Cobra trim position sending unit

[Fix'em]

I've got a question about the trim position sending unit on my old 88 Thundercraft stern drive. My trim gauge seems to not work at all, so last night I started to investigate.

I removed the 2 small bolts that secure the trim sending unit to the gimbal housing and pulled it out to test the resistance. With an ohm meter at the plug inside the engine compartment, I get readings from 0 to 198 ohms, which according to the manual is perfect.

However, when I went to re-install it, I got confused. On the trim sending unit is a plastic arm which appears that it should be hooked up to or at least rubbing on something to direct the arm up and down as the drive moves, yet, when I re-install it, there is absolutely NOTHING touching it. Should there be a linkage between this plastic arm and the drive, or should it touch the drive and be directed up and down? I must be missing a part or a cable or something. Maybe this is why the gauge does not work. It is up in the red zone all the time.

 

[HT32BSX115]

Re: Cobra trim position sending unit

Howdy,

We're talking about part numbers 86-88 in the upper right of the pic below...

That "sender" is just a simple variable resistor with one side connected to ground and the other side going to the gage.

The gage has it's other terminal connected to 12v.

It's been a while since I had that thing apart but #88 should be contacting the pivot housing. Look for a spring (#89) [did you lose it?]. That would keep the arm against the pivot housing.

You might need a flashlight and a little mirror to see in there.... I sold my Cobra gimbal so I can't go out there and take a picture of the trim sender....


Regards,


Rick

 

[Fix'em]

Re: Cobra trim position sending unit

Hi Rick.

That's exactly the assembly. #86-88. Mine is still all intact with the spring and all and the resistance variance is good. It's just that the arm doesn't touch the pivot housing at all. It's almost a full inch away from touching anything. I find this odd. I can't mount it wrong. There are only 2 bolt holes and it can't go in backwards.

 

[HT32BSX115]

Re: Cobra trim position sending unit

I seem to remember the control arm actuated well against the spring when it was installed. That would mean that you would have to move the arm all the way against the spring as you put it together.

 

[Fix'em]

Re: Cobra trim position sending unit

The spring on this particular model is mounted behind the arm and locks into the sender housing so that the arm, in theory, applies pressure to the drive housing. My spring is in place and the arm has good tension and movement.

Maybe there is some kind of trick to installing it that I am not aware of. I'll try again when I get home today. It was getting dark and I was working with a light outside last night when I gave up.

 

[Fix'em]

Re: Cobra trim position sending unit

The two screws you are talking about should be sitting in slotted holes that allow you to adjust the sendors position.

Agreed. There appears to be about a 1/4 in of adjustment in both screw holes. But even at the max, the arm just doesnt touch anything.

I'm thinking maybe I have to push the arm up so it has tension on it and points up before I attempt to install it or something. There has to be a trick to it. LOL.

 

[Fix'em]

Re: Cobra trim position sending unit

Yes, the outdrive is installed, although I did have it off recently to rebuild the engine.

Perhaps I messed something up when I re-installed it?

 

[Fix'em]

Re: Cobra trim position sending unit

Exactly. The travel is limited by a plastic "tab" on the housing. It only has about 90deg. of alloted movement.

I'll take some pictures tonight if I can't seem to figure it out. I am going nuts trying to think of what I missed.

 

[Fix'em]

Re: Cobra trim position sending unit

Ok. So, I figured it out. Wow. I feel a little less intelligent after this one.

I re-installed the trim sending unit and still it was nowhere near touching the drive housing, so I decided to lower the drive and sure enough it contacted the switch and it started to show properly on the gauge.

Here is where I think I went wrong. I was working with the drive up approaching trailering height. I assumed that at that height that this is where the gauge would indicate the red zone. However, it turns out the gauge only indicates what I now assume is the safe operating zone. As soon as the drive is tilted up more than about 15-20 degrees, the needle is pegged at the end of the red zone. Is this normal? I still need to fine-tune the sending unit with someone in the boat watching the gauge for me following the procedure in the manual, but I don't think that will make that much of a difference.

 

[HT32BSX115]

Re: Cobra trim position sending unit

Some gages will indicate up to the point where you should not be running it.


I would certainly not want to run the engine when the tilt angle is more than the drive will turn left and right. You run the risk of damaging a U-joint. You don't want to know what happens in the gimbal when a U-joints breaks.

 

[Fix'em]

Re: Cobra trim position sending unit

Some gages will indicate up to the point where you should not be running it.


I would certainly not want to run the engine when the tilt angle is more than the drive will turn left and right. You run the risk of damaging a U-joint. You don't want to know what happens in the gimbal when a U-joints breaks.

Well, that's what I assumed the red zone on the gauge was for, but it's not even close. At full down, let's say the drive is at - 5deg. angle to the transom. Well, by the time I'm at maybe + 10 deg. up, the gauge is almost maxed out. Heck, it's indicating half way through the white area when the drive is straight up and down parallel to the transom.

 

[VMAX]

Re: Cobra trim position sending unit

My 3.0 Cobra is the same way. The gauge is a good indicator of the drive being
"full in", and gives a good reference point once I'm on plane, so I don't need to start porpoising to find the sweet spot. I was in the same position as you are when I first installed it. Not much value for all the trouble.

 

[Fix'em]

Re: Cobra trim position sending unit

Ya, I've been asking around to those who are more familiar with boats than I and they all tell me the gauge is simply an indication of the where the drive is in the optimum operating area. When the gauge gets to the red, the boat will most likely start to porpoise up and down, indicating I'm beyond optimum angle. Makes sense. It's good for more of a "fine-tuning" then to indicate where the drive is in relation to it's total travel.

 

[rbh]

Re: Cobra trim position sending unit

try alittle white out on the gauge to mark were you want to be. it can be scratched off if things change.
rob