Need an opinion on a gauge/electrical issue on a 98 Sunsetter LX.

[H2oskiphil]

Fuel gauge has always been flaky in terms of being anywhere close to the actual amount of fuel in the tank, but when I fired the boat up for the first time this year, with a full tank, it reads just above E. Needle vibrates when I turn the key on like it should, but it doesn't climb. Tested the voltage at the red and the yellow wires on the plug to the gauge and I'm getting just under 6 volts. With a full tank of fuel, resistance across the fuel level sender is about 250 ohms, which seems to be at least in the ballpark of where it should be. Speedo doesn't work either, and I confirmed at least at the pickup end that the pickup is clear-the speedo tends to clog easy on the Mississippi, so that may or may not be related. HAven't tested the speedo beyond that. All the other gauges (oil pressure, temp, voltage, and tach) seem to be working fine.

 

Is there any way to test the MDC-1600 to see if it's faulty before ordering one (or trying to find one used)? If so specifically what should I test to figure out why the voltage to the fuel gauge seems to be so low?

 

Also, if anyone has a MDC-1600 they're looking to sell, let me know. Found an ebay listing for one that had expired last week from a 99 Sunsetter LXi, and the guy threw it away because it didn't get any bids at $50....

[justgary]

Clean the fuel tank ground at the tank and at the engine.  It should terminate at the rear of one of the engine heads.

[drh]

@H2oskiphil Full tank should read 33 ohms not 250. That's an empty tank reading. My bet is that your float has become fuel logged and doesn't float anymore - that was my problem. You'll need to pull the sender and manually move the arm to see how the gauge responds. Here are more details on the MDC troubleshooting guide. You can also test resistance at the 18 pin connector on the MDC as described here:

https://cdn.bakesonline.com/media/resourcelibrary/MedallionII_Troubleshoot.pdf

[electricjohn]

MMDC only sends an intrinsically safe 5 volts DC to the fuel sender, not 12 volts, getting in return a 4 to 20 milliamp signal. Same goes for the oil pressure and water temperature sender.

[H2oskiphil]

6 hours ago, drh said:

@H2oskiphil Full tank should read 33 ohms not 250. That's an empty tank reading. My bet is that your float has become fuel logged and doesn't float anymore - that was my problem. You'll need to pull the sender and manually move the arm to see how the gauge responds. Here are more details on the MDC troubleshooting guide. You can also test resistance at the 18 pin connector on the MDC as described here:

https://cdn.bakesonline.com/media/resourcelibrary/MedallionII_Troubleshoot.pdf

I've got a replacement sender.....I assume that if I wire it up and lift the float arm by hand I should see the gauge move correctly if the sender is the issue, right?

[drh]

Correct...the gas gauge will have a delay in it's movement FYI.

[H2oskiphil]

Appreciate the feedback on this. Hooked up the replacement sender and the gauge worked again, so swapped the senders out last night and everything seems to be working! I'm a little concerned about the accuracy (the boat was full when I put it in the water 2 weeks ago, but we did an hour-long idle cruise and a ski run since then, and it's still showing completely full), but we'll get that part dialed in.

Now if I can just get the analog speedo working......it worked last fall, but it's deader than a doornail this year. Checked the pickup and cleared it out as best as I can, but it's still not doing anything.

[electricjohn]

Most importantly, you want it show empty when empty.

[justgary]

3 hours ago, H2oskiphil said:

Now if I can just get the analog speedo working

Try swapping the plug with any other working gauge to make sure the harness is good.

[H2oskiphil]

The previous owners replaced the right analog speedo with a Perfect Pass. I'm pretty sure the wiring plug that originally went to the right speedo is still hanging from the harness in the dash....I may try plugging that one into the left speedo just to see if it works. If it doesn't, then it's probably the speedo head, and if it does, it's probably the left side pickup, correct?

[justgary]

1 hour ago, H2oskiphil said:

The previous owners replaced the right analog speedo with a Perfect Pass. I'm pretty sure the wiring plug that originally went to the right speedo is still hanging from the harness in the dash....I may try plugging that one into the left speedo just to see if it works. If it doesn't, then it's probably the speedo head, and if it does, it's probably the left side pickup, correct?

It doesn't matter which plug from the harness you try; they all have the same digital signals on them.  Mine only had one speedometer and a 1-2 switch on the dash.  I replaced that with PP and kept the big tachometer.  I had thought that '98 and '99 were very similar.

Your issue may be inside the MDC box.  You can take it apart and resolder the board to possibly bring it back to life.