[Fixed!] 1998 Nissan Frontier with KA24DE -- Temp Gauge

Great, thanks.

Remember, in order to go full hot the wire needs to short somewhere. Sometimes, however, there is a temperature threshold where these needles will display full hot. You do need rule out the possibility of actual hot temperatures and air bubbles in the coolant or low coolant.

it would be helpful to use a scan tool to watch the coolant temp that is being sent to the PCM during your drives and compare to what the gauge is telling you.

"You do need to rule out the possibility of actual hot temperature..." Yes... so about that. This afternoon, I opened the radiator cap and the air relief valve. The coolant was all the way to the filler neck and in the overflow bottle it was just over the max line. I started the truck and let it warm. Had steady coolant coming out of the air relief valve. Closed the valve and topped off the radiator (which had gone down when coolant was pouring out of the air relief valve).

I back out of the driveway into the street. At that time, the gauge pegs hot. Great! That's what I need! I pop the hood and jump out and disconnect the gauge. Gauge goes cold when disconnected. Oops. Not what I was expecting. Plug it back in, gauge goes hot. Dang. Pull back in driveway and run to get my meter to ohm the sensor, but then it went back to normal. Crud. But, the evidence suggests that my gauge may actually be functioning. Kind of embarassing.

I like the scan tool idea. I have a friend with a scan tool and I'm going to try it. It's a snapon Modis. We tried it a while back and it said it could not communicate with the truck. But, it will read in generic obd mode. I don't know why that is. I'll see if it can read the sensors in generic mode. Thanks for the idea.

So, it's apparent whatever is going on is being caused by the temp sensor. One thing to remember is that new doesn't mean good. Where did you get the sensor? Most auto parts sell junk electrical parts that are often bad right out of the box.

At this point it appears that either the sensor is bad or there is something going on inside the manifold that is causing the sensor to indicate full hot, or short out.

I suggest getting a good quality replacement, either OE or Wells (WVE.) A little known secret is that Autozon's Duralast brand is often Wells reboxed under the Duralast name. Those are the only electrical parts I'll buy at the parts stores.

As for the scan tool, any OBDII scan tool that supports live data should be able to see the coolant temp. The reason you couldn't get the Modis to talk with the vehicle is that you likely were connected to the normal 16pin OBDII DLC. There is another diagnostic port, usually found under the dash or in the fuse box. It takes a proprietary Nissan connector but the Modis should talk to it if he has the Nissan connector for it. I have to use this on my 98 200SX. In the early days of OBDII they were still using their proprietary diagnostic connectors in addition to the OBDII.

I got the sensor from a Nissan dealership. Nissan part. I thought that since the sensor acted properly so far (when I could ohm it, it has checked out so far, but I haven't yet caught it when it was reading hot) and that it went cold when I unplugged it (showing me it's not necessarily shorting), that the sensor was looking good and evidence was pointing to me having air in the system or something similar.

Good information about the extra diagnostic port. I have seen something similar on a 1999 BMW. Something about the changeover from OBD1 to OBD2. I'll look into it. We were connected at the 16 pin "regular" DLC, so I'll look for the other one. I doubt he has the connector for it, though, but I'll check.

See this video for the Nissan diagnostic port. In the first 20 seconds of this video you can see it in the fuse box, below all the fuses.

As for this issue, I'm at a loss. Have you been able to do the gauge sweep self diagnostic while it is acting up? This seems to be an intermittent so you have to be able to test while it's acting up. If you test the gauge sweep while it's working you really don't get a good test out of that.

I would also like to measure the voltage on the sensor wire while it's acting up, as well, and compare it to when it's working normally.

To me it looks more like a open in the wiring than anything else.

But an open would cause the gauge to go full cold, just as it does when he unplugs it. To fail full hot needs a short.

I was thinking possibly a short near or in the cluster but that wouldn't explain why when it happens he can unplug the sensor and it goes full cold.

Possibly a bad power or ground to the gauge but I would think that would affect the other gauges since they share power and ground.

I'm thinking chafed wire somewhere that's shorting and moves when the connector is pulled off.

I agree with John on intermittent short to ground which gets disturbed when the sensor is unplugged being a possibility. Wiggle test with the sensor connected and an assistant watching the gauge will probably find that.

I'm guessing this gauge is a simple voltage divider circuit with R1 in the cluster and R2 being the temp sensor, with the gauge measuring the voltage between R1 and R2. So the less likely, but does fit the symptoms, cause is an intermittent high resistance on the R1 side. When R1 increases R2 will pull the voltage down and when R2 is unplugged the gauge sees full voltage. If that is what's happening I can't think of an easy way to diag it conclusively....

To rule out something in the gauge cluster I'd unplug it and drive it with the gauge on full cold and see if it acts up. If it doesn't act up when unplugged then one can only assume it's not an issue in the gauge cluster.

Because this is not going to be an easy one I downloaded the actual 98 Frontier manual from Nicoclub. Here are the actual diagrams for this truck. I think my attention would be at the harness and the connectors. Looks like there is a connector inside the cab, over on the passenger side under the dash. Maybe it's got some water or corrosion in it? Just trying to come up with something we've missed. You're close on this one, just need to get a few more checks in while it's acting up. The intermittent nature of it can make it difficult.

Matt T wrote: ... So the less likely, but does fit the symptoms, cause is an intermittent high resistance on the R1 side. When R1 increases R2 will pull the voltage down and when R2 is unplugged the gauge sees full voltage. If that is what's happening I can't think of an easy way to diag it conclusively....

Assuming this is the issue, it would be in the gauge cluster. Some tapping on the gauge cluster might help. A few posts back, I suggested taking a voltage measurement at the sensor connector but I suggested taking it unplugged which probably wouldn't yield any results (no current flow, no voltage drop across R1.) Would it help to take a voltage measurement at the sensor connector while plugged in and compare during failure and during normal operation?

You might be able to find a used cluster and plug it in and see if the symptoms change.

This is a bit anecdotal but I have a neighbor who has a 97 Pathfinder which is why I had the Pathfinder manual at the ready earlier in the thread. His tach reads 8,000 rpm most of the time. I haven't spent any real time diagnosing that for him but I did notice that tapping on the gauges did yield changes in the needle, leading me to suspect a bad gauge cluster on that one. While not the same truck, I'm sure it's similar.

John Clark wrote: Assuming this is the issue, it would be in the gauge cluster. Some tapping on the gauge cluster might help. A few posts back, I suggested taking a voltage measurement at the sensor connector but I suggested taking it unplugged which probably wouldn't yield any results (no current flow, no voltage drop across R1.) Would it help to take a voltage measurement at the sensor connector while plugged in and compare during failure and during normal operation?

Agreed on testing voltage unplugged. Not near enough current flow with a 10 meg ohm DMM. Testing voltage with a fairly high resistance to ground might work. Would need to experiment to find a resistor which draws just enough current to pull the voltage down slightly. Then test when the fault appears to see if the resistor pulls the voltage lower than normal. Or if the OP has a DMM with a low impedance mode that might be enough to load the circuit.

First off... you guys are excellent. I don't know what else to say. Secondly, a status update... I have been riding around with my multimeter in my truck. Whenever it pegs hot, I stop, jump out and ohm the sensor. Originally, the sensor would ohm at 120-130 ohms, which would be normal temp and should not have the gauge reading hot. When I put the connector back on, the gauge always goes back to normal. BUT, lately when I jump out and ohm it, I've gotten as low as 60 ohms on the sensor. THAT would show hot on the gauge, I think. It's right down about where the coolant would be reading 212 F. Bottom line, I'm still stumped. I've also had readings where I jump out and as I'm ohming the sensor, the resistance is climbing from low to high, as in I can literally watch it climb on the meter.
I had seized on the idea of using the scan tool to read the other sensor (the one the ECU uses) to compare the two, hence my silence lately. The two sensors are right next to each other, so in theory they should read the same temperature. After you guys showed me where the proprietary connector was, I thought "Wow. Been there the whole time and I never knew it." The Nissan 2 adapter needed to connect was on backorder, so I'm waiting for that to show up. Once I can talk to the other sensor, I'll report back in. Thanks again, folks. This issue is not the end of the world, I just want to solve the riddle, whether it's the sensor, wire, gauge, or if it's actually getting hot!

One thing I just thought of us when was the last time the coolant was flushed and changed? If you don't remember buy a antifreeze tester. They are cheap and you can eliminate antifreeze as a possible cause. The other would be a sticking thermostat. Which I have run into a time or two. Also some millimeters are sold with a temperature test lead which you could put on same area as coolant sensor to see if sensor is reading correctly.

Just a thought

Antifreeze was changed about a year ago when the service place replaced the thermostat. I have seen the multimeters with the temp probe as well... may end up with one before it's over.

So, put the scan tool on it and read the Engine sensor. It never moved off of 181 degrees F. While it unwaveringly sat on 181 degrees, the gauge read full hot, then after a while returned to normal. Conclusion? Pretty sure the truck is telling me it's not hot.

It's the gauge, right? That makes sense, but here's what I don't understand... while the gauge read hot, I ohmed that sensor. It read 47 ohms. That is low resistance and a reading that I would expect the gauge to register as hot. Why is the sensor reading such a value? I check the resistance by putting the positive lead on the sensor and the negative lead on the battery negative.

Pics of the scan tool and gauge are below.

Gauge while scan tool at 181