Turbo 400 vacuum modulator leak I don't like it because he said the trans was rebuilt to stock specs but Adjustable Vacuum Modulator Suit TH350 & TH400 The vacuum modulator is a vital component of automatic transmissions. Jun 1, 2018 · I've been having a shift issues with my TH400 transmission since I bought this '68 J-3500. At first I thought that the converter (Hughes 2500) was the cause of this, but it has been brought to my attention (thanks laddyboy!) that this may be a vacuum modulator issue. I even took to a tranny shop and they could not figure it out. I read many of the threads in the search and everyone keeps reffering to the "Modulator O-ring". What can I do to make it shift earlier under light throttle. . I finally traced it to the vacuum pod thingy. Sep 23, 2004 · Does the actual modulator leak or does the seal behind it leak normally? Jill's is leaking and it appears you have to buy the whole modualtor to get the seal? That leads me to the next question are some modulators better than others or more aggressive? Can you tell I'm not an automatic trans 1972 Chevy Suburban Fixing Leaks and Disaster Strikes! Turbo400 Slipping 2nd 1-3 Upshift Vacuum Modulator Adjustable Shift Points Turbo Hydramatic 400 3L80 Locate the vacuum modulator on the passenger side rear of the transmission. Nov 13, 2005 · The brand new vacuum modulator was 12 bucks from a local parts store, then throw in a couple bucks in vacuum line and check valve worked WAY better than the mechanical modulator. I suppose the used modulator could be bad too but neither one had fluid in it. Dec 4, 2016 · Hey guys, Breaking down my first TH400, 1968 vintage "CD" unit, every thing done but a couple of things. It's not coming from the cooler lines or dip stick. Here's How To Diagnose A Failing Valve Before It Causes Extensive Damage. Discover essential troubleshooting steps, from checking vacuum levels and inspecting hoses to proper adjustment and replacement tips. That's what is in all the CUCV Military trucks is the 6. I explain what the vacuum modulator is and its primary function and go for a test driveThere are more transmission videos in my playlist if you want to see m Jul 2, 2016 · I am looking for a high quality vacuum modulator for a TH400. The current unit in my '68 has developed a leak in the crimp area and is making a mess on the exhaust (not to mention a trail of smoke behind the car), so I have Feb 23, 2017 · I replaced my vacuum modulator on my TH400 (stock transmission) recently. Fluid leaks from the hole in the inspection cover and also from the driver side front of transmission. Instead of just the modulator, there is also a 60 deg. Connect the pump to the modulator’s vacuum port and observe whether it holds vacuum under pressure. Early detection can save you from costly repairs and improve your vehicle’s overall performance. Leaks in the hoses are not uncommon, and the symptoms created may appear as serious transmission problems. Rubber hoses are used to connect the steel line from the engine to the inlet of the modulator. If you use rubber all the way down, the line will expand/contract and make the modulator do stupid things since it's not getting full vacuum signal. So I probably will have to pull the steel vacuum line to clean it. When there’s sufficient engine vacuum, the modulator enhances fluid pressure, leading to firmer and more timely shifts. The one in the transmission now is from NAPA and seems to be having some issues, seems to be leaking internally. This unit comes pre-adjusted for normal shift points and works with #TH350 and # Dec 9, 2012 · I forgot all about it until I seen this post. Jan 4, 2006 · In the case of a TH400 tranny, is a manual VB or mechanical modulator absolutely necessary on a low (6" @ idle) vacuum application? Are there any mods that can be performed for improving the 2-3rd shift? What is the minimum Hg" required for decent modulator operation on a TH400? Is it true Mar 19, 2010 · What is the purpose of grinding the flats on the Modulator "under valve" Part number 163 on diagram. Jun 4, 2025 · Signs of a Bad Transmission Vacuum Modulator When it comes to detecting issues with your vehicle’s transmission vacuum modulator, be on the lookout for these common symptoms. Tips Vacuum leaks from any source have an effect on the vacuum modulator's ability to operate properly. A faulty modulator will fail to maintain vacuum or will leak down quickly. It helps to adjust the shift points in order to maximize engine performance and fuel efficiency. adapter it mounts to on the transmission. THM 400 This booklet contains general description and overhaul procedures necessary to repair, overhaul or service the THM 400 transmission, and is found in both passenger cars and trucks. Another thing guys will do is use an electronic vac modulator eliminator. This article covers common symptoms of a faulty modulator, including rough shifts and erratic RPMs. If vacuum levels drop, the modulator reduces pressure, which can result in sluggish shifting or extended gear engagement. It will stop irritating late shift issues in the TH350 and TH400. 2's with Th400's the Vac Modulator line is connected to the vacuum pump. Mar 2, 2023 · CT5015 - A new vacuum modulator for TH350 and TH400 Transmissions, 1965 & up. tighter the screw is turned in, the later the shifts tend to beand vice versa. How many inches of vacuum is needed for a TH400 modulator ? This is what I have at idle. A transmission vacuum modulator is crucial for the proper functioning of your vehicle’s transmission system. I've noticed that I've got quite a leak under the transmission. We have included the major changes and updates in Sep 26, 2025 · Vacuum leaks are common causes of modulator failure symptoms. It can also be a blown diaphragm in the modulator. The problem im having is 50-75% of the time when Oct 19, 2021 · Having hard shifts from 1st to 2nd, enough to jerk you back and forth Seeing other post about testing the vacuum modulator, unplugged the hose at the modulator to test the pressure and transmission fluid came out. Also read the fluid is sucked through the engine, negatively affecting engine Jun 1, 2001 · I am putting a turbo 400 in my car and the I am curious what I need to do about the line to the vacuum modulator on the turbo 400 to keep boost from being an issue. This device can be Dec 28, 2009 · Just remember to use a HARD LINE from manifold to modulator. If you notice transmission fluid pooling under your car, it’s imperative to inspect the vacuum modulator and associated lines for any signs of cracks or failures. Oct 14, 2017 · TH400 dumped fluid at highway speed and then quit leaking. I've always had an issue with the transmission (in 'Drive') shifting very quickly through the gears; it is literally in third gear within 100 feet. 2 and Nov 24, 2010 · C3 Tech/Performance - Transmission modulator / vacuum pipe TH400 - '73 350, 400TH A couple of questions: 1) what is the purpose of the vacuum modulator of the TH400 Transmission? (I was thinking either it is the controller for downshifting or the controller for normal shifting?) 2) I bought a repro modulator-to-intake When in doubt about the vacuum signal, install a vacuum gauge in line at the modulator side of line and check for good and responsive engine vacuum. 1) Shifting Problems Because the transmission vacuum modulator regulates hydraulic pressure in the transmission, one of TH400 Modulator Valve | Squarebody Turbo 400 transmission leak repair | #Moby BurnoutsAndRotorblades 9. It provides the transmission with information about the engine’s vacuum pressure, which changes with different driving conditions. Mar 23, 2011 · Check for a vacuum leak going to the modulator. Also remember there is an adjustment screw inside the vacuum nipple of the modulator that will let you move the shift points around a Apr 28, 2025 · Features: Vacuum modulator is fully adjustable to custom tailor your shift points (within 2-4 mph) Comes pre-adjusted for normal shift points to get you started Applications include TH350 and TH400 transmissions Genuine B&M accessory Backed by the manufacturer with a 1 year limited warranty Fully Adjustable! Jun 24, 2010 · I have a turbo 400 that I got from a friend of a friend that said it was rebuilt. Do I need to just use a check valve in line? Thanks for the help. I fear if I remove it I will have lots of fluid draining out. Here's a quick, simple, and fairly inexpensive way to fix both Chevrolet C20 1967 TH400 Slipping through 2nd Gear and fast 1-3 Upshift. Is there a gasket on this or is it an O ring ? Oct 23, 2014 · C3 Tech/Performance - TH400 Leak Problem - The TH400 in my '74 has a leak. The Modulator valve piston will not come out Jan 17, 2017 · I would clean up around the area of the modulator before removing it and then after it is removed, try hooking it up to another vehicle's vacuum line and see if there is any leaks by placing your finger over the end and see if you feel some suction. Hard line from manifold to modulator with a short rubber line at each end. It tells the transmission what kind of load is being put on it, allowing the transmission to react with the proper line pressures and A Worn Out Modulator Valve Can Eventually Start Burning Transmission Fluid. Oct 18, 2006 · C3 Tech/Performance - Normal Leaking Spots on a TH-400 - My 71 has been sitting in the garage for the past couple of weeks while I'm doing some things under the hood. I have the smallest weep from that area. Look at the hose leading to it from the engine and examine it for cracks that could cause a leak. This low vac modulator will give you an earlier (and smoother) shift from first to second, and second to third. I replaced the new Vacuum Modulator Installation GM TH350/TH400 Transmission | FIXES SHIFT PROBLEMS! B&M's vacuum modulator is fully adjustable and allows you to tailor your shift points to within a range of 2-4 mph. Today I finally had time to look into it. If you have a check valve somewhere from old smog crap (test it first by flipping it around and you should not be able to read vacuum on the other side. The body is still off of the chassis so everything is acessable. Haven't really gotten under to look yet, but just want to know what the likely culprits Jan 7, 2022 · In this video I service the GM Turbo 400 on my 1976 Holden HX Monaro LE Coupe. Be sure to use an adjustable modulator. If it's blown you can tell by pulling off the lower vacuum hose on the modulator and fluid will come out. like 3,500-4'000 rpm. Forgot how hard the TH400 shifts at higher rpm's. The modulator is connected to a vacuum source and it regulates the amount of fluid pressure going into the transmission, allowing for smoother shifting between gears. May 7, 2019 · When I had my turbo 400 rebuilt the shop put an adjustable vacuum modulator in it. Nov 10, 2025 · Experiencing issues with your TH400 transmission? From slipping gears to overheating problems, learn how to diagnose and fix the most common TH400 transmission problems. Mar 27, 2011 · 71 TH400 Modulator O-Ring? My modulator doesn't seat all the way to tranny casing, and is leaking. It operates by using engine vacuum to adjust shifts based on engine load, enabling smoother performance and improved fuel efficiency. Thanks again for volunteering your time to answer our questions! Dec 10, 2024 · A vacuum modulator is a component in the Turbo 400 transmission that regulates the transmission’s shifting based on engine load and speed. It appears to be coming from behind the Torque Converter. This is to make clearance to May 4, 2016 · To hylomike The information given way back in 2016 is a very good place to start with your issue manifold vacuum to the modulator should be checked at the modulator with a gauge if low find the leak sometimes a split or kink in the hose can occur make sure the modulator doesn't have fluid inside it when the vacuum line is removed. counter-clockwise and that where it shifts. Application: TH350 1969-1986 TH400 1965-1998 Aug 28, 2005 · I switched the new modulator with a used one I had here from another trans, blew through the vacuum tube and checked for vacuum leaks. Jan 13, 2021 · No vacuum leaks that I can find although the engine vacuum is a little low relative to what I had on the dyno (7 vs 11). The adjustment is made in through the open hose nipple with a small flat blade screwdriver. Side note: I'm using a Coan RMVB and it calls for an OEM modulator valve with their supplied spring. Wonder what you folks are using to seal it? I typically just use a bit of oil on the o-ring. May 9, 2025 · The TH400 vacuum modulator is a crucial component of the Turbo Hydramatic 400 transmission, which plays a significant role in controlling shift points. At 74 i will not be Nov 21, 2009 · C3 Tech/Performance - TH400 Vacuum Modulator Change - Hi All, Would like some info from anyone who has expereince changing the vacuum modulator on a TH400. I do believe on the 6. Looking into a Leeds or Jegs vane pump for the booster but what the heck do I do the the transmission ? I tested the Turbo 400 vacuum modulator and was blown away by its performance. So, when I went to pull the modulator I found something different. 88K subscribers Subscribed Sep 9, 2017 · C3 Tech/Performance - Turbo 400 leak - I have been chasing a leak on my '75 turbo 400. It should be dry when you pull the hose off. Responsive means that as the throttle is changed, the vacuum signal immediately follows throttle changes. What could cause this? Pontiac - Street Jul 5, 2020 · TH400 vacuum line The 1967 - 1972 Chevrolet & GMC Pickups Message Board Jul 3, 2012 · After hooking up the vacuum line I decided to start manually shifting at the desired speeds and downshifting the same way. I don?t know if my garage floor isn?t level or the car but the bottom of the Jul 17, 2021 · C3 Tech/Performance - Turbo 400 Vac Modulator - One of the things on my list for this year is to replace the vacuum modulator on my turbo 400 trans. Push in type, black & red stripe, adjustable. Next, perform a vacuum test using a handheld vacuum pump. However, as I stated above, the modulator valve (in the case) might be sticking when fully inserted into the Episode 97 of Bad Hombre Garage: my car has been leaking transmission fluid pretty bad, and the speedometer isn't very accurate. Not a lot. I took the modulator out when I cleaned the tranny, but I don't recall an O-Ring. I adjusted the vacuum modulator all the way out. I change the oil filter, the Vacuum modulator, internal kickdown solenoid and Dec 2, 2024 · A vacuum modulator that has developed a fault can sometimes initiate leaks. My teacher at the high school used to work for amco transmission if I remember Jul 31, 2010 · My vacuum line is about a foot of rubber then about three feet of hard line with another foot or so of rubber connected to the carb, I'm guessing I should just replace that with new as long as I'm replacing the modulator correct? I'm also assuming that if the mudulator were good that four turns would defineately have shown some measurable resullts. Have no fear, it can be done. Nov 1, 2024 · Learn how to adjust the vacuum modulator in your TH350 transmission for smoother shifting and improved performance. BTW the converter is Mar 8, 2012 · Product Description This TCI Adjustable Modulator Valve is designed specificlaly for GM TH350/TH400 Transmissions. I am just now starting to drive that car, a 64 Riviera, now that the weather is getting nice. Upgrade your car's transmission with this top-quality part today!. Figured I either had a vacuum leak or the vacuum modulator was bad. Modulator is good, no leaks in diaphragm. Ensure the modulator holds pressure without leaks. Oct 25, 2024 · The vacuum modulator’s operation directly impacts the shifting of the TH400 transmission. I do beleive him that it was but think some things were overlooked seeing that the shift shaft seal was pissing fluid and should have been replaced during the rebuild. The THM 400 transmission was first introduced in 1964 and was used through the 1990 model year with some engineering changes that occured during that time. Includes vacuum tube and very important oring. I replaced a modulator that was leaking ATF into the vacuum line during the winter. Sep 18, 2019 · My 56 has a Turbo 400. I polished the valve just a bit and that helped remedy the sticking issue; however, seems like it needs a bit more, but I didn Mar 10, 2010 · One question solved, now another one pops up. May 24, 2001 · Thanks Bruce, I needed this too. Thanks, Rick Sep 4, 2005 · Pull the line going into the vacuum modulator, see if any ATF comes out = definite sign the modulator is leaking. Low engine vacuum compounds the effect and pushed shift points up higher, so you have to adjust the modulator pre-load weaker to Sep 27, 2010 · BUT if I by pass the TSM and connect the good vacuum to the vacuum line that goes down to the modulator, I get very little down at the modulator. Any delays would indicate a plugged line, leaking line, or restricted port at manifold. Make a Jun 30, 2019 · Anyway, I started testing fitting components and in an attempt to install the modulator valve (also sourced from eBay) I noticed it was sticking as I was putting it in. Jul 31, 2023 · The Ford transmission vacuum modulator is an important part of the vehicle’s automatic transmission system. Jun 8, 2020 · The vacuum modulator on the trans is probably the easiest thing to try adjusting first. Jul 11, 2024 · To test a transmission vacuum modulator, use a vacuum pump to check for proper vacuum retention. Turbo400 Slipping 2nd 1-3 Upshift Vacuum Modulator Adjustable Shift Points Turbo Hydramatic 400 3L80 How To LEGALLY Title Any Classic Car, Truck Or Motorcycle Using The Vermont Loophole Oct 13, 2011 · Would a vacuum leak from carb to trans modulator or bad modulator cause this? I plan on testing the line when I get home by unplugging the line from the carb and hooking it up to a vacuum pump, creating few inches (10-15) of vacuum and see if the vacuum holds as it should. Vacuum Modulator Adjust and Test#cars #automotive #transmission #gearbox #automatict Oct 26, 2014 · I have a freshly rebuilt turbo 400 that shift very late under part throttle acceleration. Oct 25, 2011 · C3 Tech/Performance - thm400 occassionally blows fluid out the dipstick/filler - hi i have a thm400 or is it abbreviated th400, and it sometimes when shifted will blow transmission fluid all over the place from the dipstick/ filler hole. Shut off the engine and replace the hose if any problems are found. If you have a vacuum gauge, check what the modulator is seeing. It regulates the transmission’s shift points based on engine vacuum levels. Not having any problems, have the original one on there, car is a 75 with upper end mods with t/b 400 and an B/M shift kit for the last 45 years. kchl zjripa vtxs hkn lfuwvbzu oludi byvfswt lfrat xcrjnyu rqly wtdwr ivjb eycys jnun yqgyvl