View Full Version : Next step in troubleshooting temp gauge with a mind of its own

Jeff d
06-29-2013, 03:01 PM
'00 Maristar 230 w/ LTR

My temp gauge has been all over the place for 2 seasons now. Last season it normally worked fine but would occasionally peg out at the max temperature for no apparent reason (usually on the first run of the day). Initially I'd freak out, shut it down and check everything out but the engine didn't appear to be abnormally high. I'd then start her back up and the temperature would read normal again.

This season it has normally just been at "0" (Or whatever the lowest reading is) pretty much all of the time. Sometimes I'll turn the key on with a stone cold engine and it will say 210+ degrees though.

I've pulled apart the dash and cleaned every connection for every gauge with Deoxit connection cleaner/lubricant per the recommendation of another forum member but that didn't help.

I've removed the 1 wire connection from the sender at the left rear of the engine and grounded it out and the gauge doesn't move so I think the sender is good and that points towards the gauge or MDC. Actually when I tested this the gauge decided it was going to point straight up at key on and never dropped off even with the wire disconnected.

Can I check resistance or voltage at the gauge or at the MDC? If so what values should I be looking for?

My fuel gauge also has a bit of a mind of its own which could be related if it's an MDC issue.

06-29-2013, 04:52 PM
Do you have this?


Jeff d
06-29-2013, 05:08 PM
Do you have this?


No, and I didn't realize that the gauges were interchangeable like that. Aparently i can just swap plugs with another gauge and see if the problem follows the connector or the gauge. Thanks.

Jeff d
06-29-2013, 07:09 PM
Well, the sender checks out according to that guide. It should be 300-1000 ohms when "cool" and it's at 600 ohms at around an 85 degree ambient temp. I ground the tan wire and the gauge doesn't move at all.

The gauge never "resets" as described in that guide either. It's supposed to "return to low and vibrate" at key on. It just picks a somewhat random point and vibrates.

I didn't pull the dash apart to swap the connection with another gauge as it's a pain to pull apart. I was able to reach up in there and unplug/replug the gauge but that didn't do any good. When I get time I'll pull the dash apart and do that because I want to confirm that it functions and that I don't have a bad MDC.

I'm not convinced that the MDC isn't failing though because the more I think about it I've got issues with 3-4 gauges. 1) The speedometer never worked since I bought it 3-4 years ago. I forgot about this because I replaced it with Perfect Pass 2) The fuel gauge is erratic. 3) The tach generally works fine but I do recall a couple of times when it dropped down to zero while running and then bounced back up. 4) Now the temp gauge issues.

Either the MDC is screwed up or I have a very high failure rate on these gauges. Does MC still sell gauges for boats this old? There's not anything on eBay that's anywhere close to what mine look like (chrome rings/off white background). Is there anything you can do or a service that offers repairs for these gauges?

06-29-2013, 07:18 PM
My '00 230VRS is in the same boat (pun intended). When I first bought it in 2008 the oil pressure gauge would go to zero intermittently. I replaced the sending unit and eventually the gauge was just always dead. Then all of my gauges would intermittently turn off (go to zero). They'd sometime come back with a restart of the engine or if we were cruising in chop and suddenly caught a big wave under the hull that'd jar everything.

That led me to believe it was a ground, so I checked 'em all and I've got good grounds and a dedicated heavy-gauge ground that runs all the way to the battery terminal. I've also heard that if one gauge goes bad they'll all go down, so unplugged the oil pressure gauge and the symptoms persisted (inconsistent loss of gauge/return of gauge function).

In the end, I haven't messed with the pdf I sent you the link for 'cause my boat is in storage 300 miles away, but I'm guessing mine is the MDC. From what I've seen they're somewhere between $200-$300 direct from Medallion depending on PN and (I think) you can still buy the MDC-1600, PN 6607-20007-01 F units for our boats. I'm heading out of town for about a month, but when I come back I'm just gonna bite the bullet and order a new computer to see if that does the trick.

Jeff d
06-29-2013, 07:27 PM
I actually had oil pressure gauge issues but my sender was indeed bad. I replaced the sender and it's been fine ever since. I may just buy a plain old Faria temp gauge and hook that up in "front" of the MDC. Then I'll just let my gauges fail one at a time and replace them with conventiona gauges. I've already got a perfect pass and depth finder that don't match.

What exactly does the mdc do other than control the gauges? If I'm already not using it for the speedometer what would I miss out on if I just bought a set of standalone gauges and wired those up the way gauges used to be wired up?

Edit: I've done about 5 minutes of research but what if I just buy a gauge set like this:

Just hack off the MDC connector and wire them up directly to each sender. I know that the oil pressure sender is pretty standard and not Medallion specific. I'd guess that the temp sender is the same. Voltage is voltage so there would be no issue with the gauge there. I'm not using the speedometer so I'd just try to resell that gauge on eBay, no issue there. The depth finder isn't integrated into that system. What about the tach? Is the tach signal wire from the computer/ignition system pretty standardized? The only other thing I'd need to research some more is the fuel sender. Even if I have to change out the fuel sender I'd probably still come out ahead vs. a new MDC + a couple of Medallion gauges (and I'd likely have a hard time finding matching gauges). These gauges in white would match my Perfect Pass gauge better. Thoughts? The way I look at it I'd be eliminating a failure prone complex piece of electronics and getting better overall reliability for less money and around the same amount of time to install (Just a few wires to splice).