And like Turbo suggests, what happens to the volt readings above 4k and when switching on/off the trim switches?

BT :cool:

Jrock, make sure that when you make the blue wire hot on the pump that the drive goes up. That is the high pressure side. After that, you will need a pressure gauge to measure the UP pressure of the pump alone, and the pump when it is connected to the cylinders. You could have a cylinder that has a bad shock valve/piston in it, or a bad hi-pressure relief valve on the pump.

Volt meter changes and so does every other gauage when the trim switch is activated. But, only the trim switch, not the trailer switch.
Since we're on the subject of electrical, the audio alarm goes off when you hit a wave hard and fast. The tach needle gets buried when this happens. Once you let off, the alarm goes off and the tach goes back to the right rpm.

This is a 1992 26' seebold with a 502/465hp 650 hours on it.

Does the trailer switch work with the key on? I know you have a single, but it's possible that the trailer switch doesn't have power when with the key on (it's been done before, such as with the dual power trim control box from Quicksilver/Mercruiser) and then the trim limit switch breaks connection internally under thrust load. Try putting a jumper wire across the trim limit connections for the pump to disable trim limit.
Other Possibilities:
1) Wrong trim out line connection to the pump. I know you said you tried switching them, but it's the first thing I check when somebody says it won't trim out with thrust load. The trim "out" line connects to the left port on the pump.
2) Did you replace the trim pump assembly with the proper one? I believe that the standard tab trim pumps are set to lower max pressures than drive trim pumps.
3) Extreme length drive spacers. Spacers change the leverage on the trim cylinder increasing the pressure needed to trim out under load.
4) It's been mentioned before, but if you don't have enough voltage available to the pump, the electric motor doesn't have enough strength to overcome the pressure. Check connections, actual running voltage at the pump, etc.

Thanks for all your support.
We'll try some of the suggestions this weekend.

Found the problem. It was a broken ground wire. That was a very expensive wire. I now have a good used trim pump assembly for sale, cheap. Thanks for all your suggestions.

How was broken ground wire keeping trim from working only over 4K?

:confused:

The ground wire was likely not completely broken, or he was picking up a ground through another way. The more RPM's that are put on the engine the more load that is required to trim up and thus requires more power and your bad ground will cause more and more problems.

Jon