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Discussion Starter · #1 ·
My son's 68 Mustang, 200 with C4, was running when it lost all gears. No noise, just doesn't move. We had another C4 so my son tried swapping out the pump, no change. Then tried swapping the valve bodies. Still no change, no gears at all. I found out how to test the transmission fluid pressure and bought a 0-300 psi gauge. Absolutely nothing, gauge did not move no matter what gear was selected. From that I would assume that the pump was bad. Is there any other reason why the pump would not pick up any fluid and at least build some pressure?
 

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Discussion Starter · #3 ·
The trans in the 68 had been rebuilt before we bought it. The previous owner had it rebuilt on his drive home. The internals looked good so we just thought we would try swapping the pumps. That didn't work so my son tried swapping the valve bodies since that could be tried easily. Just drop the pan and swap them.

My next thought is to rebuild the pump, but I was wondering if there was anything else that might keep the pump from making pressure.
 

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Air in the oil circuits will cause no pressure. However, since you presumably checked the oil level both before and after the VB swap, that cause is not likely, unless both filters are plugged.

When pumps go bad, they are typically not rebuildable unless you have special equipment to do the job.

When you fix your transmission, be sure to use the VB with the original transmission. There are over 40 different basic kinds of C4 VBs, and many of them do not interchange with each other. If you happen to swap in an incompatible VB, the transmission may not shift properly.
 

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No pressure at all is a pump issue . The test port is between the pump and the vavebody. If the pump is turning it should show at least some pressure .
I'd say you have 2 different problems
1. Loss of drive due to a trans issue
2. Bad install of the pump possible broken gears . Not many other reasons why you have no pressure at all..

Time to go right in and sort whats wrong. I'd then go back and use the original pump and V.B if you can .
 

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Are the drive lugs definately driving the pump? Seems odd to have 2 bad pumps. Was the same filter used when you swapped VB's? As mentioned a blocked filter would give the same results- no suction = no pressure.

Tony
 

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Not to hijack, but, what is/are causes for pump failure? gear wear or the housing the gears ride in, or both? Anyway to check a pump or pump components (measureing or otherwise) to verify they are good or bad before assembly?
 

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Discussion Starter · #9 · (Edited)
Go ahead, it's not really a hijack because I have the same question. I want to reinstall the original pump and VB now that I have the gauge and see how they perform. But if possible I would like to check the pump before reinstalling.

Also, used a new filter when we swapped VB.
 

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Not to hijack, but, what is/are causes for pump failure? gear wear or the housing the gears ride in, or both? Anyway to check a pump or pump components (measureing or otherwise) to verify they are good or bad before assembly?
Most common reason for pump failure is incorrect installation of torque converter.

Other than that, the gears, which are the only moving parts in the pump, wear out. Since they cannot go bad without damaging either or both the pump body or stator support, one or both of them are also bad.

Externally, the stator shaft (cast iron) is easy to break.

Internally inside the transmission both the direct and forward drums ride on the drum tower on the back side of the stator plate, and either of those drums can cause damage to the drum tower. Finally the thrust washers/bearings between the pump and drums can wear out causing damage to the pump.

Although possible, the bushings for the input shaft in the pump bore can go bad, but I have never seen that problem.

Eyeball the wear surfaces of the pump. If they are scored (if you feel anything when you run your fingernail across them) the pump needs to be replaced.
 

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Discussion Starter · #11 ·
OK, an update. Found the reason for no pressure. Pump had a broken gear. Put another used pump we had in the trans. Now shows 120-150 psi. You can see the pressure drop and come right back up as we shift thru the gears. Still have no movement at all in any gears. Any ideas on what to check next?

Note when the trans initially failed it had given us no inclination that there were any issues. Just lost all gears, no noise, just quit moving.
 

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Bad torque convertor, broken input shaft or striped splines on input shaft or in front drum.
 

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OK, an update. Found the reason for no pressure. Pump had a broken gear. Put another used pump we had in the trans. Now shows 120-150 psi. You can see the pressure drop and come right back up as we shift thru the gears. Still have no movement at all in any gears. Any ideas on what to check next?

Note when the trans initially failed it had given us no inclination that there were any issues. Just lost all gears, no noise, just quit moving.
Did you re-use your old input shaft? If not, was the replacement shaft EXACTLY the same as your old one? Using the wrong shaft will mean no motion past the pump.
 

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Discussion Starter · #15 ·
Well, we had it working. Pressures seemed good at 150 to 180 psi but it was acting like the vacuum modulator was bad. Did not want to shift on it's own. The vacuum hose looked suspect as it was collapsing so we got some new hose as well as a new modulator. Topped the fluid off since we lost some installing the modulator. Seemed to solve the shifting problem. The car was driven about 10 miles and now seemed to have problems again. Checked the gauge and pressures had dropped. Maybe 90 psi initially, increase the rps and pressure would go up some but the gauge would be bouncing all over the place from 30 to 150psi. Drop back to idle and it was around 30-40 psi. What could cause it to drop like that? Started it up to put it back in the garage and it had enough pressure to limp into the garage without using much throttle. Let it set for a while and restarted it. Fluid level was ok and pressure was 120 psi. Gave it some gas and then looked at the pressure and it had dropped back to 40psi.


Any ideas?
 

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Discussion Starter · #16 ·
I have been reading other posts and wonder if it might be in the valve body? If the pressue regulator is sticking would that explain why the pressure drops so much when you drop the rpm back to idle?

Thanks
Jimmy
 

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If you broke the pump it may have sent metal into valve body. Take it a part and clean.
 
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