81f100custom Posted July 31, 2019 Share Posted July 31, 2019 First I will say you may want to check the resistance on the temp sender to see if it is actually running hot. So anyway for the past 30 years or so my temp gauge stayed around N or O on Normal and here just in the last few months it now wants to stay around the A and L of Normal. So me being lazy I guess for lack of another term did I get the manuals out and check anything of course not the gauge says its hot so must be. I have not changed anything on the cooling system in many years other than a water pump and the antifreeze so just figured the age of the components it must be the cap so got a new one and what do you know no change at all. Ok no problem got to be thermostat brand new one put in no change what so ever. At this point I get the manual out now and start reading up some on the cooling system. Along this time I get the infrared thermometer out with truck sitting idling at operating temp and start checking all over to see what the temps are running. The highest I could even get a reading was about 205 and had to really try and find that. The thermostat I put in was a original temp Motorcraft 192 degree. All around the sender I was reading 186 to 197. The top radiator hose right out of the thermostat reading about 186 or so and going into radiator about 168. Bottom hose out of radiator back into motor right at 140. So me being an electrician by trade I understand the wiring pretty well and the book says check with a 10 ohm resistor for the hot side and a 73 ohm resistor for the cold side of gauge. So I just happen to have a decade box so no problem I dial up my 10 ohm and 73 ohms and the cool side is almost perfect the hot side seems to go a little father than the manual says it should be not to bad. So no problem right must be the sender if it reads good with the correct resistances. Do I keep checking like I should of course not get a sender and put that in and what do you know it reads right at the edge of normal like it has been. Ok start digging in further and figure out on the decade box that around 38 to 40 ohms put my temp needle right where it used to read. I come home from work today after driving the truck 30 miles do not shut it off pull the temp lead wire off put ohm meter on sender and what do you know it reads 37 or 40 ohms sitting idling with the ac on also so that is perfect. I hook the decade box back up to temp lead wire set and 38 ohms and the temp gauge reads perfect and the lower end. Put temp lead back on sender that just read 38 ohms with volt meter and temp gauge goes to far right. Thought must not be grounded good enough so grounded temp sender straight form battery negative with jumper no change. Thought it was going to be the IVR or the gauge but reads perfect with the resistance like the shop manual states and also not having any issues with the fuel gauge either as it works off the same system. So my question is what am I missing here or overlooking? Link to comment Share on other sites More sharing options...
Gary Lewis Posted July 31, 2019 Share Posted July 31, 2019 I don't remember what decade boxes are rated for, but there's ~1/2 an amp of current going through the wire and sender at full scale. (As you know, I=E/R and the ICVR's average voltage will be ~5. So 5/10 = .5) Is your decade box rated at that? It has been a long time since I played with one. If not, you may be getting a false reading. I know that the power resistors I bought to use for instrument testing get HOT. Beyond that, I'm lost. I can't see why what is happening is happening. Link to comment Share on other sites More sharing options...
grumpin Posted July 31, 2019 Share Posted July 31, 2019 I don't remember what decade boxes are rated for, but there's ~1/2 an amp of current going through the wire and sender at full scale. (As you know, I=E/R and the ICVR's average voltage will be ~5. So 5/10 = .5) Is your decade box rated at that? It has been a long time since I played with one. If not, you may be getting a false reading. I know that the power resistors I bought to use for instrument testing get HOT. Beyond that, I'm lost. I can't see why what is happening is happening. I don’t know if this is the same. I just put an aftermarket gauge on my truck because my stock gauge is sometimes showing high, as in pegged. Running around on the flats the new gauge stays at 195°. The stock gauge had been acting ok after installing the new gauge. Drove over the mountains the other day and the new gauge got up to 205-210° (maybe higher, fuzzy memory suddenly) while climbing, and the stock gauge pegged all the way to the right. Funny, before I climbed the second pass the new gauge showed 195° and the stock gauge came back to the L in NORMAL, where it usually runs. The new gauge showed 205-210° while climbing and the stock gauge went up a little. Don’t know the history of my radiator, may change it. I don’t like the temp. going up while climbing through the mountains when I’m empty. My other gauges work good as well. HTH. Link to comment Share on other sites More sharing options...
81f100custom Posted July 31, 2019 Author Share Posted July 31, 2019 I don't remember what decade boxes are rated for, but there's ~1/2 an amp of current going through the wire and sender at full scale. (As you know, I=E/R and the ICVR's average voltage will be ~5. So 5/10 = .5) Is your decade box rated at that? It has been a long time since I played with one. If not, you may be getting a false reading. I know that the power resistors I bought to use for instrument testing get HOT. Beyond that, I'm lost. I can't see why what is happening is happening. Looking on the decade box I am using I only had to use one scale to get the 38 ohms I was looking for to drive the gauge and it is rated at 250ma or .25 amp. So 5/38 = 0.1315. If I recall the different scales have different ratings. Link to comment Share on other sites More sharing options...
Gary Lewis Posted July 31, 2019 Share Posted July 31, 2019 Looking on the decade box I am using I only had to use one scale to get the 38 ohms I was looking for to drive the gauge and it is rated at 250ma or .25 amp. So 5/38 = 0.1315. If I recall the different scales have different ratings. Since your box is rated for that current I can't explain what is going on. Given that, I'd put an aftermarket gauge on like Dane suggested. My favorite is a mechanical gauge since you can get them with a 270 sweep rather than the 90 degree sweep of an electric gauge. The wider sweep makes reading them much easier. But they are also harder to install as you have the long tube that has to go through the firewall. Link to comment Share on other sites More sharing options...
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