Preface: I know that many consider the factory gauges useless but I hate to have inoperative or unreliable stuff so please bear with me...

Since December, the dash indication never moves past (right of) the 'N' in Normal under any driving conditions. It will indicate a colder condition, i.e. coasting downhill, but never hotter, i.e. prolonged power uphill.

The gauge indicator wire and overtemp (light) annunciator wire are both connected to their respective sensors.

When the coolant heater is plugged in and the key is turned to RUN (before start/GP warm up) the gauge will move from the full-scale left deflection to indicate ambient temperature and then stay in the same area after start and for the rest of the drive.

After driving if I put my hand on the radiator it's hot. I won't have a thermal temp sensor until late this week but it's hot enough not to keep you hand there for more than a second.

A new Motorcraft thermostat was installed by me after a coolant service.

If I disconnect the gauge indicator wire and stick a paperclip in receiver and touch it to the engine block, the temperature indicator does NOT move to the hot position with the key in RUN; it will read about ambient coolant temperature. I didn't start the engine, just key in RUN.

The fan clutch has a little drag in it. It does not spin freely but is not unmovable. If I tried to spin it, I would get about one fan blade rotation and it stops. It spins at startup and idle but I never looked at it at higher RPM conditions.

Appropriate amount of heat in the cab.

Any ideas or other areas for me to consider?

Thanks in advance.

 

Im not sure what range the factory temp gauge operates in, but whenever i have a temp gauge issue i grab a new sender. They dont last forever, and usually when they start getting bad they read low,lower, lower until they dont read at all. If that doesnt fix you up, snag a gauge out of the junkyard. I assume you have traced the wire for any obvious frays / breaks.

 

Yes, I opened the loom containing the two wires (gauge and light) and traced them as far back towards the firewall as I could and they were in good shape.

I assume the bottom of the sender/sensor has direct contact with the coolant. Can I do a quick component swap without having to flush any coolant? There's no thread sealant or other stuff on the threads, just metal to metal, similar to the tach sensor connection correct? I would plan to thoroughly clean the exterior as best I could beforehand.

 

Just be quick, and you can pull it and instal with minimal coolant loss, for some obvious and other less obvious reasons, dont do this while its hot. Make sure the system is not pressurized when you go to do it. The sensor is brass, so should seal fine, but a little teflon tape never hurts anything.

 

LOL be fast and do not open the radiator cap or water will really be flowing, I just changed mine and the pressure made me loose grip on the sensor never to be seen again . I got mine from Autozone if you have one around you, part number TU25 with sealing tape already on it and the gauge works great now, before it was not reading at allllllllllll....

 

Haha. It looks like I have a Monday project before more snow. I see Napa down the road has one...part MPE TS6628SB. The local Autozone has a bunch of cheerleaders working there more interested in updating their status than working with customers, but thank you for the part number confirmation.

 

You can check the gauge by turning on the key, you don't have to start it, grab a piece of wire and put it in the plug and ground it and the gauge should move all the way over to the H marking..

 

I tried that today and the gauge didn't move to the high side. But I tried it with a paperclip (all I had - away from home). I may give it a shot with some 12ga wire when I get home.

ETA: Unless there is something in the cluster blocking the movement of the gauge needle or problems in wiring I can't readily see, I would think that the gauge is alright and just indicating the faulty temperature the sensor is giving. If the gauge was bad I wouldn't expect any movement out of it. I hate to throw parts at anything but for under $10, it's reasonably worth a shot. I'm sure the sensor is original.

 

Sounds like you have a bad connection in between the sender and guage, if the connections are corroded you may get some movement but not full sweep due to resistance. First I would pull the dash cluster out and clean the connections on the back of the gauge. I had corroded connections on my 83 F150 that caused the fuel gauge to be inoperative. I pulled it apart, cleaned all connections with a copper brush, and applied electrical grease. It fixed the problem and has been working great for the past 13 years.

Hope this helps

Bill

 

It may also be something blocking the movement of the gauge. Pull it out and make sure you don't have a dead bug in there.

 

when properly functioning the cooling system is very good, and can take some time to heat up, my van is just warmed up by the time I get to work, (about 12 miles) so on very cold days I block 30% of the rad with card board. it helps it warm faster. I remove it on longer trips. just past the o of normal is the warmest ever..

 

I give plenty of warmup time before I drive it and by the time I get in, the heater is usually blowing warm or hot air already at idle and once I drive it I have to turn the blower or temperature down because it gets to be a sauna. I'm going to troubleshoot that wire with a more acceptable wire to try to get the gauge to move full scale hot. Pulling the cluster is really a last resort for me because a light always conveniently burns out on me after past removals...

 

That gauge needs to peg, or close to it, on the hot side when the sensor wire is grounded. (Key on). If it does not there is something wrong with the gauge or the wiring to it. Until you fix this anything else you do will be pissing into the wind.

 

Is there a trick to get the cluster out? I removed the bezel, removed the screws, but I can't get forward movement of the cluster by more than about 1". I don't want to force it and risk anything worse...

 

I don't know if the '91 still has a speedometer cable, but if it does (and it sounds like your problem) then you have to reach behind the cluster and disconnect it before you can move the cluster out.