heat gain. They also both confirmed that when travelling uphill in 3rd, a steep grade will cause you to slow and lug and manual downshifting is proper around 40 mph. I will be seeing Jeff again in February so I will check with him again.
Thanks,
Jeremy Knezek
1976 Glenbrook
Birmingham, AL
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| Re: GMC Driving fundamentals question [message #294027 is a reply to message #294014] |
Mon, 18 January 2016 12:43 |
Bob de Kruyff
Messages: 4260
Registered: January 2004
Location: Chandler, AZ
Karma: 1
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jknezek wrote on Mon, 18 January 2016 07:49Loffen wrote on Mon, 18 January 2016 04:36And please do not ignore what I said in the second post, if the transmission is like you say in the first post it has serious problems and I would recommend that you took a talk with Manny
Maybe I'll get a chance to talk to Manny about it if he gets to Dothan, but both Jim Bounds, who put in the tranny, and Jeff at Alex Sirum told me Manny does not hook up kickdown switches because they cause too much heat gain. They also both confirmed that when travelling uphill in 3rd, a steep grade will cause you to slow and lug and manual downshifting is proper around 40 mph. I will be seeing Jeff again in February so I will check with him again.
I realize Manny is the transmission expert, but even though I usually downshift manually on grades when needed, the kick-down switch is nice if you need to make a passing maneuver and don't have time to fool around with the shift lever. Also, if adjusted correctly the switch in conjunction with the modulator and governor should never allow your engine to lug.
Bob de Kruyff
78 Eleganza
Chandler, AZ
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| Re: [GMCnet] GMC Driving fundamentals question [message #294038 is a reply to message #294014] |
Mon, 18 January 2016 15:39 |
Ken Henderson
Messages: 8726
Registered: March 2004
Location: Americus, GA
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Unless he's changed his policy since I spent time at his place in June
2013, Manny installs new downshift solenoids during his rebuilds. I don't
remember for sure whether he connects the leads to them, but I think he
does. At any rate, it would be trivially simple to attach the plug when
changing the ATF. But, he DOES NOT recommend their use since the manual
downshift creates higher operating pressures and consequently reduced
chance of clutch slippage.
Ken H.
On Mon, Jan 18, 2016 at 9:49 AM, Jeremy wrote:
> Loffen wrote on Mon, 18 January 2016 04:36
>> And please do not ignore what I said in the second post, if the
> transmission is like you say in the first post it has serious problems and
> I would
>> recommend that you took a talk with Manny
>
>
>
> Maybe I'll get a chance to talk to Manny about it if he gets to Dothan,
> but both Jim Bounds, who put in the tranny, and Jeff at Alex Sirum told me
> Manny does not hook up kickdown switches because they cause too much heat
> gain. They also both confirmed that when travelling uphill in 3rd, a steep
> grade will cause you to slow and lug and manual downshifting is proper
> around 40 mph. I will be seeing Jeff again in February so I will check with
> him
> again.
> --
> Thanks,
> Jeremy Knezek
> 1976 Glenbrook
> Birmingham, AL
>
> _______________________________________________
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Ken Henderson
Americus, GA
www.gmcwipersetc.com
Large Wiring Diagrams
76 X-Birchaven
76 X-Palm Beach
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| Re: GMC Driving fundamentals question [message #295236 is a reply to message #293963] |
Mon, 08 February 2016 11:53 |
jknezek
Messages: 1057
Registered: December 2007
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gbarrow wrote on Sun, 17 January 2016 11:33Jeremy,
With my 455 and 370 final drive 2900/3000 rpm is 69/70 mph and 14/15 inches of vac on the flats at 300 ft elevation. I try to keep it between 2700/3000.
I'd be interested in your vac readings on the flat at 3000 rpm with the 355 gear.
Had a short drive with the new vac and tac gauges. I was between 2600 and 2800 RPM between 60 and 65 mph (my speedo isn't real accurate and I didn't have the GPS in the coach for this short run). Vacuum was in the 13-15 inch range. These readings were on the interstate flats. I use my electronic cruise control a lot, and when we hit moderate hills you could see the vacuum drop to 8-10 to maintain the speed.
We have a longer trip planned, mostly flat, in the near future. Unfortunately I will be towing the HHR. Should be interesting to keep an eye on it. I did adjust the carb a bit for what I was seeing, it seemed to need a touch of advancement to get the idle where it should be. This summer I'll be interested to see if the little extra advance causes any run-on.
Thanks,
Jeremy Knezek
1976 Glenbrook
Birmingham, AL
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| Re: GMC Driving fundamentals question [message #295265 is a reply to message #294014] |
Tue, 09 February 2016 02:15 |
Ken Burton
Messages: 10030
Registered: January 2004
Location: Hebron, Indiana
Karma: 10
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I have a Manny transmission and my kick down switch works as designed by GM.
That said, I have found and corrected many out of adjustment kick down switches that do then do not work. It is a simple, no tool, 10 second fix to bend the accelerator pedal rod back in to position so the switch functions correctly.
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana
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| Re: [GMCnet] GMC Driving fundamentals question [message #295273 is a reply to message #295265] |
Tue, 09 February 2016 09:40 |
sgltrac
Messages: 2797
Registered: April 2011
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Doesn't the vacuum mechanism on the trans downshift the trans when a
specified amount of vacuum (adjusted at the mechanism) is present while
also telling the trans to shift up as well?
Sully
77 royale
Seattle
On Tuesday, February 9, 2016, Ken Burton wrote:
> I have a Manny transmission and my kick down switch works as designed by
> GM.
>
> That said, I have found and corrected many out of adjustment kick down
> switches that do then do not work. It is a simple, no tool, 10 second fix
> to
> bend the accelerator pedal rod back in to position so the switch functions
> correctly.
> --
> Ken Burton - N9KB
> 76 Palm Beach
> Hebron, Indiana
>
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Sully
77 Royale basket case.
Future motorhome land speed record holder(bucket list)
Seattle, Wa.
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| Re: GMC Driving fundamentals question [message #295300 is a reply to message #293882] |
Tue, 09 February 2016 17:52 |
JohnL455
Messages: 4447
Registered: October 2006
Location: Woodstock, IL
Karma: 12
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No kickdown and old GMCers with poor hearing is a great way to have them lug the engine on long grades. It also prevents you from quickly escaping a sticky situation where you need to floor it and have both hands on the wheel. With no kickdown the GMC is a real pig.
John Lebetski
Woodstock, IL
77 Eleganza II
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| Re: GMC Driving fundamentals question [message #295308 is a reply to message #293882] |
Tue, 09 February 2016 18:27 |
JohnL455
Messages: 4447
Registered: October 2006
Location: Woodstock, IL
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I think Dick P stated that lugging is worse on an Olds. Up or down hill it should upshift at a set RPM and not overspeed the engine.
John Lebetski
Woodstock, IL
77 Eleganza II
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