Temp Gauge Pegging--help trouble shooting please

[Gulfster]

I have a 1999 Epic X22 (on the registry--just got it and am very excited to be part of this community.  Wonderful boats). 

The temperature gauge pegs (full hot) when I turn on the key.  It makes the clicking/spinning sound and then pegs.   

I tried installing a new sender, but it did not fix the problem.  I tried a new gauge, and again, it did not fix the problem.  All of the other gauges work.  I switched the 5-pin oil pressure plug and plugged it into the temp gauge. Did not fix the problem.  I switched the 5-pin fuel gauge plug into the temp gauge and it did not fix the problem; the temp gauge still pegs.
 
I read the threads about gauges and overheating and it looks like this is either a bad ground wire or a bad MMDC.  How do I figure out if it is the ground or the MMDC without hauling the boat to a shop and having them put it on their MMDC diagnostic machine?


If it is a bad ground going into the MMDC, why do all of the other gauges work?  (FYI--there is water in the area where the battery is located.  Is this normal?)

I really appreciate any feed back and would love hearing from anyone who has had similar issues.

Best, Josh  

[lilfoo2225]

did you read the mdc trouble shooting in maintenance section? theres a section about the temp gauge.

[Gulfster]

I did, thanks.  I've followed the procedure but am still confused.....

[Wet-N-Frugal]

Gulfster,

First, the water under the battery is normal... These boats have a low spot there, and you'll find the bilge pump is in the forward end of it. To minimize the puddle size, after every outing we park the boat on a STEEP hill , and let the majority of the water drain out the rear plug.

So far your troubleshooting of the temperature gauge has been darn thorough. In your case, since the other gauges are working, and you've swapped gauges between connectors, I don't believe the MDC power or connector is the problem. Note: However I do suggest the Power wire upgrade (up-size) from the battery to the dash... it eliminates a lot of flakey behavior; String new (#8 or #10) wires in parallel with the old for both the +12V and Common. Now, from what you've described, the issue is either between the sender and MDC, or the MDC has blown input. According to the MDC documentation, the tan wire runs to the sender. Did you do the ohm test described in the troubleshooting guide? This will confirm the signal wire is sound and the sender is good. Should you not get the readings described, first check the continuity of the wire alone. If it checks out. And the ohms thru the sender (regardless if its new) checks out.  Then it's time to focus your attention directly at the MDC.

-WNF

[Gulfster]

Thank you very much Wet-n-Frugal.  I'll try all of that tomorrow and let everyone know.  This community is great!  Best, josh

[Gulfster]

And now for the rest of the story:

OK, here goes for anyone who is experiencing temp gauge problems.  Mine pegs when I turn on the key.

1:  I replaced the temp sender.  This was not the problem.

2:  I replaced the gauge.  Again, not the problem. I also swapped out the 5-pin plug from a working gauge to both the original gauge and the new gauge, again to no avail.

3:  I followed the MDC troubleshooting procedure.  At first it looked like it was the sending wire from the sender to the MDC because the OHM meter showed it was grounded.  I then discovered that someone had run a ground wire from the battery and spliced it into the sending wire (between the sender and the MDC).  Once I cut this wire, the OHM meter showed it was not grounded.

4:  I tested the 18-pin connector and got a reading of about 800 on pins 1C and 2C, which is within the range of 1,000-300 specified in the MDC troubleshooting guide.  This established that with the scabbed on ground wire disconnected the sending was working correctly and the wiring was not defective.

5:  I tested the output from the MDC to the gauge.  With the key on and the engine cold, it read 5.75 Volts.  The normal range is supposed to be 3-6 Volts.  From this, I can only conclude that the the MDC output is fried from having a direct ground wire connected into the sending wire (tan).  The MDC is sending the high-end of voltage thereby telling the gauge to peg even when the engine is hot.  I am assuming that the MDC is supposed to start around 3 Volts cold and increase towards 6 Volts as the engine warms. 

What does anyone think?  Am I wrong?  Best, Gulfster.   

 

[lilfoo2225]

it says to use ohm's when testing cold and warm. less than 200 ohm's when warm.

[Wet-N-Frugal]

The MDC Doc says than when the Tan signal is grounded (0V) the temp gauge should peg at full hot... thus the higher the voltage reading the cooler the gauge reading. The thermister in the sender is decreasing in ohms (between signal and ground) as it gets hotter, thereby pulling the signal closer to ground as it gets hotter.

What do mean you checked the voltage between the MDC and Gauge?  I'm guessing you measured the voltage on the yellow wire expecting to see the fuel sender voltage. The system does not work that way. Yellow wire sends digital signals. If you plugged a different gauge into that connector and it worked, there is nothing wrong with it... the computer is sending (outputting) digital signals to all the gauges, that's why you can swap gauges between connectors and they still work.

When you first apply power to the temp gauge, does it go to zero and rattle like the other gauges?

-WNF

[Gulfster]

WNF, the MDC troubleshooting guide says that 5-pin plugs send different volts depending on the color. Yellow (data) is 3-6 volts.  Red is 7 +-.5.   I tested the 5-pin to the temp gauge and it met this spec, with yellow at 5.7 with the engine cold.   But I did no know that higher voltage should be lower temp.   

I plugged the 5-pin from working gauges into the temp gauge and it still pegged. I have not plugged the temp gauge 5-pin into another gauge.  Should I? 

When I turn on the key the temp gauge rattles and then pegs full hot. 


Thanks, Gulfster. 

[Wet-N-Frugal]

I thought you had done this already, but swap the connectors going into the temp and volt gauges. Does the problem follow the gauge or the connector?

 * if the volt gauge continues to work  AND the temp gauge begins to work, there is something very curious about your system...  but don't mess with it, for everything is working.
 * if the volt gauge fails AND the temp gauge works, then there is a problem with the temp gauge connector/connection to the MDC.
 * if the volt gauge continues to work AND the temp gauge doesn't, then there's nothing wrong with either of the connectors/connections to the MDC...   Further, if the TAN signal wire from the sender is producing the required response (low voltage when hot, higher voltage when cool), then I would summize there's a problem with the Temperature circuit (input) internal to the MDC computer.

-WNF

[wakejunky]

do you have the four gauge melenium dash or the old style dash?

[Gulfster]

The old style dash.   

[Wet-N-Frugal]

Gulfster,

It doesn't matter if its a new or old dash, they both work very similar.

To Clarify:
You did the right thing by measuring the voltages at the connector.  Yet the yellow wire functions as a communication line between the computer and gauge. The high speed digital signals on it have ONLY two voltage levels: near 0V and above 3V. When you put the average voltmeter on it, well it averages... thus it comes up with a reading somewhere between 3V - 6V. 

Any luck with switching the gauges??

-WNF

[wakejunky]

based on your testing the mdc is fried as you stated earlier. someone or something let the magic smoke out of the box. you have done the correct testing. The input signal is correct. and all other gauge data on the data wire is working. the mdc is all that is left.

 I'm sure capt rick will chime in or you can contact him, he is very knowledgeable

[Gulfster]

WNF, first, thanks for all of your help--it has been great. Today I tried your latest suggestion and switched the 5-pin connectors for the temp and the volt gauges.  The volt gauge worked perfectly and the temp gauge failed (pegged).  Based on this, and the fact that I get the correct reading for the 5-pin connector, I agree that it has to be internal to the MDC.   

Again, I really appreciate all of your and everyone else's help.  This Toyota community is great. 

PS--my wife was not happy with me about buying the boat until Saturday when we all went out.  Everything is fine now! She and the kids loved it. 

Best, Gulfster.