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1980 - 1986 Bullnose F100, F150 & Larger F-Series Trucks Discuss the Early Eighties Bullnose Ford Truck

Temp Gauge Pegs Out When Truck is Not Even Close to Warm

 
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Old 02-17-2019, 01:30 PM
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Temp Gauge Pegs Out When Truck is Not Even Close to Warm

Since I got my engine running good again, I figured I would tackle the next problem. On my 1984 F150 4.9L i6 with Edelbrock and DSII conversion, the temperature gauge in the cluster starts moving to the hot side even when the engine has only run 30 seconds, after starting at 40 degrees (this has only started happening recently, a while after the DSII conversion). Now, this was without coolant in the head, but that shouldn't matter, as there would be at least SOME coolant in the block where the sending unit is. it wouldn't go from around 40 degrees to 100-ish in 30 or 45 seconds after starting it. So I checked with a test light to make sure that it was indeed the sending unit (yes, I do mean the oh, so fun one to get to on the back of the block), and when I clamped my test light probe to the terminal on the unit, and touched the probe to the positive terminal on the battery, the light slowly lit up, and got brighter as time went by. Now, this was with a BRAND NEW Duralast sending unit (the one that was in it was toast, I didn't even bother to test it, see pics at end of post). So I was surprised it didn't work, as we've almost never had trouble out of Duralast parts from out local Autozone. I then added coolant to the head, guessing MAYBE it was actually getting that hot that early. But sure 'nuff, as soon as I turned the key on, the gauge starts moving to the hot position. I just assumed that I got a "lucky" sending unit. I took it out, and with it at what I thought was cold (around 60 or 70 degrees, it was still slightly warm from the engine, but was around 50-ish outside), I touched the threads to the negative terminal on the battery, test light clamp on the terminal on the sending unit, and then touched the positive post of the battery. Again, the test light slowly got brighter as time went by (it would reach full brightness after around 10 or 15 seconds). So I went back and got another sending unit. This time it was a Valuecraft (and it didn't come with sealant on the threads like the Duralast did....hmmmmm).

Next day after it getting down to mid 30s. I take the old sending unit (the Duralast one) and test it again...it tests fine. What?! I do the same thing again, touch the unit to the negative post on the battery, test light clamp to the terminal on the sending unit, and touch the positive post on the battery with the test light. Sure 'nuff, nothing happens. So I guess that it starts messing up when it gets warm. So I try to warm it up...maybe I got it up to 60 degrees. Still nothing. So I put the new unit in the block, hook everything up, and the gauge doesn't move when I turn the key on...yay!! I start her up, and about a minute after she starts, again, with no coolant in the head, and maybe a little in the block, she starts moving again. This time she moves to about the beginning of normal (even after I stop the engine), and stops. I check and the engine is not even hot (I can touch the exhaust manifolds with my bare hands, they are just warm). I test with my test light, and the light doesn't immediately come on, but after around 5 seconds, it comes on, and again, slowly gets brighter. At this point, I am completely lost, I have no idea what the problem is, but I am 98% sure it's in the sending unit, BUT I'M ON MY SECOND ONE! I ground out the wire going to the gauge, and it pegs out, almost immediately when I turn the key on (takes 2 seconds to move that far), and goes down when I take the wire off, but once it gets down the the beginning of the "normal zone" it slows down, but still goes back to cold. I turn the key on with the wire hanging in the air, and nothing happens, so I think the gauge is fine.

I am completely at a loss, I don't know what to do! I'v tried 2 sending units, I've isolated them to be the issue, or I think I have. Am I missing something? Should I try to get an OEM sending unit? I would think the OEM Motorcraft unit would be the same, if not similar to the Duralast. I'm getting tired of wasting coolant on taking the sending unit out, so I'd like to try everything I can before taking it out again.

Thanks for all the help! You all have helped me solve many problems on this truck, I hope I can help someone else in the same way someday.



The old sending unit, she was a good one!


Edit:
I was thinking, and are the temp sending units different between models with oil pressure, and charge gauges, and ones without? I know all models had fuel and temp, but I wonder if they changed between gauge setups. Also, here is what I was told by my grandfather (who is a tractor expert, we have a tractor dealership and have had similar problems): On some tractors, they have Delco gauges, and some have another brand, but with each brand gauge, there is also a different brand sending unit, and you have to have the same brand unit as the gauge, as they are calibrated for each-other. Could that be the case here? I've never heard of ford using different gauge brands, but I'm really not sure. I would think this is a problem with the sending unit, because of the way it acts when testing it, but again, I don't know.
 
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Old 02-17-2019, 03:29 PM
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Sounds like you might have a short in the sending unit wire grounding out somewhere along the way. Sounds as though it's only grounding out when it's hooked up, so check where it may be rubbing the block or head on the back side. That's all I can think of right off.
 
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Old 02-17-2019, 03:31 PM
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I thought of the same thing. The connector on the end of the wire was the only place that was showing any wires, so I taped it off, and that didn't change anything. The rest of the cable has plastic wire loom around it. Thanks for the idea though!
 
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Old 02-17-2019, 03:35 PM
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Could the wire be pinched behind the dash somewhere? How far down the terminal does the connector go when it's on the sending unit. Is it maybe grounding to the unit itself?
 
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Old 02-17-2019, 03:52 PM
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Where in the world did you get that testing procedure? What is it telling you? Hooking the sending unit in series with a testlight? That may be burning the units up. I do no know how much current a temp sending unit can stand, but the testlight may be too much for it.

The standard procedure is to take the wire going to the sending unit, unplug it from the sending unit, and with the key on, engine off, ground the wire while watching the gauge. Then unground the wire. Each time you do this, the guage should swing back and forth from hot to cold. If it passes this test, you know the wiring and the gauge are good.

The only way I know how to test a sending unit is to get a ohmmeter, and you need a temperature chart for that sending unit. Usually the factory manual has something in it. You know the ambient temperature when you pull the sending unit out, so you take the phmmeter and read the resistance between the threads and the wiring post. The resistance should jive with the resistance shown in the temperature chart for the approx ambient temperature during the test. You can also install the sensor, warm the engine up till the thermostat opens, and you can assume that is going to be close to 200 degrees. While it is still in the hot engine block you can take your ohmmeter and read the resistance between the block and the wiring terminal, it should jive with the chart for the neighborhood of 200 degrees give or take.
 
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Old 02-17-2019, 04:01 PM
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I just was reading page 100 in this link. Fuel Tank Selector & Gauges - Gary's Garagemahal (the Bullnose bible)

I did not know this, but apparently according to that all the gauges are the same. They react the same to the same resistance as the fuel gauge does. So apparently they are all the same, just different graphics on the front of the gauge. So according to that page I just read, 10 ohms is high, 73 ohms is low. So when testing the temp sensor at normal room temps, it's probably going to read in the high 70's. Reading it when the thermostat opens it 's going to read something inbetween 73 and 10.
 
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Old 02-17-2019, 05:05 PM
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Thanks Dave, I just assumed that testing procedure would work, I did not consider the resistance part. I only "tested" the units after I put them in and they didn't work (or didn't fix my issue). I grounded the wire, and the gauge went back a forth, so the wiring and gauge are good. But i'm still not sure about the units. I will get my ohm meter and do some checking tonight. I currently have another issue that just came up (while I was on my first test-run), that needs attending to, but as soon as I fix that, or while I'm waiting on parts, i'll check my sender. If it's bad, do I just replace it again? If the resistance is wrong, what do I do to correct it? We had a machine that the gauge read overheated, when the engine was a operating temperature, and we put an inline resistor (the factory manual told us the needed resistance), and that fixed the issue. Can I do the same here?
 
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Old 02-17-2019, 07:44 PM
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I have heard of some doing just that add a resister.
What do you need to fix first?
Dave ----
 
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Old 02-17-2019, 08:17 PM
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I was going for the first test run and did a pull from 0-40 ish. After that the engine wouldn't idle, and acted like a huge vaccum leak. Took me a bit to find it, but the carb apparently had some of its old gasket still on it. After cleaning it off and putting her back on, everything is fine now. I'll do some reading into my manual. I also got another sender, just incase I burnt this one up while I was "testing it" either it will fix the problem, or I start checking to see what resistor I need. Or, check the resistance of the sender, if it's fine, then my problem is somewhere else. I remember there being a resistor (or something similar looking) attached to the prited circuit on the back of the instrument cluster, but I cannot remember where it was, or what it was for.
 
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Old 02-17-2019, 09:07 PM
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The only thing that I know of on the back of the cluster is the IVR.
This supplies like 6 volts to the oil, gas, temp gauges. do they all read / work right? If not the IVR may be bad.
Dave ----
 
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Old 02-17-2019, 09:58 PM
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Do you mean the voltage regulator? It has to connectors where the printed circuit board pop on. I meant a little...bar-like thing that is around 2-ish inches long, and screwed in with 2 small screws. I think it was behind the charge light, but i'm not sure.
The voltage regulator was replaced a year or 2 ago, but it could be bad again. The fuel gauge isn't working (it hasn't worked at all, until I put a new LMC printed circuit in, now it shows 1/3 or a tank all the time). The oil pressure light doesn't work. It keeps blowing the fuse for the warning lights (even with the fuse blown, the charge light still works so idk) so I guess the positive feed wire for the oil pressure light is shorting somewhere behind the dash... that's a furture project I already had lined up, and am dreading. Will probably try replacing the voltage regulator if this new sending unit doesn't work.
 
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Old 02-17-2019, 10:43 PM
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Originally Posted by beatboxingboy View Post
Do you mean the voltage regulator? It has to connectors where the printed circuit board pop on. I meant a little...bar-like thing that is around 2-ish inches long, and screwed in with 2 small screws. I think it was behind the charge light, but i'm not sure.
The voltage regulator was replaced a year or 2 ago, but it could be bad again. The fuel gauge isn't working (it hasn't worked at all, until I put a new LMC printed circuit in, now it shows 1/3 or a tank all the time). The oil pressure light doesn't work. It keeps blowing the fuse for the warning lights (even with the fuse blown, the charge light still works so idk) so I guess the positive feed wire for the oil pressure light is shorting somewhere behind the dash... that's a furture project I already had lined up, and am dreading. Will probably try replacing the voltage regulator if this new sending unit doesn't work.
Sounds like all your issues come together in that one spot.

 
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Old 02-18-2019, 07:27 AM
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Originally Posted by beatboxingboy View Post
Do you mean the voltage regulator? It has to connectors where the printed circuit board pop on. I meant a little...bar-like thing that is around 2-ish inches long, and screwed in with 2 small screws. I think it was behind the charge light, but i'm not sure.
The voltage regulator was replaced a year or 2 ago, but it could be bad again. The fuel gauge isn't working (it hasn't worked at all, until I put a new LMC printed circuit in, now it shows 1/3 or a tank all the time). The oil pressure light doesn't work. It keeps blowing the fuse for the warning lights (even with the fuse blown, the charge light still works so idk) so I guess the positive feed wire for the oil pressure light is shorting somewhere behind the dash... that's a furture project I already had lined up, and am dreading. Will probably try replacing the voltage regulator if this new sending unit doesn't work.
Slow down a bit here.
So you only have a temp & fuel gauge and lights for everything else?
What year truck is this and what year LMC printed board did you get?
It had 1 bad IVR (yes gauge regulator) why was this replaced and did it fix it?
What have you done to look into the fuel gauge issue? If you disconnect the wire at the sender what happens?

Originally Posted by Kramercd View Post
Sounds like all your issues come together in that one spot.
I am starting to think the same thing being the temp & fuel gauges don't work.
Dave ----
 
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Old 02-18-2019, 09:32 AM
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Okay, here is the story of this truck and all her electrical issues, get ready, it's a long one.

To clarify, I have a 1984 f150 (was originally a fleet truck with a radio delete, thank you Marti report) with a 4.9L I6, converted to DSII, with oil pressure and charge warning lights, and no tach. I've owned this truck for 3 years, since I was 13 (she was/is my first truck).

When I bought the truck, the fuel gauge and A/c didn't work. We find out later that it had some form of dealer installed A/c, with a rare compressor (I've never seen one like it since) and it wasn't wired up correctly. We wired the A/c compressor to a toggle switch inside the cab (the high pressure switch is still in the line, so when the pressure gets too high, it still cuts off the compressor) and the separate blower motor to a separate switch as well. This was done 3 years ago. I would like to (at some point) re-wire the A/c to work as it should (through the panel for the heat), but I just needed it working then, and it does. I wasn't very mechanically inclined (I didn't have the know-how, or space to work on this truck yet, I was 13), so I took it to a good mechanic friend that does all the work on our vehicles which we can't do ourselves. I told him about the fuel gauge, and at that time the gear indicator in the instrument cluster wasn't working either. He called me back and told me to order a new indicator, as the cable was broke, and couldn't figure out the fuel gauge problem (it always showed empty). We fixed the indicator and a few other small things, and I drove it for a while. I brought it back with carb issues, and said take another look at the fuel gauge. He then said that it might be the voltage stabilizer on the back of the cluster, so I ordered one from LMC (they and Autozone were my main parts vendors, I have started expanding now) it was only $25 so it wouldn't hurt incase that wasn't actually the issue. We replaced it and that did nothing, he checked the wiring between the gauge, replaced the sending unit with one from Autozone, swapped it out with a used one, and nothing helped. He wondered if a genuine ford sending unit would help, but I didn't really care THAT much, so I just said "forget it" and left it there. Fast-forward to around Christmas this year, after DSII conversion, and now my oil pressure light issue. I noticed that when I would turn the key on, but not start, the charge light would come on, but not the oil pressure light like it normally would. I had taken the dash bezel out before to replace a headlight switch, so it wasn't a huge deal to me anymore (like it was 3 years ago). I replaced the bulb (even though it didn't LOOK blown, I was there and had everything apart, figured I might as well replace it. It didn't help. I, being the noob I still was, did some more checking, and noticed that the copper pads on the circuit board were scratched up fairly good, and noticed that where the connector for the entire cluster plugged in, some of it's pads were coming loose, and about to fall off. I knew LMC sold a circuit, so I checked and it was only ~$70. (I don't have the oil pressure, and charge gauges or tach, I have the lights and no tach) I get it in, install it, and it doesn't fix the light. I do notice that the fuel gauge will now move though, as I hope it might be helped my the circuit...YAY! I didn't know how much fuel I had in the tank so I didn't know if it was accurate or not. But I kept working on my oil light. Before, I had my Haynes manual open, and noticed that the oil pressure light circuit went to the fuse panel, but according to it's key, there was no fuse that sounded close to a warning light (my original fuel panel cover was missing, I later found it way under the seat). I checked the dash lights fuse ( guess it was the back-lights), but I cannot find anything else. Back to after replacing the circuit. I check my wiring diagrams again, and see that the wiring also goes through the ignition switch. I replace it, and it helps with my key problem (the key would get stuck in the start position, and I would lose accessory power). I was at a loss. I dug around more, and found my fuse panel cover and saw the "warning lamps" name and I felt like a complete first-class idiot. I replace the fuse and sure 'nuff, it works! I again feel like an idiot. I turn the key off, put my stuff up, come back and turn the key on, and the light doesn't come on again. I check the fuse, and it blew! I replace with a fuse 5 amps higher, and it blew again! So at this point, I think I have a short behind the dash, and just give up (my shop was being built, and I figured I would tear the dash out when it was done, if I had the time and didn't need the truck). I then have an issue where the temp gauge pegs out, almost immediately after I turn the key on (it is now mid January). I assume it's the sender (thought the one on the thermostat housing, but later found out that the one on the housing was for the computer, which I had taken out of the picture). When I find out that it's the one on the back of the engine, I decide to wait until I get my engine running good again (I had just had a spark plug blow up on #2 cylinder, and mess up the intake valve, but I didn't know that yet). After I figure out my running problem, and take the head off. I decide that since I have great access to the sender, I'd replace it since it was TOAST, so I do. I get everything back together, turn the key on, and BOOM it pegs out again (by pegs out, I mean goes completely to the right side of the gauge). I "check" the sending unit some with a test light (I thought I was doing the right thing, but find out later I probably hurt the sending unit more). I ground out the wire to the gauge, and it goes full tilt again, take the wire off of ground, and it goes back to cold again, although it moves much slower as it hits the beginning of the "normal zone" on the way back to cold. I replace the sender again, thinking that I got the "lucky one" and it doesn't help. Fast-forward to today. I order a voltage stabilizer (I was ordering from amazon anyway, and it was $25), and have another sending unit in my hand. I'm going to swap the unit, in case I murdered the one in the block with my testing methods, and then replace the stabilizer, and maybe that'll fix it, or help it. While I'm at it, I'll swap out the warning lamp fuse, as maybe the stabilizer is sending too much power, or shorting out the circuit for the oil light, and therefore blowing the fuse. Maybe, just maybe, it is also hurting my fuel gauge. With my luck though, it probably will only fix one out of the 3 things, and I'll be back at this again.

I am also going to compare the traces on the LMC board, and the original board, when it take the cluster out, I am 99.95% sure I got the right board, as I studied on it for a while before ordering, but I could be mistaken. The LMC part# I got was 49-6280.

Thanks for the help guys.
 
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Old 02-18-2019, 10:48 AM
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Based on LMC site that is the right part number for warning lights and NO oil or ALT gauges.
I am going to go off the path for a bit.
Have you pulled all the grounds apart and cleaned them?
If like my 81 should have NEG battery cable to frame by motor mount bracket then down to a starter bolt. This should ground the frame & motor but NOT the body.
I also have a heavy gauge wire from the motor left side to the firewall, this is the body ground.
My wiring harness has think 3 or 4 wires also screwed to the firewall close to the motor / body one. I don't know what they are for but has to be for something or it would not be there.
Also at the front of both fenders that pickup the head & parking / turn lights.
Now I keep seeing it posted there is a bunch of wires to a ground screw on the inside cab side of the firewall like behind where the radio would be but I don't remember any there on mine.
Bad grounds can do crazy things.

This blown fuse it was ok, key on and did not blow till you put the cluster back in and turned the key on.
If so could a screw be going into the harness? A bad ground back feeding can also be a cause.
That blown fuse for the charging bulb? If so when it is blown does the truck still show charging with a volt meter on the battery?
I wonder if there is an issue with the charging system and that keeps blowing the fuse? We can look into that later lets look at the temp & fuel gauge after you check & clean all grounds.

I know you tested the temp gauge by grounding the wire. If the fuel gauge is not working can you do the same test, ground wire gauge should go to full and drop back to empty when not.
This like on the temp wire would shoe if the system from gauge back thru is good and working.
A little harder to test but using an ohm meter could take a reading on the fuel sender and move the float up & down or if you know how much fuel and the range of the sender figure if it is working or not.
Another way to test the temp & fuel sender wires, A test light should blink when put between sender wire & ground or a volt meter the same way and get I think 6 volts.

Should give you some things to check.
Dave ----
 

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