2003 Mercury Pro XS Engine nearly goes dead when running and accessing trim

[geno368]

I am having some electrical problems I am working on. When I am idling and try to trim up or down, the engine looses about half of it RPM's and when I am running it really cuts the power about half...How should I troubleshoot?
Thanks in advance.
Serial # 1B963709

[trivialpursuits]

Start simple -- check battery health, terminals/connections, and cables. Trim draws a few amps so it could highlight shortcomings in the main electrical supply.

[EuropeanAM]

1B963709 was actually built October 1, 2012, and is a 2.5L 175 ProXS Optimax. I'll go ahead and move the thread to the 2.5L Forum for you.

Good suggestions above. Load test the battery and make sure you have AT LEAST 950 CCA available to you.

Verify clean connections, ENGINE CABLES FIRST, remaining lugs Largest to Smallest, and finish off the connection with a Stainless Nut or Locknut (do NOT use wing nuts).

Poor connections or improperly stacked connections are one of the leading causes of what you described.

[geno368]

Thanks for the reply...yes, I was thinking about my old efi I used to have...it is a 2012

[geno368]

I have replaced both of the cable terminals with top post direct connectors eliminating wing nuts. I did a load test on the battery and it showed good although it is a new Walmart 600 cca...The motor has always been a slow turnover since I got it but I was told the pro xs sounds a little different when cranking..
I was told by a local boat mechanic that it was possible that the wire from the alternator that has a 50 A fuse in it may be bad causing the electrical system to be running on battery voltage only. I really don't know where that wire is....anybody know?
Thank in advance...

[EuropeanAM]

It doesn't have "a fuse in it"- it's actually a fusible link LEAD or wire. The large-red output terminal wire coming from the alternator.

Very simple to check- with the engine running, check the voltage at the battery, AND at the red output terminal of the alternator. If the fusible link lead is blown, battery voltage will be lower than expected, and voltage at the output terminal will be HIGH (well above 15 volts).

You need a better battery. For a fishing application- you really should be running something with at least (minimum) 950 CCA.

[SilverFox579]

^^^^

Your battery doesn't meet engine only minimum cranking required 800CCA. See this FAQ topic for top recommendations - "BATTERIES THAT MEET THE REQUIREMENTS"

[geno368]

Thanks for the reply....I will check the link as well as replace the battery....any suggestions on a reasonably priced battery for my application?

[geno368]

That's the info I needed...I will update my battery...Thanks Silver Fox

[EuropeanAM]

Please take a few moments to look through the post referenced above. The information is outlined in a very clear format, with numerous options for you to choose from (even includes a "budget friendly" list).

[SilverFox579]

I like the peace of mind the biggest, baddest, gp31 AGMs provide, may, or may not, be your preference. I'm currently running the Northstar AGM31M.

[geno368]

Thanks again for the replies...I have now replaced my battery with a 950 CCA marine battery and also replaced both battery cables from the battery to the motor. My cranking problem seems now gone. My only issue is the power trim affecting my engine rpm's badly....how do I check how many amps the trim is pulling?

[EuropeanAM]

You can use an "amp clamp" that slides over one of the trim wires (outside of it), usually combined with a DVOM.

When you say "affecting my engine rpm's badly"... can you be more specific or give a good example?

[geno368]

When I am idling and try to trim up or down, the engine looses about half of it RPM's and when I am running it really cuts the power about half...I was running about 3800 rpm yesterday and trimmed it up and it dropped to about 3000....at idle it is reallt noticeable...I have a multimeter but am not sure which wire to use and what setting on my meter for amps...

[EuropeanAM]

Since idle is PCM controlled at approximately 565 RPM's, it's probably not really dropping to 285 RPM's (as it would have died well before that).

It is not uncommon to note a drop in engine RPM's when trimming (especially where the battery may be partially or heavily discharged). It should not, however, be anything nearly as significant as you are describing (50-150 RPM's, depending on throttle setting and battery condition would be expected).

Poor grounds on the electrical plate below the PCM (including the plate itself), or poor ground strap connections for the alternator are two common areas for problems.

[geno368]

Would it be a possibility that the trim motor is going bad?..I acts normal when I am trailering and trimming up or down.

[EuropeanAM]

Possible: Yes. Usually detectable with an amperage clamp.

Think of it this way. Your computer monitor flickers on and off. Do you (first):

A. Go to the Electronics Store and buy a replacement

B. Check the connections at and for the monitor to make sure nothing is loose?


Also should note that proper stacking order of the lugs at the BATTERY are extremely important (engine cables FIRST). And, if you have a battery switch, bypass it (for testing purposes) to see if the problem remains or ceases.

[geno368]

As I stated above, my battery and cables to the motor were replaced by a retired marine mechanic and I checked over his work and all seems good. Engine cables are first...can I check amp draw at the battery cable or somewhere else?

[EuropeanAM]

To test trim motor amperage, you need to test with the clamp over ONE of the trim motor leads.

[geno368]

Thanks for the replies...I don't have a amp clamp, so I am waiting for scheduling at our boat dealership...