How to troubleshoot my speedometer?

My speedometer is dead, all the wiring appears to be intact on the back, is there a step by step troubleshooting plan anyone knows of?

Re: How to troubleshoot my speedometer?

Check to see if the pitot tube is clogged. Remove if from the lower unit or (pitot tube assembly) and blow thru it to see if the gauge moves.

Re: How to troubleshoot my speedometer?

No mention of the engine/outdrive.... Most speedometer gauges have only a power and ground connection going to them for lighting purposes. The input to the gauge which provides the "signal" for the indication is a rubber hose. The other end of the hose is connected either to a pitot tube mounted on the transom or connected to the outdrive where there is a small hole in the leading edge.

Basically the principle is that water pressure if forced into the small hole of the pitot tube or outdrive. The water puts pressure on the air inside the rubber hose which is connected to the gauge and deflects the gauge needle for indication. The faster you go, the more pressure and the more the needle is deflected on the gauge. Clog or disconnect the pickup and the gauge indication stops.

For troubleshooting, find the speedometer pickup. Is the pitot tube flipped up where no water can enter the pickup hole? Has debris clogged the pickup on the front of the outdrive? Was the rubber hose which connects to the front of the outdrive been left disconnected after the outdrive was serviced/winterized? Any water puddles collecting at the helm while you are underway?

Re: How to troubleshoot my speedometer?

Okay..... Follow-up. I found the pitot tube and it runs into the back of the outdrive in the engine compartment. Is there a diagram available for the mechanism, so I can troubleshoot it, I blow into it and the guage moves, I wonder if it is primed over or something.
Addendum, should I just run a new sending unit since they are like 5 bucks? just disconnect the existing and re-run the tubing? Ideas, thoughts?

Re: How to troubleshoot my speedometer?

There most likely will be NO diagrams. What outdrive do you have??? Is there a speedometer pickup mounted on the transom??? By blowing into the pitot hose and the gauge indication changes you know the gauge is good and the hose is connected to the gauge. Where did you find the unconnected end of the pitot tube that you blew into?

Re: How to troubleshoot my speedometer?

I disconnected it from the inside of the outdrive, it's an alpha 1 on a 1999 3.0L mercruiser

Re: How to troubleshoot my speedometer?

More than likely you just have gunk/dirt/paint clogging the hole. I have cleaned out many of them with a wire coat hanger.

Re: How to troubleshoot my speedometer?

I just unclogged mine yesterday. Alot of junk in the lake do to high runoff in colorado. I took a piece of 3/8" rubber tubing put it on the end of a blow gun on my air hose held the other end tight to the hole on my outdrive and blew it out. Blew it out from both ends. I also disconnected tubing from speedo and blew all the water out of the tubing. It works fine know.