Faulty temperature sensor, or gauge?

As some of you might know by now, I recently got an '87 in very good shape (other than the fact that it was wrecked and totaled, and then repaired, which is how I came to buying it...). Pretty much everything is in working order, but I took into to my local shop to give it some love and general maintenance, as I'm not as mechanically gifted as some of the other fellows here...
One of the things I asked them to look at was the temperature gauge. It only got up to the first line on the gauge spectrum, which I would say it's about 10% of the way, even though it seemed to be warming up fine, as I could tell by the heat coming out of the vents.

They replaced the temperature sensor, and now the gauge does this weird thing where it will either be stuck all the way on the "C" side, or all they way on the "H" side, randomly. Sometimes it will be stuck on "C" for a while, and after driving for 20 mins or so, it will quickly go all the way to "H" and stay there for a while, until it decides to come back all the way back to "C"....

The guy at the shop was stumped by this, and because they were really busy and couldn't do anything about it at the time, we agreed that I should keep an eye on it, since the engine seemed to be running fine (not overheating), and if it kept doing it after a couple days to bring it back.
The tech said that since we replaced the sensor, the sensor was not the problem, and since information was getting to the gauge, the wire was not the problem, so maybe the gauge was the problem...

What do you guys think? Have you seen this happen? Is the tech right that it might be the gauge that is the problem? Is it possible that we got a faulty brand new sensor?
The tech also said that it might "just work itself out", that maybe there was an air bubble in the coolant (from him changing the sensor), and that it would start acting normal after a while, maybe...

Obviously he's not sure what's going on, I'm hoping one of you guys might know!
Thanks

Check the shop's work. Make sure the sensor is connected and most importantly, that they replaced any coolant lost during the sensor replacement. Low coolant levels can cause the gauge to behave erratically.

Yeah check the coolant levels URGENTLY. If you're getting air trapped in the head it'll kill the auto choke on the carb and damage sensors and potentially the radiator thermostat. They're only designed to work within a certain temperature parameter and anything over it's max normal operating temp can cause failures.

I checked the coolant, and it looks ok.

Here's the radiator cap off
Attachment 21669

And here's the coolant tank, looks like the level is on the low end, but still above the LOW line
Attachment 21670Attachment 21671

And here's the new temperature sensor, which seems to be connected
Attachment 21672


So if the coolant level is fine, and the sensor is connected, could it be that the (new) sensor is faulty? Or could the gauge be faulty?
Something's obviously not right, but is there any danger that the engine might be overheating if the sensor is not reading the temperature correctly?
My understanding of how this works is that the cooling to the engine is controlled by the thermostat, and that temperature sensor is just there to give a reading, though the gauge. Is this right, or does the temperature sensor has something to do with the temperature control of the engine as well?

Could the thermostat have anything to do with this issue?

Is it safe to drive right now?

Thanks

Hey, MrPaco ~ The sending unit is only a sensor: it doesn't have any affect or control of engine temperature, so lay that fear to rest.

It almost sounds like a ground wire is not connected good, but I'm definitely no expert on electrical matters. It is remotely possible that it is the wrong sending unit...like for an idiot light instead of a gauge.

Just my two cents :)

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Originally Posted by royster

Hey, MrPaco ~ The sending unit is only a sensor: it doesn't have any affect or control of engine temperature, so lay that fear to rest.

It almost sounds like a ground wire is not connected good, but I'm definitely no expert on electrical matters. It is remotely possible that it is the wrong sending unit...like for an idiot light instead of a gauge.

Just my two cents :)

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That's an interesting theory. You're saying that maybe the sensor is at fault, but not necessarily because it's defective, but because it might be the wrong sensor... right?
That might explain the really weird gauge readings: It fluctuates between all the way to the left at C, and all the way to the right at H. The fluctuation is not back and forth all the time, it will stay at one end for 10 minutes or so, and then switch to the other side. I haven't found any pattern to why/when it switches from one end to the other.

Glad the engine temperature is not affected. Do you think is safe to drive as long as the coolant level is good?

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Originally Posted by MrPaco

Do you think is safe to drive as long as the coolant level is good?

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Start and run it with the heat on. When it blows warm, feel the radiator top and hoses. (Watch out for the spinning fan). They should start to get warm and then hot. If they do, that tells you that coolant is circulating and it’s okay to drive although I’d want the gauge issue fixed ASAP. If they stay cold or tepid, the thermostat should be checked immediately.

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Originally Posted by FMS88

Start and run it with the heat on. When it blows warm, feel the radiator top and hoses. (Watch out for the spinning fan). They should start to get warm and then hot. If they do, that tells you that coolant is circulating and it’s okay to drive although I’d want the gauge issue fixed ASAP. If they stay cold or tepid, the thermostat should be checked immediately.

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I performed this test, and yes, the big fat hose coming out of the radiator did get hot. So that's good.
I called the guy at the shop, he thinks it's a faulty sender, which he says is not uncommon with aftermarket parts like this one. He's ordering a new one and will try that next week. I'll let you know what happens.

Here are two pictures of the gauge. The first one is after running the engine about 10 minutes, including a short drive. I then turned it off to run into the parts store to get some more coolant. The second picturesis right after turning the engine back on, as you can see from the miles, I didn't not drive any distance between the two photos.

Attachment 21673
Attachment 21674

I bet they got the wrong sender unit and it's sending a false reading...

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Originally Posted by geezer101

I bet they got the wrong sender unit and it's sending a false reading...

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I have a feeling you're right. Any idea where I could find the spec on the correct sender to make sure they are getting the right one?

EDIT: found these, any of them a better choice?:
https://shop.advanceautoparts.com/c4...36400474892571

^says it's for the 2.6 Ram 50 so it must be right.

You want the single blade sensor. The Mitsubishi P/N is MD091056 according to my old parts book. Should be the same or work for the Ram 50. The two blade sensor sends info to the computer controlling the Mikuni carb, not the temp gauge.

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Originally Posted by FMS88

You want the single blade sensor. The Mitsubishi P/N is MD091056 according to my old parts book. Should be the same or work for the Ram 50. The two blade sensor sends info to the computer controlling the Mikuni carb, not the temp gauge.

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Yessssss, thank you, sensei!
Ok, so this one then (the one that has "W/Gauge" in the description, makes sense).
https://shop.advanceautoparts.com/p/...7CL4*-49999252

BTW, I love it how the brand name of this one is "ORIGINAL EQUIPMENT", that's some genius bulls**t marketing right there
https://shop.advanceautoparts.com/p/...7CL4*-49999252

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Originally Posted by MrPaco

BTW, I love it how the brand name of this one is "ORIGINAL EQUIPMENT", that's some genius bulls**t marketing right there

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You mean you don't believe everything the advertiser or salesperson says???

Attachment 21682

there are 2 "sensors - one is a 195 degree sensor for the o/d to activate the o/d circuit, the other is the gauge sensor. The issue is both are marked sensors, and without the proper number for the gauge sensor, there is no way to know which of the single blade sensors is the right one. the sad part is beck Arnley just reboxes the parts and does not know if they are right or not. The 195 degree sensor is an on/off switch and will do exactly what you are seeing with the temp gauge.

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Originally Posted by pennyman1

there are 2 "sensors - one is a 195 degree sensor for the o/d to activate the o/d circuit, the other is the gauge sensor. The issue is both are marked sensors, and without the proper number for the gauge sensor, there is no way to know which of the single blade sensors is the right one. the sad part is beck Arnley just reboxes the parts and does not know if they are right or not. The 195 degree sensor is an on/off switch and will do exactly what you are seeing with the temp gauge.

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So you're saying that most likely what's going on is that we put on one of the sensors that is for the O/D in the spot for the gauge sensor, and that's why it's giving out the odd readings, right?
And if I follow, you're also saying that the manufacturer doesn't necessarily label them differently?

Do both sensor look the same?

Thank you, this info is priceless

yes to the wrong sensor for the gauge. The problem is the parts seller don't know the difference, so they get sold interchangeably, so you have to figure out which is the gauge sensor by cross referencing the gauge sensor with the Mitsubishi part number from a factory service manual.

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Originally Posted by pennyman1

yes to the wrong sensor for the gauge. The problem is the parts seller don't know the difference, so they get sold interchangeably, so you have to figure out which is the gauge sensor by cross referencing the gauge sensor with the Mitsubishi part number from a factory service manual.

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Found this:
http://www.beckcatalog.com/Vehicle.a...6cid&back=true
I only see the temperature gauge sensor, not the O/D sensor. Is Beck Arnely the only ones making single blade sensors for this purpose? I guess is possible that the faulty sensor is from a different manufacturer.

Thanks to FMS88 I know that the original part number is MD091056 , but not sure where to cross reference since there no such info on the seller or manufacturers page

As an aside, but related to this, after doing a lot of research on the topic I'm wondering if maybe the original sensor wasn't bad, and that I had another issue. I though the original sensor was bad because it would never rise above the first mark after the C on the gauge (it did move a tiny bit from all the way to the left when the engine was off to the first mark).
Is it possible that the sensor was right, and the engine was running really cold because of the thermostat being stuck open?

This doesn't change the fact that now I have a bad sensor, but I wonder if maybe I also have a bad thermostat too. I know that is not stuck in the closed position because the hose to the radiator is hot, but maybe is the opposite...?

yes that would mean the original sensor was good. Put it back in if you changed the thermostat.

pennyman1 and others are definitely correct. The shop installed a sensor/switch (no/off). You need a gauge sensor (hi/low). Have them fix it or get your money back and fix it yourself. With a different gauge sensor, if the gauge still registers low, the next one to accuse in this line-up of suspects is the thermostat. It may be stuck open or rated too low. The correct rating is between 190 and 195 degrees.

New sensor on order (old one is long gone), will test this theory later this week when it comes in.
Does the on/off O/D sensor have one blade like the gauge sensor? Do they look the same, other than maybe a different part number?
The sensor that is on right now, the one that is reading funny, has one blade.

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Originally Posted by MrPaco

New sensor on order (old one is long gone), will test this theory later this week when it comes in.
Does the on/off O/D sensor have one blade like the gauge sensor? Do they look the same, other than maybe a different part number?
The sensor that is on right now, the one that is reading funny, has one blade.

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Both types have a single blade. Depending on the manufacturer, they may otherwise look similar. I have no experience with the O/D switch except in ads and I wouldn’t trust their accuracy. The P/N is best whenever possible. You’ve done a thorough investigation so you’ll very likely get the right part. The shop was too busy to make sure they got and installed right part.

UPDATE- Problem solved!

As suspected, they were using the wrong sensor, and there was nothing wrong with the original one. The problem was the thermostat all along. The original sensor wasn't registering any heat because the thermostat was stuck open, and the engine wasn't getting hot enough.

Luckily we found the original sensor at the shop laying on the guy's tool tray. The one that he had ordered (twice) was most likely mislabeled in the parts catalogue he was using. I saw what he ordered and it was labeled as the temperature sensor w/gauge for this truck. The one that came in the box looked almost exactly as the original sensor except the bottom end was shaped like a bullet, the sides of the "bullet" part were brass like the rest of the sensor and the tip was gray.
I trust the guy, I think he knows what he's doing, they were just sending him the wrong part even though he was ordering it correctly, and he wasn't familiar with this particular engine. Now he knows, I guess. He didn't charge me for any of this, he just returned the wrong sensors so he'll get a refund.

Glad we were able to solve this problem. The truck is running proper now, which was the goal, but I also enjoyed the process of figuring out what was wrong with all your input. Thanks to all of you who chipped in with thoughts and suggestions, all very helpful!
I also feel good about taking this on my own next time. It's a little intimidating when you don't know what you're doing, but now that I understand it, seems doable to me.

And to end this silly thread, here's the truck at the shop. Three Rams in a row, amazing to see the size difference when they're side by side...

Cheers!

Attachment 21768
Attachment 21769

Awesome, MrPaco!!! And thanks for posting the resolve to this mystery.

Onward to the next adventure :)

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Originally Posted by FMS88

You want the single blade sensor. The Mitsubishi P/N is MD091056 according to my old parts book. Should be the same or work for the Ram 50. The two blade sensor sends info to the computer controlling the Mikuni carb, not the temp gauge.

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Old thread (?) but my temp gauge not working (again) and truck (Mikuni carb) runs rather rich ~ am ordering the two blade CTS from Rock Auto (oreillys napa autozone no have correct sensor)

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Originally Posted by xboxrox

Old thread (?) but my temp gauge not working (again) and truck (Mikuni carb) runs rather rich ~ am ordering the two blade CTS from Rock Auto (oreillys napa autozone no have correct sensor)

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The two-blade CTS is for the feed-back carburetor system and computer. Replacement might help the rich condition but not the gauge. The sending unit for the gauge has a single blade. Look at the schematic in the Haynes manual, page 12-13. The single wire to the gauge CTS is red on yellow. (Haynes may have printed the schematic for a Gen 2, but I bet it's the same for your Gen 1.)

The test for a Gen2 gauge: disconnect the wire and ground the connector through a test light. Turn key to "On" and the light should flash and the gauge operate. If only the light flashes, the gauge is faulty. If no light or gauge operation, the sending unit or gauge circuit has a fault. I bet the same test applies to your truck. If you think it's the sending unit and you can't find one, I have extra used ones you're welcome to.

Here's the gauge sending unit per my parts list, P/N MD001380:
https://www.ebay.com/itm/293183077612

Note that it has a squared off end. You don't want the single blade unit with a bullet shaped end. It's a switch, not a sending unit.

DANG IT FMS88..!

OK, sometime REAL SOON I am gonna hunt down & find the gauge sending unit..!

THANKS TEACHER :thumbup: