Hi my name is ***** ***** I will be happy to assist with this today.In some cases I may need more info to assist better.I apologize for any delay.
I assume you may have meant a new transmission needed and not an engine.If I am wrong let me know.
If the fluid level is good in the transmission one way to check the pump operation is when this occurs with the engine off the fluid level on the transmission dipstick should be higher with the engine off then it is with the engine running.If the fluid level is the same height or very close with the engine off and on then the pump is usually the cause.
The other issue I have seen is a fault with the pressure control/regulating solenoid in the transmission valve body or the valve body itself.You may not need a complete new transmission here to correct this.There is a chance a clutch is burned or wore out needing replaced inside the automatic transmission but this can be replaced/repaired instead of a complete replacement.
I would have this checked into/tested further before having a complete one installed to see if it can be repaired.I suspect either the pump,pressure control solenoid,valve body or clutch failure internally but all of these can be repaired at a less cost then a replacement transmission,unless you go with a used transmission as this can be cheaper in some cases.
I can say however if the pump is the cause this can lead to several components in the transmission becoming damaged needing repaired or replaced and in this case it is cheaper to replace the entire transmission.If the transmission mechanic isnt able to diagnose this and wants to replace the entire transmission it would be a good idea to have another hands on diagnosis to be sure of what is causing this here as this could save you some money.
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