Reads manifold vacuum at the modulator line on TH400, TH350, C4 and C6 transmissions so you can tell a bad modulator from a real vacuum leak.
If your Turbo 350 or Turbo 400 shifts late and hard, slips into gears softly, or is losing fluid with white smoke out the tailpipe, the vacuum modulator is the first suspect. Before you buy a modulator or tear into a valve body, find out what the transmission is actually being fed. The OTC 5613 kit reads manifold vacuum right at the modulator line, so you can tell a ruptured diaphragm from a cracked hose, a big-cam engine with weak idle vacuum, or a problem inside the case.
The modulator sits at the right rear of the TH350 case. Inside is a diaphragm and one spring: the spring pushes the modulator valve toward higher pressure, and engine vacuum pulls the other way. High vacuum at light throttle means low modulator pressure, soft line pressure and early shifts; low vacuum under load means high pressure and later, firmer shifts. That hydraulic signal acts on the boost valve and raises line pressure, so a bad vacuum signal changes how hard every clutch and band is held. The TH400, C4 and C6 use the same idea, which is why one gauge covers all of them.
Warm the engine, pull the hose off the modulator nipple and look inside it for ATF. Connect the gauge to the hose with the supplied barbed connector, or tee it into the line, and read idle vacuum. You want a strong, steady reading; a needle that sags or flutters points to a cracked line, a leaking source fitting or an engine that simply does not make much vacuum. Then use a hand vacuum pump on the modulator itself and confirm it holds. The full walk-through is in our TH350 common problems guide, and if vacuum is good but engagement is still slow, move on to the delayed engagement guide.
One detail trips people up during a line pressure test: GM lists the connected test at a manifold pressure in inches of mercury absolute, not inches of vacuum. At sea level, 12 in. Hg absolute is roughly 18 in. Hg on a vacuum gauge like this one. The 5613 reads 0 to 15 psi on the pressure side, so it is not a line pressure gauge; you still need a high-range gauge on the case tap for that test. The 5613 is what tells you the vacuum half of the test is right.
The gauge works on any vacuum-modulated automatic, including the TH350, TH400, Ford C4 and C6, plus general engine vacuum and low-pressure fuel testing. It has a 3-1/2 inch face with a chrome bezel and rubber boot, scales of 0-30 in. Hg and 0-70 cm Hg vacuum and 0-15 psi and 0-100 kPa pressure, a 24 inch hose, an in-line barbed connector, a tee, a conical nozzle adapter and a universal fuel line adapter, all in a hard case with a reading chart under the lid. See the full TH350 parts collection for what you may need after the diagnosis.
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Doing this repair right usually means replacing these at the same time. Same transmission, ships together.
No. The pressure scale reads only 0 to 15 psi. TH350 line pressure runs well over 100 psi, so a line pressure test needs a separate high-range gauge on the case tap. Use the 5613 to confirm the vacuum signal during that test.
A healthy engine at idle should show strong, steady manifold vacuum at the modulator hose. The exact number depends on the engine and cam. What matters most is that the reading at the modulator matches the reading at the manifold and does not flutter or bleed off.
Pull the hose off the modulator nipple. ATF in the hose or in the nipple means the diaphragm has failed and engine vacuum is pulling fluid into the intake. Low fluid with no leak and white smoke support it.
Yes. All of them use a vacuum modulator, and the gauge simply reads vacuum at the modulator line, so the same test applies.
GM specifies manifold absolute pressure, measured from a perfect vacuum, while a vacuum gauge reads down from local atmosphere. At sea level 12 in. Hg absolute is about 18 in. Hg on this gauge, and the conversion changes with altitude.
