Volvo MD22L Erratic temperature gauge problem

[Rich Stidger]

OK, wizards, I think this will be a tough one to solve. Bear with me, this is a long post.

I have the subject engine with the deluxe panel in my 1997 Hunter 40.5. Several years ago my temperature gauge which normally reads 185 F began to randomly creep up to 200-220 and then return to normal. This is at constant rpm and sea conditions.
My initial troubleshooting showed that the water strainer is clean and nothing fouling the prop. My IR thermometer showed that the temperature at the sender on the engine was at 186 F. So I conclude that my sender/gauge circuit is faulty. At least I know that I don't have a real over-heat issue. BTW, the temperature idiot light which uses a different sender has always been off.

The common method to troubleshoot these circuits and test the gauge and sender is pretty simple. Removing the wire from the sender should cause the gauge to read LOW (pegged at minimum). Shorting the sender wires together should cause the gauge to read maximum HI (pegged).

Both of these test passed. I conclude that the gauge is operationally normal and that the wiring between the gauge and the sender is not at fault.

Now it looks like the sender is faulty. But the fault is not consistent. Sometimes it works normally (as judged by the gauge) and other times it reads way off. Every time it reads high I have checked the engine temperature with my IR thermometer and have found the actual temperature to be normal- about 185 F.

So I ordered a new temperature sender, planning to install at the next opportunity.

This year, 2023, the failure changed. At initial startup this spring, the temp gauge was pegged HI before starting. I started the engine and while it was running I disconnected the sender wires and reconnected them. The spade connections are clean and tight. The gauge still was pegged HI but after a few minutes the gauge showed a temperature that was normal. Then within 10-15 minutes, the gauge showed HI (pegged) again.

After stopping the engine, I disconnected the sender wires and the gauge read LO (normal). I shorted the wires and the gauge read HI (pegged)- normal. So everything I have done indicates that the gauge and wiring is OK and that the sender is faulty.

I install the new temperature sender. At the initial start the gauge reads HI (pegged). This is the same faulty indication as with the original sender. I started the engine and within a few minutes the gauge was reading normally.

The next day after the engine has cooled to 70 F, I made some resistance measurements. The new sender on the engine read 476 ohms. The original sender read 482 ohms. Both were at room temperature. Reconnecting the engine sender the gauge read normally at LO (engine not yet run)

SO NOW I AM CONFUSED. It appears that the gauge is normal and both the original and the new sender are in agreement. This leaves the wiring that is somehow intermittent.

Do any of you have experience with this kind of issue and can suggest my next trouble shooting steps?

 

[rgranger]

Electric problems are the worst.

Sounds like an intermittent electric short. It could be corrosion under the insulation of the wire harness.

In your case, the fault has to be in one of two places, the wires from the sender or something in the display. Wires are cheaper and easy to swap out. I'd check that first. If you have any inexpensive wires in your shed, you could beta-test this hypothesis before buying more expensive marine-grade wire... if the problem persists after the test, reinstal the original wires and start looking at the display.

I've been chasing down a ghost "short" on my stinkpot. Sometimes the ignition switch seems totally dead but most of the time, she starts right up. I've changed the battery, battery cable, solenoid, and ignition switch. I've disconnected the batter switch and cleaned all of the connections and cleaned all of the connections in the engine.

And still I get the problem every now and again. So the only things left in the system is the wire harness from the ingnition switch, something internal to the battery switch, the MOB kill switch or something internal to the starter motor.

 

[Ralph Johnstone]

OK, wizards,

Don't know if I fit the prerequisite, but here goes. Many, may tides ago, I had a similar problem that when the water temp gauge increased, so did the oil pressure gauge increase. Bull, as I would think that as the oil temp increased along with the water temp (think I got that right), I would expect the lower viscosity to reduce the oil press. Both went up and down together.

Finally traced it down to a less than perfect ground connection. All instruments were connected to the same ground wire as the amperage was so small. Proved this with a dedicated wire connected to the (-) post of the battery. Don't know how many times I tripped over that damned wire laying on the floor.

Do I loses my wizard status if this doesn't work ?

 

[JamesG161]

I have same engine and a more Deluxe Panel.

There is a wiring harness with a screwed connector at the Panel.

I recently found my connector un-screwed, not by me, ever.

Wild Azz guess...
Clean that connector.

Jim...

 

[Rich Stidger]

Don't know if I fit the prerequisite, but here goes. Many, may tides ago, I had a similar problem that when the water temp gauge increased, so did the oil pressure gauge increase. Bull, as I would think that as the oil temp increased along with the water temp (think I got that right), I would expect the lower viscosity to reduce the oil press. Both went up and down together.

Finally traced it down to a less than perfect ground connection. All instruments were connected to the same ground wire as the amperage was so small. Proved this with a dedicated wire connected to the (-) post of the battery. Don't know how many times I tripped over that damned wire laying on the floor.

Do I loses my wizard status if this doesn't work ?

In my case the oil pressure and voltage gauge don't change and read normally. But your point to a common ground wire is interesting. Assuming (very dangerous I know) that there is a common ground for all of the gauges, and only the temp gauge is indicating badly, maybe if it is a ground wire fault it is in the panel itself between the temp gauge and the other gauges. I will pull out my crummy pictorial schematics and see if this scenario makes any sense. And I have wire onboard that I can run to the panel directly, but unless I can wiggle a wire and cause the fault, there is no way I can prove that I have found the actual problem.
Thanks Ralph, Wizard status intact.

 

[JamesG161]

Here is a picture...

Part#27

 

[jssailem]

Temp issues are a bugger. Sounds like you went through the electrical system side. You checked the sender. The resistance difference does not sound like it would be out of the range for the gauge. You might compare the sender data with the gauge requirement.

This anomaly is reported as first occuring when the engine was in operation. Have you checked your engine thermostat for proper operation and hoses for blockage.

 

[Rich Stidger]

I have same engine and a more Deluxe Panel.

There is a wiring harness with a screwed connector at the Panel.

I recently found my connector un-screwed, not by me, ever.

Wild Azz guess...
Clean that connector.

Jim...

I have removed that panel many times to change the tachometer of which failures are abundant. I also replaced all lamps in the gauges with LED bulbs. Every time I disconnect and reconnect that panel I spray the contacts with DeOxit cleaner. But I will check again.

 

[Rich Stidger]

<snip>This anomaly is reported as first occuring when the engine was in operation. Have you checked your engine thermostat for proper operation and hoses for blockage.

Since the first start this week, the problem and come and gone several times. No apparent rhyme or reason. And in the past my IR thermometer has always indicated correct temperature. Also, since the temp gauge was pegged HI before the engine start, the issue is clearly with the faulty gauge circuit.

 

[JamesG161]

Part #15 is part of electrical groups, lights up the Small lights.

Jim...

 

[JoeWhite]

I am with rgranger. Run a temporary wire externally from sender to the gauge and see what hsppens.

 

[Ralph Johnstone]

Sometimes the ignition switch seems totally dead but most of the time, she starts right up.

Sorry, but you're looking at an ignition booster relay to overcome the connection harnesses.

 

[Rich Stidger]

UPDATE:
Basically I am stuck right now. Periodically I turn on the key but not start just to observe what the temperature gauge shows. Sometimes it pegs HI which is the failure mode for the cold engine, and sometimes it moves off LO and shows a temp under 100 F which is normal.

Right now it is repeatedly showing a normal indication. So until the gauge or circuit shows a fault, I can't begin to troubleshoot it. I hate intermittents.

I will not be back aboard for a week, and hopefully the problem will re-appear and that I will have time at that point to troubleshoot.

I will keep this thread posted as to my testing. Thanks to all who offered ideas and methods of attack.

 

[Ralph Johnstone]

Periodically I turn on the key but not start just to observe what the temperature gauge shows. Sometimes it pegs HI which is the failure mode for the cold engine,

Correct me if I'm wrong (which is becoming more common) but is the thermistor resistance at the sensor directly or inversely proportional to its temperature ?

I'm still hung up on this faulty ground thing and wondering if a combined high resistance (thermistor + high resistance wire) is contributing to the high reading ?

 

[Rich Stidger]

Correct me if I'm wrong (which is becoming more common) but is the thermistor resistance at the sensor directly or inversely proportional to its temperature ?

I'm still hung up on this faulty ground thing and wondering if a combined high resistance (thermistor + high resistance wire) is contributing to the high reading ?

Good question Ralph. The resistance of the sender is inversely proportional to the temp. So a high resistance in the sender leads would cause a LO reading on the gauge. The typical temperature sender resistance vs temperature is:
100 F 450 ohms
175 F 99 ohms
250 F 29 ohms

Senders for oil pressure or fuel level have different resistance endpoints.
I think the most likely fault that I will eventually find will be a bad ground at the gauge. I believe that the gauge and sender form a bridge circuit and I think a open or hi resistance in one of the bridge legs will cause a bad reading. As soon as this fault rears it's ugly head I will try to prove my theory.

 

[Ralph Johnstone]

Aw crap. That does not help at all. I found my ancient scratchings from years ago when I was hunting down my intermittent problem and confirms your values above :

This is where I found the actual problem in my VDO setup so many years ago :

If there is a high resistance wire between point S at the gauge terminal and the ground wire at the temp. sensor, you would have problems with LOW temperature readouts. Also, there is no wire at the sensor ground as the FIP thread in the block is all you get for a connection. Through the magic of electricity, a high resistance fault at the gauge terminals caused my problem and my gauge to read high.

I would think our gauge and sensor setup are similar.

I think the most likely fault that I will eventually find will be a bad ground at the gauge.

Seems likely. If not, you've really got a headache .

 

[odaydokay]

I would not rule out an intermittent blockage in or around your heat exchanger and hoses. Piece of pencil anode moving around, piece or two of water pump impeller. Neither difficult to check out.

 

[Rich Stidger]

I would not rule out an intermittent blockage in or around your heat exchanger and hoses. Piece of pencil anode moving around, piece or two of water pump impeller. Neither difficult to check out.

The bad gauge reading comes and goes without ever starting the engine.

 

[jssailem]

The bad gauge reading comes and goes without ever starting the engine.

That type of gremlin leads one to focus on an electrical issue. Not an easy issue to diagnose. I generally look for the least complex cause. Wires and contacts causing intermittent connection. Moving from there to faulty components.

I feel your pain Rich.

 

[JamesG161]

Piece of pencil anode moving around, piece or two of water pump impeller. Neither difficult to check out.

No pencil Zincs on his engine. Only shaft Zincs.

Jim...