I just bought an '88 F-250 4.9/M5OD. The temp gauge was reading very low and, not knowing what was done to it before, I replaced the thermostat. It had a 195 in it and I just put a new 195 in. It's got heat and the truck runs fine.

I pulled the temp sender wire off (the one way back on the block, underneath the manifolds) and the gauge reads below "C". I grounded the sensor wire, turned the key on briefly, and the gauge went to "H".

I measured the resistance of the sensor with a cold engine, ambient temp ~95F: 240 ohms. I ran the engine for a few minutes and checked it again: 150 ohms. The Haynes manual shows 177 ohms at 212F and 2.4K ohms at 86F...way different from my readings. And off the wrong way. I went to Autozone and measured ~350 ohms on their Duralast brand temp sensor; it was probably 70F in the store. Their sensor wouldn't fix my problem. I took the instrument panel out and measured the gauge supply voltage and it was battery voltage. I also measured the resistance from the gauge sensor connector to ground with the sensor wire attached to the sensor and it was within 5 ohms of the disconnected sensor reading.

So I took the gauge cluster out and hooked it up to a 12V supply and a pot. This is what the gauge is showing (N, R, and L are the letters in the middle of the gauge: NORMAL.)

C: 32 ohms
N: 20 ohms
R: 18 ohms
L: 14 ohms
H: 8 ohms

Bad gauge or maybe the Autozone sensor - and the sensor in the block - are the wrong ones for this truck? I saw TSB 95-13-2 that shows it 74 ohms cold, 9.7 ohms hot. It would be much closer than what I have now. But if the Autozone sensor is what I'd be getting at an aftermarket store, I guess I'd have to buy one from Ford. I'll measure one at a NAPA store to be sure. Anyone run into this problem? What sensor fixed it?

 

I caught myself answering this post with no useful info. Sorry.

 

My temp indicator recently started to act up; it sometimes stayed low, only moving up a needle width or two, but the truck heater felt like the engine was at normal temps…. but then I would look at it a few minutes later and it was close to my “normal” reading or maybe a needle width higher.

I did the usual tests: grounded the sender wire, replaced the push on wire connector with another and cleaned connections. I had a spare used temp sender I connected up and used a cig lighter to heat it for a static test. Before I actually got around to changing the temp sender on the engine block it “self healed” and the problem went away. I now suspect the thermostat was sticking open a little or had some crud on it to limit its ability to close tightly. The truck is my daily driver.

On my truck the temp gauge always stays just to the left of the N in normal, if it gets above that point I know the thermostat is running wide open. That is really all the useful info the gauge can provide accurately. I don’t know what temp my thermostat is, might be a 180 deg.

You never did say what position the needle was in, just said it was reading “very low”. So where is the needle at with the truck warmed up?

Regarding you testing with a 12 volt supply--the instrument cluster has a built in Voltage Regulator (VR) that drops the voltage to around 6-7 volts. The voltage regulator works like a blinker (can) switch and produces a square wave, 0 to supply voltage, with the engine not running it would be 0-12v or so. And, like a turn signal blinker, it needs normal current flow to work correctly, meaning if one of your turn signal bulbs is burnt out the other one just stays on and doesn’t blink. I am not sure how you could bench check this as it would depend on what a “normal” duty cycle of the instrument VR should be.

I think you might be over analyzing this, if you need or want an accurate gauge you will have to switch to a capillary tube style of gauge.

Amazon.com: Equus 8442 Water Temperature Gauge: Automotive


Or just add some resistance in parallel to the sender to put the needle where you want it to be.

Jim

 

Jim, you're right, I should have said how the gauge was acting. It never gets to the "N" in NORMAL. And it's erratic, going from below "C" to almost "N", back and forth while driving down the road at highway speeds. But the truck runs fine and I'm pretty certain that the cooling system is working fine too. No problems other than the damn flaky gauge reading.

There are three terminals on the back of the gauge. Measuring the sockets in the instrument panel that those terminals fit in, I get battery voltage, ground, and the temp sensor resistance. I haven't put a scope on it but I'm reading zero volts AC between ground and the battery voltage connector. Are you saying that the two-gauge cluster (temp & voltmeter) has an internal voltage regulator (maybe a switched-mode power supply)? The cluster itself seems to have just a normal, battery voltage input. I hope so because otherwise the battery voltage I'm measuring on the instrument panel gauge connectors should really be a 5 or 6 volt square wave. I assume that the voltage regulation is internal to the gauge or the cluster.

I wish I could put a resistor in series with the temp sender. The problem is, I have too much resistance; high resistance = cold gauge. Making matters worse, I checked three senders at NAPA, two at Checker/O'Reilly, one at Autozone, and even went to a Ford dealer, and all read ~ 300 ohms at room temperature. Checker didn't have a third sensor that they list, Motorcraft #SW2328, which crosses with Ford #F1SZ10884A, the sensor listed on the TSB (and the sensor I checked with my ohmmeter at the Ford dealer.) But they got the Motorcraft from a local warehouse and I'm about to go pick it up.

I hear you on buying an aftermarket gauge. If it were just me driving the truck I would do exactly that. The problem is, this truck is going to be driven by lots of people, my elderly parents included, and I want it bone stock. The truck will spend most of its time sitting at a home I bought for my parents for them to spend their winters and it's an eight hour drive away from me. It will also be driven by brothers/sisters/inlaws...people who know nothing about cars. I just want the stock gauge to work.

I'm gonna go pick up that sensor at O'Rielly's now. Thanks for your help, I appreciate it!

 

A resistor in parallell like Jim said would give less total resistance (if my memory serves). HIt up radio shack for a cheap rheostat to try it out. I'd first look into the voltage reg, it attaches to the back of the cluster I believe.

 

The 3 terminals on the gauge got me thinking so I reread your first post and you have an 1988 truck, I just assume it was an 80-86 at first as that is the years for this forum… so what I said was based on an 80-86 inst cluster which only has 2 terminal on the temp gauge, power and GRD through the engine sender.

Here is a picture of the back side of my 86 inst cluster. The VR is just below the 2 top right black removable bulb holders. Yours will look different of course.
More fixit ideas in my next post. Jim
.
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So the add a “resistor in parallel” to lower resistance won’t work if the issue is an erratic gauge reading. It looks to me like the gauge is just losing it ground path through the sender… is the sender installed with Teflon tape covering the threads? To trouble shoot the ground issue just add a ground. It is the same P/N engine sender on 75-96 engines.

I don’t know when Ford gave up on the idea that our trucks would go back to a 6 volt system, or maybe they just ran out of 6 volt parts… but I would be surprised if your truck still had the VR on the back side of the inst cluster.

I also stock like the stock instruments idea, it is simpler and cleaner looking
Jim

 

Jim, your comment made me realize that I posted in the wrong forum. My truck is an '88 and the instrument panel is very different from the picture of yours posted.

Jim & Bashby: In PARALLEL! What a great idea! Thanks!

After running around to NAPA, Autozone, Advance Auto, the Ford dealer, and finally O'Reilly's/Checker, I found a sensor that's better. Borg Warner part #WT386. But since the dash is in pieces, I haven't been able to take it out on the road yet to see if the gauge is going to act erratically. At idle it seems to work fine.

With the old sensor the 195F thermostat would open and the gauge needle would be between "C" and "N". Even on the road in 95F weather, the needle would never get to "N". With the new sensor, the WT386, the needle is right on "O" when the thermostat opens. Much better! Not perfect, but better. I did find teflon tape on the sensor I replaced and I didn't use it on the new sensor.

With all that running around I found that none of the parts stores nor the Ford dealer was giving me the right part. The Ford part is F1SZ-10884-A and it reads over 300 ohms at room temp. Too high. Same with NAPA, which lists three different sensors, and Advance Auto. A sharp guy at O'Reilly's spent some time and crossed a Motorcraft #SW-925 (which was discontinued) to the Borg Warner part (BWD) #WT386. On one diesel Ford site (F250 4BT Buildup) it shows that the WT386 has 24 ohms resistance at 220 degrees. They also show another sensor that would fit, a WT324, with 17 ohms resistance at 220 degrees F. This sensor would probably be the better choice but the only problem is that it has a bigger 10-24 stud for the connector; the WT386 has an 8-32 stud. The sensor is such a pain to get to...I don't think I want to adapt the connector to a bigger stud. With the WT386 at least the gauge is reading in the normal range. And who know, tomorrow I might scavenge some pots of a an old TV to see if I can easily crank the needle up a little bit more. It would be easy enough to wire one in parallel to the sensor wire behind the dash.

Thanks again.

 

Roader and other experts, what do you think about this?

In my '89 Ranger, the temp gauge never has been good, with the needle lingering near C. I recently decided to work on this, and I found a manual indicating that the gauge expects 73 ohms cold (or maybe 100 degrees F) and 9.7 ohms hot. After various tests of the gauge, the power to the sender and the sender's resistance, I bought a new sender, BWD WT7245 from Advance. It wasn't much help, going down only to 56 ohms with the truck warmed up idling.

So I got creative and soldered a 68-ohm resistor into the back of the instrument cluster to reduce the resistance on the feed to the gauge. The needle started at a less-than-ideal position above the C but below the "NORMAL" range bar on the gauge. After I drove around for a while, it went up to the "N" in "NORMAL." This was encouraging, but I suspected that the new sender was worse than the old one. I tested the two side by side in a pot of hot water. At 104 degrees F, 180 degrees F (maybe close to the thermostat setting) and 204 degrees, the old sensor had 13 to 9 ohms less resistance, so I put it back in the truck.

With the old sender and the 68-ohm resistor still in place, the needle started at the bottom of the normal range bar -- understandably higher than it had started with the new sender. I drove around. I was encouraged when the needled crept up past "N," then proceeded through "O" and "R." I hoped it would stop on "M," but ... no! It slowly made its way all the way up to the red warning marker below H, past the normal range bar. I don't see how the change in senders could have made the needle go so drastically beyond the "N," where it had stopped with the new sender. In my hot-water tests, the old sender had only 6 to 9 ohms less resistance than the new one near the hot end of the temperature range.

So, my options:

1. Leave the old sender in and disconnect the resistor, risking that I'll never get it resoldered, because my soldering iron barely melts the solder. Presumably that would restore the gauge to its original non-working state, but maybe my fiddling has improved some electrical connectivity.
2. Leave the resistor in and swap in the new sender, presumably going back to a maximum gauge reading of "N" in "NORMAL."
3. Leave the old sender in and change to a different resistor, greater than 68 ohms.
4. Buy the BWD WT386 sensor suggested here and probably use it without the resistor, despite the parts stores' warning that it does not "fit" my vehicle.

Your thoughts?

 

has to be oem coolant sensor or gague won't work