Coolant Temperature Guage Reads Low

[cmturner]

On my 2006 Safari Cheetah (350 hp Cat) the coolant guage slowly rises to 150, but never goes any higher. My Cat manual says it should read at least 195. When I touch the temperature sending unit lead to ground, the guage needle pegs. I replaced the temperature sending unit with a new one from Monaco, installing it using Teflon thread tape--still topped out at 150. I cleaned off the tape and applied a paste thread sealer before reinstalling it--still topped out at 150. I replaced the temperature guage with a new one from Monaco--still tops out at 150.

A Cat dealer checked and found the engine to be running at 200 degrees--while the guage still topped out at 150.

Any ideas?

[wolfe10]

Welcome to the FMCA Forum.

Sounds like you have done the correct diagnostics, starting with a confirmation that the engine is running at the correct temperature.

Are any other gauges reading "non-normal"? If so, check the power supply to the gauges.

Troubleshooting this is really a question for the Monaco techs, as they speced and installed the instruments. It is unlikely that you have multiple bad sending units, so the gauge or voltage to the gauge are the likely suspects.

[Guest BillAdams]

Has the gauge ever read properly since you owned it?

[chucknewman]

I thought Monaco continued with Intellidrive digital unit installation that Safari had pioneered with? Then, maybe not. It would have given you the exact temperature used by the ECM.

Bill's question is an important troubleshooting step. I had a defective gauge on a GMC P-30 chassis years ago. Being a pre-OBDII capable gas engine and not having other test devices at the time, I thought the gauge was accurate. I spent a lot of time and money on engine overheat issues that did not exist.

Since then, most gauges I've compared to the ECM data did not read accurately. Anywhere from 10 degrees to 30 degrees in error in actual temperature. And if they are within 3% accuracy, you still cannot tell exact temperature due to their lack of scale resolution.

You posted "When I touch the temperature sending unit lead to ground, the guage needle pegs." That is what it should do and indicates a high resistance somewhere in the line going to the gauge. Probably a poor crimp connector. But you still could have a defective gauge.

I would go to Radio Shack or Home Depot and purchase enough 14 or 16 awg stranded wire to run from the engine compartment and through a window or door to the dash area. Remove the factory lead at the temp sensor and gauge, and replace with the new temporary wire. If the gauge reads about 200 degrees with the engine at normal temp, you have a wiring issue. If it reads 150, you have a defective gauge -- if all your other gauges are OK as Brett mentioned. If your other gauges are not reading correctly, then forget my info here. You have voltage or ground issues.

[hermanmullins]

cmturner,

I read where you put teflon tape and then used thread sealant. If I understand sending units they are temp. or pressure switches and use the block for ground. I beleive the sending unit will either have MPT threads or have a 'o' ring seal. You might try to clean the threads so there a metal to metal connection.

I had a problem with my oil guage on my Monaco. I changed the guage and the sending unit and my guage still pegged. The guage was made by "Bedee" and when I called them they were quite helpful. The oil pressure sending unit I installed (from Cummins) did not solve my problem. My idiot light on the dash goes out just as soon as I start the engine. I understand that Cummins has two oil sending unit and the new replacement is a superseeded P/N that is only an off/on unit that only tell you if you have pressure or not and has to have the ECM reprogramed. That did not work either. My conclusion is that I have a bad wire some where in the harness. Some day soon I plan to replace the sending unit with a pressure sending unit and run a wire from the guage to the sending unit.

[chucknewman]

Herman made an important point I totally forgot about -- multiple temp (or other type) sensors. My Safari has three coolant temperature sensors. One for the ECM. One for a dash light. And one for the mechanical gauge. Two of the sensors are analog type with a variable voltage that changes with temperature. These are for the ECM and gauge inputs. The third sensor is a switch type that closes contacts upon reaching a certain temperature. Both types are common in all vehicles I've worked on; large and small.

The point being a shared sensor of any kind (temperature, pressure, flow, etc.) is an engineering problem of divided currents, multiple shared impedances, etc. It is much easier, cheaper, and more reliable to have a single transducer feed a single input load; be it a computer, a relay, or a light.

These devices are installed by the chassis maker, not the engine maker. The engine may come with one sensor for oil, coolant temp, coolant level, etc. for the engine ECM. But the sensors for the gauges and dash lights are installed by the chassis manufacturer and in my experience typically are not "grouped together" in the same place on or around the engine.

This is information for someone initially trying to troubleshoot a gauge issue. In your case you have already determined the correct sensor by grounding the sensor lead and noting the full scaled gauage.

Let us know how it turns out.

[cmturner]

I've been off-line for a while, but I appreciate everyone's input. My temp. sending unit has pipe threads (no o-ring), so I assume it needs pipe dope. The guage has performed the same way since we bought the rig in Jan., 2010. All the other guages work fine. I can use an extension cord to "hot wire" the sender to the guage, but I don't know how to check for power to the guage; it has four leads and I don't want to damage anything. (Two RV techs advised me to "Don't worry about it", but I'm having trouble doing that.) Any more advice about checking the guage?

[dickandlois]

You mention that the gauge pegs when you ground the connection at the sensor,that tells me its an analog type gauge.

If you have a volt meter,measure the voltage at the sensor and gauge when the engine is cold.

Then run it till the gauge reads 150degs. read the voltage at the sensor and the gauge. is there a difference in the voltage reading and how much ?

should you read a difference,then you could run a wire from the sensor up to the gauge removing the OEM wiring for this test, what is the temperature and voltage reading.

Should your gauge read close or at the expected temp reading,there is some resistance in the wiring between the two points. That is where things could get interesting,just finding the wire in a harness that is like a dish of spaghetti.

What we call a fox and Hound tester is a device that places a tone on the wire and a receiver that picks up that tone can help isolate the circuit.

Rich.

[cmturner]

Coolant guage reads low: Thanks to all who sought to help me with this problem. I had spoken with around 20 RV technicians at RV shops, Monaco, and RV seminars: "The trouble has to be a loose connection." On advice from an audience member at a recent RV seminar, I talked with a technician at Bedee Guages. He said that they had issues with Monaco installing temperature sending units which were incompatible with Bedee guages and the proper sender would likely solve my problem; he was right. For $16 I bought a new sender--VDO #323-419/NAPA #701-1813. Now my temperature guage works perfectly.

Carl

[dickandlois]

Carl, Thanks for taking the time to reply with the fix for the temperature issue and the replacement part numbers. Takes some of the steepness out of other readers and posters learning curve.

Relaxing and safe travels. Rich.