As seen in Speedboat Magazine’s
June 2024 Issue.
502 Mag to 525 EFI
Dear Bob:
I have a 2000 29 Fountain with a Mercuiser 502 Magnum that has been damaged by being frozen. I am getting ready to swap the motor and install a 2004 Mercury Racing 525EFI that I purchased which has been recently rebuilt. What concerns should I have with this motor swap? Is the wiring harness and mounting the same for both motors? Please provide me with any information you think would be helpful. Thanks!
Charles Miller
Sandy, Utah
This should be a very easy upgrade to your boat. The 2004 model of the 525EFI uses the same 10 pin wire harness connection as your 502 Magnum. All the senders and warning systems will work with your current gauges and engine warning alarm. Later model 525EFI engines are equipped with the 14 pin connector. If your engine has the 14 pin connector, you can purchase a harness adapter from Mercury Marine. If the adapter is used, it is necessary to run a separate dedicated 10 gauge power supply wire to the helm for all accessories because the hot wire in the 14 pin system is not capable of supplying the boat’s power needs. The earlier model 525EFI was equipped with an Analog Gauge Interface (AGI) which allows the use of your current gauges without any modifications. The later model 525 with the 14 pin harness is not equipped with the AGI. One would need to be added to the harness for analog gauges to function correctly.
The 525EFI is traditionally supplied with the plate style motor mount. You can use this mount by installing “L” angles vertically to your stringers that are in turn bolted to the engine plate mount. Your Fountain may have had the same style plate mount used at the factory to install the 502 Magnum. If so, it might be better to use the mount off that engine for the 525EFI. If the 502 Magnum has feet mounts, you can also remove the plate mount from the 525EFI and install the feet mounts off your 502 Magnum. It is better to solid mount the higher horsepower engine instead of having the rubber feet if so equipped. You can either install new “L” angles that are higher on the stringers or purchase after-market feet that are solid replacements. It will be easier for you to use some type of adjustable mounts when aligning your engine. It is imperative that you use an alignment tool to adjust the motor height during installation.
1. Early design Mercury Racing 525 EFI Harness with 10-pin plug and AGI.
2. Late-model Mercury Racing 525 EFI with 14-pin plug.
3. Mercury Racing 525 EFI Header without tailpipe.
4. TCM ST450 Muffler Kit for CMI Headers.
If your 502 Magnum has the standard exhaust system (without riser spacers) and the exhaust tip holes in the transom are in the “standard” locations, the CMI “sweeper” headers and standard tailpipes should fit to the same dimensions. If the exhaust holes in the transom are not in the standard location, the tailpipes will have to be built or modified.
If your boat is currently equipped with switchable exhaust such as Silent Choice or Captain’s Call, I would delete any switchable system that requires the water jacketed tail pipe to be shorter, and remove the “bull horn” that routes the exhaust through the propeller hub in order to prevent possible exhaust water reversion that is sometimes an issue with the more aggressive cam profile. A block off plate will then be needed to cover the exhaust hole on the inner transom. If you desire to have a switchable muffler system, the TCM ST450 switchable tips could be added. Alternatively, several types of full time mufflers are available that are not too restrictive for the higher horsepower engines.
Your 502 Magnum package came with a Bravo One outdrive, which may be OK if you are not an abusive operator. The 525EFI package came with an XR drive. Upgrading to an XR or similar aftermarket drive, such as an IMCO SC or a Teague Platinum drive, might be something to consider.
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Check out the Teague Tech Library for diagrams and schematics of how to correctly install and perform various repair and upgrade projects. At Teague Custom Marine, we don’t ‘just sell parts’, we know how to install everything we sell. When it’s time to tackle a project, count on Teague Custom Marine to have the part and the answer.