Just installed a new vacuum modulator because tranny wasn't shifting into third. Does anyone know the shift points for this tranny? It shifts into third gear now but isn't shifting until about 50 mph, seems high to me. I did some research on the TH400 and some have said the shift points for that tranny is 7 and 35 mph. I know I can still adjust the modulator, but I do not believe that much. Any suggestions? Thanks in advance.
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TH400 Transmission Shifts points
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scooter421
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Have you verified you have good vacuum at the pump? I replaced my modulator hoping that was the problem but it turned out to be the vacuum pump. Prior to installing the new pump my transmission was shifting at about 48mph. Now, it's shifting around 35 to 38 mph.
Scooter.
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Scooter.
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chevymike
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Yeah, my first M1010 had a problem with bad vacuum lines and modulator. I did the lines first and then had to do the modulator.
dependable
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There is the modulator on the tranny, and vacuum adjustment valve to the left of IP on top of engine, this can be adjusted by loosening screw rotating it some. The vacuum pumps do occasionally go bad, but I'd double check everything else first, those pump heads are getting hard to get last time looked. Eight times out of ten it's the lines in my experience.
A vacuum gauge would be a worthwhile investment at this point, the only way to know if correct amount is getting to the modulator.
A vacuum gauge would be a worthwhile investment at this point, the only way to know if correct amount is getting to the modulator.
Very useful information, thank you. I am still learning on the diesel curve. I tried to locate a pump, you are right, not going to find one easily. Through my research, so far, there are two vacuum modulators for the TH400 with a 6.2 diesel. There is the one the parts store sold me and another with a white stripe on it. I am going to change my new modulator for the one with the white stripe as suggested on several sites as the proper one to run. I am also buying
a vacuum gauge. Will update here after I make the change to the new modulator etc..Thanks to everyone for all your help and patience.
a vacuum gauge. Will update here after I make the change to the new modulator etc..Thanks to everyone for all your help and patience.
dependable
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Usually it is not the modulator either , although possible. Did you check the rubber double connector on the adjsutment valve on top of motor? This is often cracked. It can be replaced with two single hoses.
cucvrus
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A quick test. Remove the vacuum hose from the top of the vacuum pump at the rear of the engine. Start the engine at idle. You should hear a chirping sound. Carefully get a step and put your finger over the hole on the vacuum pump while the engine is idling. It should suck fast gently. If no suction and no chirping pump is bad. At this point good luck finding a vendor that stocks them. Be Safe and report back on your findings.
I believe I have replaced all rubber lines related to the system but will recheck when it is not freezing outside, no garage. The transmission was initially working when I bought the truck and then slowly shifting into third gear at a high speed (50mph) and then no third gear. I did check the pump, there was a constant gentle vacuum, but I didn't hear a chirping sound. I found another pump off of a parts truck for free, don't know if it is good. I think that I am going to try the new modulator with the white stripe. Now, as far as this modulator with a white stripe is concerned, it is also shown for the TH400. I compared the two online and they are different. The white striped modulator appears larger/longer especially in the area of the neck/throat that meets the rubber hose. While doing my research online, many of the older Chevys vehicles that had this transmission that were being resto moded experienced this problem if they chose to keep the TH400 and not change over to the 700R4. Many had installed the same modulator as I did and later switched to the modulator with the white stripe with more success as far as the proper shift points being much closer to 7 and 35 mph. I will post my findings when I install the other modulator next weekend when it will be hopefully, warmer.
Jake59
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Hi,
Did you finally get better results with the new modulator?
When I purchased my M1009, the vacuum line from the vacuum pump to the vacuum control valve on the IP was not even present. The ATF smelled burnt... I assume this is related to the high RPM shifting that was going on?
I have now put in fresh ATF and new silicone vacuum lines where needed.
Everywhere I read it says I should have about 21" Hg vacuum at idle.
I have checked the vacuum pump and at idle I get a reading of 19" Hg now, after checking all vacuum lines, all seems well, sealed and tight now.
According to TM 34 the adjustment of the vacuum control valve is made using a special tool and with the accelerator partially open (I guess about 80% or so?)
What confuses me most however is this: reading in TM 34, it says that the opening point is at 8"Hg, but when I apply vacuum directly to the vacuum modulator valve, it only begins to open at 15" Hg vacuum and reaches full open with 21.5" Hg.
Is this the wrong vacuum modulator valve I have in my TH400 or is this correct and is 19" Hg at idle sufficient or does that indicate a vacuum pump - or other problem?
Does anyone have any data on the amount of travel the vacuum modulator valve vs. amount of vacuum or alike, so I can check that my modulator?
Or can anyone for comparison give me have the vacuum readings of their TH400 vacuum modulator valve?
The vacuum modulator valve on my TH400 was painted all white and has following information stamped on it:
HRD
G4
581
Google did not learn me anything...?
Thank you,
Jake
Last edited:
muttly
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I replace my vacuum pump earlier this year with a re-manufactured Cardone #64-1008. I believe it is for a ford engine but it is the same pump on a different pump drive. Just remove the screws that hold the pump onto the ford drive, attach it to your drive and discard the two extra screws. They are available on E bay from $70.00 up. I did some shopping and got mine for $35.00
shotty
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I just installed a rebuilt transmission in a truck that was shifting kind funny, but mostly leaking a ton of atf. It would downshift when you floor it but then immediately upshift back in to 3rd. When I went to get a new modulator for it the auto parts stores showed a unit with a red stripe in their computer system when I asked for an 84 chevy k30 with 6.2 unit., but when they gave it to me it was a no stripe adjustable unit. You can change the modulator shift settings by turning a small screw in or out where the vacuum line connects. I left it in the default setting and the truck shifts great. Something to ponder, maybe snag one of those instead of the non adjustables?
Since this whole health thing, I put the truck on the back burner. I did buy the adjustable modulator, not much change. I am probably going to go through the pump and lines again when it is not 100 degrees outside. I may try the vacuum switch suggested by cucvrus if the pump in junk, we'll see.