98 ranger 2.5L 5spd 2wd

Shortly after having a new timing belt put on my truck (mud got stuck behind the timing covers and caused the belt to snap) i noticed that the truck was running cold after driving 35 miles at highway speeds with the A/C on. the temp gauge was reading below the normal range. i thought at first that truck was over cooling but when i put the heater on it blew hot air.

My question: How can i test the temperature sending unit, the gauge, and the associated wiring? i had the factory service manual but i can't for the life of me find it. i ve tested Honda sensors with the multimeter, thermometer and hot water but i had a chart stating temperature and corresponding voltage.

Help i hate not knowing what's going on with my truck.

Thanks
Guy

 

Gauge temp sending unit should measure, approx 75 ohms full cold, 10 ohms full hot.

To check the gauge, disconnect & ground it's lead to the sender through a 10 ohm resistor & it should indicate about mid scale where it would normally read when warmed up.

 

pawpaw lemme see if i got this right.

disconnect the sensor. put one jumper wire between the connector and the incoming power and the other jumper wire (with a 10ohm resistor) in line between the sensor and the connector ground terminal.

The gauge should then read full hot as in "the car is overheating" or full hot as in the car is running at its normal "hot" temperature.

thanks for the help Pawpaw

 

How can 10 OHMS be full hot, and mid scale???? If you ground the temp sending unit lead, it should go to full scale (HOT)

 

i am thinking what bob is saying. i have always just grounded it out and if it when full peg than the sensor is working

Matt

 

I just checked the instrument panel schematics for my 1999, and it says 16 OHMS for HOT, and 275 OHMS for COLD are the specs on the temp sending unit.

 

What I was attempting here was a simple way to test & isolate your percieved problem.

Well what I was intending to suggest, was to remove the temp gauge lead from the temp sending unit, then connecting the temp gauge lead to the engine block, through a 10 ohm, or so, (close value) resistor & see if the gauge will move upscale, off it's cold setting, towards where it normaly runs.

Grounding the temp gauge directly to the block could slam the meter movement, so I was just suggesting you be kinder to it, by grounding it through a resistor, rather directly to the engine block, to prevent slammng the meter movemen & also to get an idea if the gauge is working ok.

The resistance values I posted for the sending unit were values from my repair manual, for the (temp gauge) sending unit.

Maybe my manual has a misprint for the sending units cold resistance value, as it seems it doesn't jive with Bob's numbers, although the hot numbers are fairly close.

Rockledge caught a misprint from this manual for me last year, on oil pressure, so it wouldn't surprise me if there is another misprint error.

Any way, ground the wire through say a 10-20 ohm resistor & see if the gauge moves off the cold value, up sacle, if so, it's likely ok & thats what your wanting to know I believe.

Now to test the temp sender, with the gauge wire removed, measure the senders resistance to ground, with the engine fully warmed up & it's resistance should be somewhere around the values Bob & I've posted, say in the 10-16 ohm range. If it's much higher than that, the gauge won't read upscale as it's supposed to, so the sending unit is likely bad, or it's ground connection through it's threads isn't good, maybe corroded.

Let us know how it goes.

 

Pawpaw, if I remember correctly, half-scale on the temp gauge corresponds to a sensor resistance of about 100 OHMS, which should be a normal temp.

 

Ok good input Bob, then it seems to me, if he grounds the gauge lead through a 100 ohm resistor, it should read about mid-scale & then he'd know if the problem was with the gauge, it's wiring, or the sending unit, if it's resistance is considerably higher than 100 ohms.

Seems to me Rockledge just posted a temp sending unit temp/resistance profile, in the past week or two, on another thread in this forum, but I've not located it yet.

I had itended to book mark it & print it out to post in my manual, because it had so much good detail, thats lacking in my manual & ths question is ask so often, but got distracted & forgot to do it, then couldn't find the thread again.

Gettin old s a bummer sometimes!!!! lol

I'm gonna do a search right now & see if I can find & link it.

On Edit: Well what I thought I remembered wasn't for the gauge sending unit, it was for the Computers engine temp sending unit.

SO I won't post a link, lest someone would get the two confused!!!!

 

Tell me about getting old!!!!! lol!

Yes, Joe (Rockledge) is a wealth of information!! Just for some electronics trivia, a 100 OHM resistor color code is BROWN-BLACK-BROWN-(gold or silver)

 

so grounding out the circuit will cause the gauge to read full hot.

what the best way to do this?

my first thought was to just throw a jumper wire in the connector while i have someone watch the gauge. Or is there a better way?

how long can i leave it grounded before it causes any damage? it seems to me that by throwing a jumper in the circuit would blow a fuse

Thanks again if i get some answers in time i get do this Sunday seein as its really my only free time. whatever happens i'll come back here and give ya an update

Guy

 

Well the best way, may be to measure the sending units cold & hot resistance values, that Bob posted.

If the cold, or warm resistance is higher than those numbers, your problem is likely with the sending unit, or it's ground through it's threads & sending units are more often the problem than a faulty gauge.

If you decide to ground the gauge directly to the block, without a resistor in series with the wires connector, have someone watch the gauge & do it quickly as you suggested.

I don't know how long you could leave it connected directly to ground, before it would damage the meter movement, or even if it would damage it.

Maybe Ford already has a current limiting resistor in the gauge circuit, to protect the meter movement, but I doubt it, as they don't like to spend another ten cents if it isn't necessary!!!! lol

I really didn't mean to confuse this issue so much, with the resistor in series, with the gauge lead, idea.
I was just trying to suggest a simple & kinder way to check the meters movement, without slamming it, by grounding the lead directly to the block.

Using a jumper lead, from the engine block, to the sensors wire, is the same thing as grounding the wire directly to the block. Both are direct connections to ground.

However, if you insert a resistor of the proper value in series with the gauge wire ( directly between the connectors electrical connection & the engines block), it'll simulate the sending units resistance for the gauge & if the gauge & it's wiring are ok, it'll move off "cold" & indicate up scale.

Bob says his repair guide shows a 100 ohm resistor, in series with the gauge's connector, to ground, should cause the gauge to read about mid scale.
Then removing the 100 ohm resistor & inserting a 16 ohm resistor in series, should cause it to indicate around the "hot" reading.

SO, between testing the temp sending units cold & hot resistance value, with your multmeter & simulating a temp sending units warm & hot resistance, with the proper value fixed resistors, you should be able to check the temp gauge & thereby isolate your low temp reading problem.

So let us know what you find.

 

i'm Sorry about not getting the results to you guys. Last friday i got a promotion at work from assistant to Technician (i work at a honda dealership here in miami). So instead of having all Sunday to work on this temp. sending unit thing i spent most of the day "Investing " in tools and getting my small existing tool set gathered up for work the next day.

But don't work the problem will get fixed and will let you know how it went.

Thanks for you Guys Help


Guy