Temperature gauge not working

Gents, My temperature gauge is not working.  I read through all the other posts on the subject, and I'm still stumped.  My 85 4x4 with 300 I6 has had the smog delete, replaced the YFA carb with a YF, replaced ignition with an HEI distributor, and I have a mess of wires that don't go anywhere.

However, the temp gauge didn't work before I did all that, and I had previously checked the sending unit on the heater hose housing (fine) and removed the dash and checked the gauge itself (fine).  Called it a day and went on to other things.

Now that everything is running fairly well, I decided to tackle the gauge again.  Sending unit OK, but installed a new one anyway.  Gauge still OK (proper resistance).  I even traced the wiring from the sending unit through the firewall into the giant plug under the dash.  The wires have continuity.  I even checked the gauge from that same plug, and had the proper resistance.  But the gauge still reads dead cold.  Will not even move.

My sending unit has the two wires.  I know with a single wire unit you can ground it and see the gauge rise.  Should I be able to get a reading on the gauge from the sending unit plug?  If so, what kind of reading?

I also read on the forum that there is a sending unit on the exhaust manifold?  I put in new intake and exhaust manifolds, and didn't install anything in either one of them that was electrical.  I figured it was an oxygen sensor, so I didn't bother since I don't have to deal with the computer anymore.

Any help would be appreciated.  And, by the way, I'm going to update my signature with all the changes I've made in the truck.

The gauge itself is probably bad at that point. They aren't really a gauge anyway, just a dummy gauge with a needle that moves if it gets any kind of reading at all. Never trust the factory gauges.

Get yourself a real gauge.

Have you read the Troubleshooting tab here: Documentation/Electrical/Gauges?  Try the tests there.

But, a Bullnose sending unit for temp should not have two wires.  So you may have the wrong unit.  In the tests you'll see that the temp sending wire should be a red/white.  With the key on if you short that to the engine the gauge should go to full scale.  That proves the ICVR, gauge, and wiring are good.  If so, it is the sender.

And I'm assuming the fuel and oil pressure gauges work?  If not, it may be the ICVR.

Don't give up on the gauges.  While I run aftermarket gauges I also have my factory ones working nicely.

You have Sensor (for the computer you no longer have) and Sender (for the gauge) confused.

On a 4.9l the Sender should be found on the rear RH side of the cylinder head.

300 six temp sender looks to be on the passenger's side near the rear of the block.

I'm pretty sure that is the head, Gary.
The block starts below the exhaust manifold(s)

But it wouldn't be the first time the EVTM is wrong.

Jim,

The temp sender for the gauge is actually in the block. I was going to grab one from a 300 at the junkyard one day and remember seeing it there. Here's a random photo from the interweb...



You're 100% right about the 2-wire sensor up front. That one is for the EEC-IV system.

Gary Lewis wrote

Don't give up on the gauges.  While I run aftermarket gauges I also have my factory ones working nicely.

Gary, I agree on the stock gauges. They do work nicely if everything is set up and working correctly. After having some temp gauge issues due to an aftermarket sender I tried, I did a lot of testing with an IR temp gun, and the coolant temp gauge is quite useful. Same for the oil pressure gauge...since I had my engine broken in on a dyno, I knew my pressures cold, hot, and and idle, which all show differently on the factory gauge. They do have decent range. I've also had to replace the ICVR too.

Now, I'd agree that the DC gauge is all but useless lol, and the fuel gauge can be far less than linear, but with a lot of patience and screwing around, I got my new sender to behave quite nicely with the stock gauge. Took more work than it should have, but it was worth it in the end.

Cory,
I was admitting my ignorance, and saying that the '86 EVTM on this site may very well have it wrong.

I'm not too well versed in 300...  

ArdWrknTrk wrote

I'm not too well versed in 300...  

Neither am I, but I just happened to look at a 1983 with a 300 at the junkyard a few months ago, an both manifolds were missing so I had clear view of the temp sender. Otherwise I wouldn't have known.

In any case, I'm anxious to hear the next chapter in this story now that the OP knows where to check. I imagine that sender is quite well hidden from normal view with a completely intact engine.

Look where the bold arrow is pointing in that illustration from the parts catalog.  

Taking bets that the sender isn't even plugged in?
Broken wire counts.

You guys are awesome!  Pictures are worth a thousand words.  I found the sending unit, and sure enough, no wire was attached to it.  I rooted around the bell housing and discovered an unattached wire with the appropriate "plug" on the end.  I grounded that wire, turned on the key, and I have a temperature gauge that works!

I've got some stuff apart on the engine right now, so I can't test the sending unit, but I will by the end of the day.

Once again, thanks very much.

Congrat's!  Well done.  

Great that it was an easy, no $ fix, Rob!  

Hope the rest of your engine work goes as smoothly this afternoon.
That 300 is about a perfect engine for a 150 work truck.