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Old 04-25-2008, 06:13 AM
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Trevor Trevor is offline
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Transmission Line Pressure Testing.

Transmission Line Pressure Testing.

Some notes for consideration.

I have been involved here with technical matters for several years and it has amazed me that transmission line pressure has been referred to so little in regard to transmission problems, particularly their diagnosis. It is strange that the quite simple equipment required, is not mentioned as being in the hands of, and regularly used by all involved with transmissions.

Most when investigating an electrical problem, would immediately check on the availability of adequate battery power. The important functions within an automatic transmission, rely on hydraulic power. Surely it is sensible to confirm that such is on tap, as a first consideration.

The manual states --- “If clutch or brake band shows signs of slippage, or a shifting sensation is not correct, the line pressure should be checked.” If this was carried out as a diagnostic procedure, prior to a specialist becoming involved, I am confident that much less needlessly spent money would change hands. Prior knowledge can be valuable.

Both the transmission proper, as well as the four wheel drive transfer section, can be tested separately. A testing outlet screw connection is available for each. Both systems rely on a pulse modulated solenoid valve for pressure regulation.

Main line pressure is controlled/regulated via solenoid valve “A”. This valve in particular, is subject to very arduous duty as it is cycled some fifty times per second. It is known to be commonly the first to fail. The valve seat can become worn and leak, or become contaminated, both causing a loss of pressure.

Unfortunately, although the diagnostic system can record an electrical malfunction, or a jammed solenoid, a faulty seat will not result in a signal. This remains an important anomaly. A physical test requires no more than a pressure gauge, connecting nipple and flexible tube. Pressures run at below 200 PSI, or say 2,000 kPa.

Transfer system pressure involves solenoid valve “C”, which operates in the same fashion, but has a much less arduous duty cycle. Here pressures of less than 125 PSI, or say 800 kPa are involved.

All of this is covered within the manuals, transmission section three, performance test. It would be greatly appreciated if someone with US manuals, could scan and publish here the relevant pages covering line pressure testing. It is possible that detail may differ from the manuals I have.

There could well be someone with the sort of interest required, to set up a gauge or gauges, so as to observe and record typical running pressures. This could prove to be an interesting and valuable experiment. Phil and Nomake are already investigating the control signals involved, and such a project would usefully complement their findings. Any takers?
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As a child, on cold mornings I gladly stood in cowpats to warm my bare feet, but I detest bull$hit!
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