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New to the group. I am bringing back to life a 1976 98 with 89k miles. It's been off the road for 15 years. I need a vacuum hose diagram. can anyone help?
Welcome. If you are bringing a 1976 back to life, the first thing to get is a factory 1976 Chassis Service Manual. This has all the information you need to repair and maintain this car, including THREE pages of different vacuum hose diagrams, which depend on engine and model and accessories. In addition, the vacuum hoses will be different for A/C, cruise, and other vacuum-operated options, which are also described in the CSM. Don't cheap out - "free" information on the interwebs is worth exactly what you pay for it. There are over two dozen of these 1976 manuals on ebay right now, starting at $8.00.
Welcome. If you are bringing a 1976 back to life, the first thing to get is a factory 1976 Chassis Service Manual. This has all the information you need to repair and maintain this car, including THREE pages of different vacuum hose diagrams, which depend on engine and model and accessories. In addition, the vacuum hoses will be different for A/C, cruise, and other vacuum-operated options, which are also described in the CSM. Don't cheap out - "free" information on the interwebs is worth exactly what you pay for it. There are over two dozen of these 1976 manuals on ebay right now, starting at $8.00.
I have the manual. But can't find where the modulator valve hooks to the vacuum port. And how much vacuum it takes to work. Thanks for your reply.
Whether for this project, or the next, I'm glad you have the book.
My olds 98 is a whole lot older and simpler than your '76, but if you could describe the modulator valve you're working on and which vacuum port you'd like to hook it to, we might be able to help you.
The short version is most accessories need manifold vacuum and your car should have several vacuum ports on the intake manifold to provide that. Usually you can "T" off of one of them, or simply reconnect (or replace) a hose which has cracked, split, fallen off or been taken off by a previous owner. I would advise against using a T fitting on the brake booster line since that's safety related. But most other ports are usable either with manifold vacuum or ported vacuum on the carb or thermostatically controlled vacuum with a signal from the water jacket on the front of the intake manifold.
While you're in there, take a good look at all your vacuum hoses. They're pushing 50 years in a hot environment. Look carefully for splits, cracks, holes, cuts, burns and other problems that could be causing your engine to run rough and/or preventing your accessories from working well.
How much vacuum? Well your engine should show something between 14-18 inches of mercury on a vacuum gauge at idle. If you have that, it'll be plenty to get functional accessories to do their thing. So you get the vacuum gauge and a rubber hose, hook it up to the intake manifold and see what you've got. If you don't have that level of vacuum signal, the engine may not be running all that well. If you do have steady vacuum at the intake, and competent hoses, then the problem may be in the accessory itself. Many/most of these accessories had rubber diaphragms that the vacuum sucks against to get them to move levers or do other stuff. If those diaphragms have cracks/splits/holes/leaks, they aren't going to work. Or work well. That includes your brake booster...
If it were me, I'd focus on getting the engine and any heater/ac vacuum controls right first, then start working on vacuum controlled accessories like cruise control, etc...
The car is not shifting right. Thats why i think the modulator valve is nor working. I will check the vacuum at the manifold and if it is good I will hook to it. Thanks for the help.
I have the manual. But can't find where the modulator valve hooks to the vacuum port. And how much vacuum it takes to work. Thanks for your reply.
Turn to page 6C-23 in that manual. The two center diagrams on that page are for the Ninety Eight, 49-state on the left and CA emissions on the right. Both of those diagrams have a line that says "TO TRANS MODULATOR" in the lower left of the diagram.
If the transmission modulator is not working right, replacements are available and the job is not bad at all. This modulator has one of the rubber diaphragms inside which moves every time the car gets driven. So they do need periodic checks or replacements. If I recall they're not expensive either. Like cheap enough to do as a test if the car is not shifting right. Others here can correct me, but if the 2nd to 3rd gear shift is your problem, this is deinfitely a good cheap fix to try. I don't believe the modulator has any effect on the 1st to second gear shift.
The modulator on a TH-400 is located on the passenger side of the transmission and is easily identifiable by the vacuum line (presumably) running down into it from the factory it was typically a combination of specifically bent steel line going from the passenger side of the intake manifold probably with a rubber connection up top down the back of the passenger side of the engine where it terminates connecting to another rubber line connecting it to the transmission modulator.
Get the part, which should come with (or you should get) a replacement O ring to seal it against the transmission on the inside. Then just make sure you've got a sound vacuum connection running from where ever it starts on your intake down to the modulator. If the shifting doesn't get better, at least you've ruled out the modulator for not much money.
If the transmission modulator is not working right, replacements are available and the job is not bad at all. This modulator has one of the rubber diaphragms inside which moves every time the car gets driven. So they do need periodic checks or replacements. If I recall they're not expensive either. Like cheap enough to do as a test if the car is not shifting right. Others here can correct me, but if the 2nd to 3rd gear shift is your problem, this is deinfitely a good cheap fix to try. I don't believe the modulator has any effect on the 1st to second gear shift.
The modulator on a TH-400 is located on the passenger side of the transmission and is easily identifiable by the vacuum line (presumably) running down into it from the factory it was typically a combination of specifically bent steel line going from the passenger side of the intake manifold probably with a rubber connection up top down the back of the passenger side of the engine where it terminates connecting to another rubber line connecting it to the transmission modulator.
Get the part, which should come with (or you should get) a replacement O ring to seal it against the transmission on the inside. Then just make sure you've got a sound vacuum connection running from where ever it starts on your intake down to the modulator. If the shifting doesn't get better, at least you've ruled out the modulator for not much money.
Hope that helps.
Chris
Good write-up, Chris. I'll mention there exist Automatic Transmission Modulators (ATM) which are adjustable, as well. I'm not privy to which manufacturers; yet, I've noted several individuals who have installed them. I think you can simply turn them CCW &/or CW to increase or reduce shift point(s). I've replaced a couple (perhaps several) but never installed an adjustable type ATM.
