Coolant temp guage gremlin

Austintacious · May 29, 2020

Hey guys, I've been having a strange issue with my factory engine coolant temp gauge. It started about a year ago. When i first start the truck, the needle pegs all the way to the right past full hot immediately. after running the truck down the road for 10-15min it suddenly will jump to around where it should although it still reads a bit high. Before it started it would normally sit between the "N" and "O" on the normal. now, when it does work it sits between the "M" and "A". I replaced the sending unit. No change. I tested the wire for breaks or shorts. all good. I gave up on it and added an isspro aux gauge to my pillar and just lived with it. Last weekend I replaced my water pump and noticed there was another small plug on top of the water pump right in front of where the factory sending unit was. So i decided to move the sending unit for the isspro from the body of my coolant bypass filter to the water pump. Now the isspro is doing the same thing as the factory gauge. it just pegs full hot and will stay there until i've run the truck a bit. both the factory unit and the isspro seem to "snap" back to where they should be within 30 seconds of each other. I thought maybe it didn't have a good ground so i ran a test lead off the - on one of my batteries. Still didn't change anything. Anyone have any suggestions? I'm completely stumped. I would think if it was an electrical issue they wouldn't work at all right?
Truck is a mostly stock '95 with 182k on it Any help would be appreciated Thanks!

Bugman · May 29, 2020

You are not mistaking the engine oil temperature sender for the coolant temperature sender are you?

I only have one port for the coolant temperature sender on top of the water pump. There is another one down lower on the right side of the block for the warning light.

Austintacious · May 30, 2020

It's definitely the coolant temp sender. I'll attach some pics so you can confirm but im 100% sure that's it. also what the gauges are reading when if first start it and after my drive to work(about 12mins @ 55 MPH). My truck has the older international water pump and does not have the pressurized cooling system with one of the big brass radiators if any of that makes a difference. My truck is a '95 year model but was actually built mid '94 so it has some odd stuff most newer OBS's do not. but if you can see it in the picture, the smaller probe in the front is for the isspro and the fatter one hiding right behind it is the factory unit.

Bugman · May 30, 2020

That's a strange one.

I have a 203 degree thermostat in mine and it runs between the R and M on the stock dash gauge. Usually if a gauge pegs it is a solid ground on the wire between the sender and the gauge.

What happens if you pull the wire off of the sender and then start the engine?

Austintacious · May 30, 2020

Bugman said:
That's a strange one.

I have a 203 degree thermostat in mine and it runs between the R and M on the stock dash gauge. Usually if a gauge pegs it is a solid ground on the wire between the sender and the gauge.

What happens if you pull the wire off of the sender and then start the engine?

it does the same thing but never starts working at any point.

Austintacious · May 30, 2020

that's what keeps stumping me. if its a short, why does it start working after it warms up? dead shorts usually stay that way in my experience. and the fact that the darn near brand new isspro is doing the same thing now makes me wonder what the common denominator is......

Austintacious · May 30, 2020

Austintacious said:
it does the same thing but never starts working at any point.

sorry I meant just stays on cold unless I ground it.

Bugman · May 30, 2020

I'd try another sender if it doesn't do anything when the wire is disconnected.

Austintacious · May 30, 2020

Bugman said:
I'd try another sender if it doesn't do anything when the wire is disconnected.

That make sense for the factory unit but still doesnt explain the isspro acting up as well. is there any other good place on the block to put the sender for the aux gauge? when i had it on the bypass filter, i didnt ever seem to read as hot as it should(around 170f) when the engine warmed up. I assume the coolant looses some heat on it's way to the filter

Austintacious · May 31, 2020

So i swapped the factory sending unit with a new one. still no change on the dash gauge.

Bugman · May 31, 2020

Then it may be the gauge in the dash itself.

Didn't you say that your ISPPRO did the same thing?

rusty1161 · May 31, 2020

Since both gauges have the same problem, could it be an engine ground problem?

Rusty

Bugman · May 31, 2020

It sounds like a weird enough problem to be a ground issue.

Did you use pipe dope or teflon tape on the new sensor?

You can also try adding a jumper from the negative post on the battery to the sender

Sent from my SM-J737V using Tapatalk

jschildm · Jun 4, 2020

The fact that it changes after a bit has me wondering if its related to the glow plugs/relay or WTS light. Thats really all i can think of that switches after the engine is one for some short period of time.

lincolnlocker · Jun 8, 2020

I would guess a ground issue. They aren't tied to the same wiring are they?

live life full throttle

Kowboy · Jun 10, 2020

Is this immediate right after you start the truck? Once warmed up if you shut it down and restart in 10 - 15 minutes do you have the same issue?

Do you have access to a IR temp sensor/gun with a 15/1 or 30/1 spot to distance ratio? The better ratio will give you a smaller area for a measurement. 30/1 is a 1" spot at 30". If so you could shoot the temp in the location of the sensor and get an idea of the temp that the sensor is seeing.

Has the water pump been replaced with a newer style? I seem to remember that there was some issue with the old style pump with the short stem t stat and a potential over heat issue. It has been to long ago and I do not remember the exact concern with the older pump.

I would lean toward an electrical issue until you said that the Isspro did the same thing.

One other thought is do you have the correct temp sender? There are two options for our trucks. One with a red band, and one with a green band. I just went through a similar issue last year on my 95 F350. When I would first start it the temp would be fine but after 15-20 minutes it was reading 80% of full scale. My IR temp gun was only showing about 185F. My sensor had failed and I picked up a new one and it did the same thing. I thought I should have checked the dash gauge. But I did more digging and found that the replacement sensor was not the correct one.

"Ford used 2 different styles of temperature sensors on the 7.3L engines that look the same but function differently. If you touch your sensor wire to the chassis and your temp gauge reads full scale, you will need the 6-016 sensor. If it does nothing, you will probably need the 6-015 sensor.

There is also one difference in appearance that may be hard to distinguish on the sensor once is has been installed on the engine for years. There is a colored ring in the sensor around the single electrical pin. One is red and the other sensor is green. If you have a green colored ring, you need the 6-016, red is the 6-015 sensor."

Link to this info:
https://www.dieselorings.com/1994-1...s-parts/6-016-coolant-temperature-sender.html

KBMKVIII · Jun 10, 2020

Kowboy said:
Is this immediate right after you start the truck? Once warmed up if you shut it down and restart in 10 - 15 minutes do you have the same issue?

Do you have access to a IR temp sensor/gun with a 15/1 or 30/1 spot to distance ratio? The better ratio will give you a smaller area for a measurement. 30/1 is a 1" spot at 30". If so you could shoot the temp in the location of the sensor and get an idea of the temp that the sensor is seeing.

Has the water pump been replaced with a newer style? I seem to remember that there was some issue with the old style pump with the short stem t stat and a potential over heat issue. It has been to long ago and I do not remember the exact concern with the older pump.

I would lean toward an electrical issue until you said that the Isspro did the same thing.

One other thought is do you have the correct temp sender? There are two options for our trucks. One with a red band, and one with a green band. I just went through a similar issue last year on my 95 F350. When I would first start it the temp would be fine but after 15-20 minutes it was reading 80% of full scale. My IR temp gun was only showing about 185F. My sensor had failed and I picked up a new one and it did the same thing. I thought I should have checked the dash gauge. But I did more digging and found that the replacement sensor was not the correct one.

"Ford used 2 different styles of temperature sensors on the 7.3L engines that look the same but function differently. If you touch your sensor wire to the chassis and your temp gauge reads full scale, you will need the 6-016 sensor. If it does nothing, you will probably need the 6-015 sensor.

There is also one difference in appearance that may be hard to distinguish on the sensor once is has been installed on the engine for years. There is a colored ring in the sensor around the single electrical pin. One is red and the other sensor is green. If you have a green colored ring, you need the 6-016, red is the 6-015 sensor."

Link to this info:
https://www.dieselorings.com/1994-1...s-parts/6-016-coolant-temperature-sender.html

Good info right there.
Thank you for sharing

lincolnlocker · Jun 10, 2020

KBMKVIII said:
Good info right there.
Thank you for sharing

Holy crap! Agreed!!

live life full throttle

Austintacious · Jun 21, 2020

Hey guys, sorry I've been trying to help my parents get moved into a new house and been crazy busy. So I have tried grounding the factory one to both the frame and negitive terminals. If the wire is hooked up to the unit, no changes but if I disconnect the wire and ground that then gauge pegs full hot. Water pump is the older international type. That's what was in the truck originally so that's what I put back in. I did use some pipe sealant on both sensors. I removed them, cleaned up the threads and reinstalled them dry. Still not working right. If I run the truck enough to get the gauges to work again, if I turn it off and restart it, both gauges are where the should be. If it sits more then 20 minutes then they start acting up again. I was not aware there were 2 different senders. That sounds like it may be the issue with the factory unit. I will check it when I get back home. I do not think the glow plugs or wts are affecting it. I can watch the voltmeter jump up to 14 when the glow plugs kick off and neither gauge changes. To add another wrench to the works, I've noticed since I've been here in Texas, the isspro started working properly again. The only thing I can figure is the ambient temperature has something to do with it?? Its been in the upper 80s here and it has been close to freezing back in NM. My dad has one of those laser thermometers from harbor freight but not sure how accurate it is but I will give it a shot when I get home. Thanks for all the help. Definitely gives me some more things to check.

Kowboy · Jun 24, 2020

One other thought I had was if the proper thermostat is installed, if it had been replaced before this started? I know that the original water pump in my 95 took the short stem t-stat. When I had to replace the pump I used a later model pump and that takes the long stem t-stat. I no longer have the old pump or I would check if the long stem could be installed in the old style pump. I know that the short can be installed in the newer style pump and cause the coolant to not circulate as it should. Just one other thing to look at. Hope that you get it back to normal. It would be nice to know the solution. Good luck!