1989 Mercury BlackMax 135HP Questions

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Any Mercury experts out there that can tell me what this wire is and module on the back of the outboard? It is disconnected and I do not know what it is used for.

 

[Figz]

Does anybody out there have any head schematics that they can provide me with?

 

[Figz]

Is this the trim sensor?

I’m looking at my wiring diagrams and a wire that is brown with a white stripe seems to be the trim sensor wire. I’m trying to figure out how to get my trim gauge to work, but I’ve tested this wire with a tester and there’s no power going through it when I move the trim up and down.

 

[Figz]

Now that the boat has been out a couple times, some of the gauges are either failing or are not working. The boat HAS been sitting for 26 years, could the gauges just be going bad from putting them back in service after being idle for so long?

A new temperature gauge was purchased and it is working. However, the RPM gauge was working and is now not working after the temperature gauge replacement. The tilt trim gauge is still not working at all.

And for the fuel gauge, the sender in the tank is bad on the inside so I have a replacement on order.

What are y’all’s thoughts on any of the gauges? Specifically RPM?

 

[jumbofordman]

First pic is the trim sensor.

I would just replace all the gauges with a matching set. Get a GPS speedo and never worry about a pitot tube again, plus it'll actually be accurate.

 

[Figz]

First pic is the trim sensor.

I would just replace all the gauges with a matching set. Get a GPS speedo and never worry about a pitot tube again, plus it'll actually be accurate.

Thanks for the reply!

Since the last update, I replaced the fuel sending unit in the tank and the fuel gauge is now reading properly. I have an RPM gauge on order and the trim gauge. I guess I might just need to get a new sensor. Are the sensors known to go bad?

Right now as gauges are going bad, I am replacing them with genuine NOS Mercury Quicksilver gauges.

The only gauges right now that are not Mercury Quicksilver are the fuel gauge and the speedometer.

I’ll send a picture of the gauges that I am getting from when I get home as I have the complete mercury book that came with my engine.

 

[Figz]

Gauge Update:

Working:
Temperature
Speedometer
Fuel

Not Working:
RPM
Trim

After receiving a NOS Mercury Quicksilver RPM gauge and installing it into the boat, on the first click of the key when the engine was cranking, the gauge was reading RPM. However, the gauge only worked for that split second and then it went right back to zero and showed no response while the boat was running or cranking.

At first, I thought that my wiring must’ve been bad so I rewired all the gauges. Rewiring the gauges did not change any outcome. During some point of my rewiring, the RPM needle fell below zero a significant amount, but then when I click the key to accessory, it, jumped back up to zero, and when I cranked the boat over the gauge jumped to RPM, but then died right back again.

Initially, the old RPM gauge is working before I install installed the NOS temperature gauge. Now that the temperature gauge has been installed the RPM gauge is having difficulty working properly. I have not yet tried to run the RPM gauge without the temperature gauge hooked up or just the RPM gauge by itself. I might try to do that next.

Does anybody have any ideas of what could be causing this intermittent RPM gauge?

 

[jumbofordman]

Gauge Update:

Working:
Temperature
Speedometer
Fuel

Not Working:
RPM
Trim

After receiving a NOS Mercury Quicksilver RPM gauge and installing it into the boat, on the first click of the key when the engine was cranking, the gauge was reading RPM. However, the gauge only worked for that split second and then it went right back to zero and showed no response while the boat was running or cranking.

At first, I thought that my wiring must’ve been bad so I rewired all the gauges. Rewiring the gauges did not change any outcome. During some point of my rewiring, the RPM needle fell below zero a significant amount, but then when I click the key to accessory, it, jumped back up to zero, and when I cranked the boat over the gauge jumped to RPM, but then died right back again.

Initially, the old RPM gauge is working before I install installed the NOS temperature gauge. Now that the temperature gauge has been installed the RPM gauge is having difficulty working properly. I have not yet tried to run the RPM gauge without the temperature gauge hooked up or just the RPM gauge by itself. I might try to do that next.

Does anybody have any ideas of what could be causing this intermittent RPM gauge?

So the RPM is reading while cranking, but not while running. Sounds like a wire isn't connecting when the switch is in the run position, but it is in the start position. Time to start tracing the wiring diagrams for an Merc outboard and see what the difference is in the wiring path in both key positions.

I read your post again and I'm going to speculate something is wrong with the grounds on the gauges (one gauge is grounding through another one.)

 

[Figz]

So the RPM is reading while cranking, but not while running. Sounds like a wire isn't connecting when the switch is in the run position, but it is in the start position. Time to start tracing the wiring diagrams for an Merc outboard and see what the difference is in the wiring path in both key positions.

I read your post again and I'm going to speculate something is wrong with the grounds on the gauges (one gauge is grounding through another one.)

Thank you for the reply.

I have been troubleshooting this issue for about a month and a half now maybe more and it is very frustrating. At first, when I got my new gauges all the gauges were working except the trim gauge which was known to be inoperable now all the gauges have slowly died and no longer operate.

Even if I only hook up one of the gauges to the harness, it is still inoperable. Example is the temperature gauge. The only gauge that is working is the fuel gauge, which is on a separate circuit going from the battery to the tank.

I am beginning to suspect that it is probably an issue with the rectifier on the engine itself. As right now I am stumped of what the issue could be. I have wiring diagrams for the boat and I’ve checked them over and over and I’ve traced all the wires all they’re all grounded correctly. None of them are corroded.

The gauges light up when the key is working, so I know that the gauges are all getting power, but the actual senders on the gauges don’t seem to be working or giving any type of read like they used to. When I ground out the gauges, the needles move and when I put power to the senders, the needles move, but they just don’t seem to be working on the boat.

The part that gets me is that they all worked at one point and now they have slowly stopped working after about a month of use even though I’ve gotten a brand new temperature gauge and RPM gauge.

And yes, the RPM gauge during the first initial crank of the key, the needle jumps from zero to maybe about 100 or 200 RPM and then it falls back down to zero.

 

[Figz]

So the RPM is reading while cranking, but not while running. Sounds like a wire isn't connecting when the switch is in the run position, but it is in the start position. Time to start tracing the wiring diagrams for an Merc outboard and see what the difference is in the wiring path in both key positions.

I read your post again and I'm going to speculate something is wrong with the grounds on the gauges (one gauge is grounding through another one.)

All of the wiring on the back of the gauges are daisychained together and have one power and one ground going to them until they go back to the wiring harness, which supplies a ground and power. My wiring goes from the harness to the RPM gauge to the temperature gauge to the fuel gauge to the trim gauge to the speedometer, and then terminates there.