308 temp guage question

1. Replace the sensor and see what temp goes to.
If still goes to high temp….
2. Replace the gauge and see what temp indicates.

 

Martin's post #9 here:

https://www.ferrarichat.com/forum/threads/308-water-temperature-gauge-gone.150071

gives the expected resistance of a new 104628 water temp thermister at various temperatures. You could try replacing the thermister sender with a 1W fixed resistor to see if the gauge reading is close, or not, to the temperature in Martin's table -- and maybe do it at a couple of points -- like maybe 150 Ohms (211 deg F) and 420 Ohms (158 deg F). If the gauge is OKish, then improving the fans per:

...a refreshed radiator and large, powerful, well-ducted electric radiator fans mount directly to the radiator fins would be a good upgrade over the stock, unducted, spaced away radiator fans.

might be the next step of desperation

 

Your fans are on and the water going IN to the radiator is 154? That seems wrong. If the feed to the radiator is under 180 then the fans shouldn't be on at all.

 

You can get a more accurate reading of the T-stat housing if you spray it matte black over a couple square inches.
rad hose is not good indicator, the black rad metal inlet is a good spot.

I have a theory, I believe these are gauges with a bias coil*, so they read about the same whether or not the battery voltage is 14.5, or 10. I suspect the ground that the bias coil is uses is getting flaky over the years, causing the gauge to read hotter than it should. Your thermistor ground, the temp sensor, is still normal though. That bias coil problem ground may be on the instrument cluster itself, or in the gauge.

*One coil on each side of the bottom of the needle, one goes to temp sender, one goes to chassis ground.

Doug

 

I was wondering if the water circulation to the radiator was reversed. I'm not sure what the implications of this condition would be. For sure the "inlet" temperature would be less than the "output" temperature. But would it actually have a noticeable effect on cooling efficiency? The fans would come on when the engine is hot and probably stay on forever, no amount of fan cooling would matter because the sensor would see engine water and not cooled water. Anyway, other than a bad temp reading from your instrument, it's the only explanation I could come with.

 

Hi, I also have a 1984 308 (GTS version) and have the exact same issue. I tried some of the same things as you, but then I saw a water gauge on ebay for a reasonable price. After I installed it, the needle never went above mid point or only slightly above but never again to that second line past the midpoint. However the replacement gauge was made for an earlier version of the 308 ( I think a late 70's 308) and it has different graphics where it says "Water" on the actual gauge instead of the symbol like on the original. So I actually put the original one back in as I confirmed the gauge was off and didn't like the look of the mis-matched gauges. Hope this helps.

 

Some information on a similar problem: In this case it was indicating too low. The gauge was the issue.

https://www.ferrarichat.com/forum/threads/308-water-temperature-gauge-gone.150071/#post-147743936

 

My 1983 308 GTS QV has the same problem, identical indication as Post #1, but my gauge fluctuates between normal and the higher reading. I'm guessing ground issue on my car.

 

I had the same problem on my 84 QV, coolant temp gauge would often read high, even if the fans weren't on. Sometimes the gauge would just drop to normal from a high reading, then gradually go up again.

Last winter I took the instrument panel out, and cleaned the connector of every single wire, bulb, gauge on the back of it. Fine sandpaper and emery boards, then used Stabilant-22a contact enhancer when I reconnected them. I also added another ground wire from the back end of the ground wire on the instrument panel harness to an empty stud under the dash.

My hot running problem went away completely. My coolant gauge was fine, but all the wiring connections were slightly dirty and gray, just 30 plus years of electrical connector buildup. I'd strongly recommend doing the wire harness cleanup before investing in new gauges, senders, etc.

While cleaning up the instrument panel harness, it's also a really good time to convert to LED bulbs - works great to make the gauges actually visible at night!

From this thread 308 gts dash led conversion I posted a bunch of detail in post #15 about cleaning up the harness, including this:

Gordon

 

There are two key factors affecting IR thermometers. The first is emissivity - different materials and coatings have different emissivity properties and the second is geometry - the distance from the object being measured. A more expensive IR thermometer will have an emissivity adjustment and a table with typical ratios for materials and it will also have a dual laser that will converge at the optimal distance. Without those you are getting a generic reading that can vary quite a bit from bare metal to rubber hose so for this purpose you need to pick your targets carefully and look at the results skeptically. Especially when you get a result that just plain doesn't make any sense.

In regards to this discussion if you want to suspect the gauge I think the thing you need to ask yourself why is it okay when I'm driving and only reads high when I'm still and the fans are running? One possible issue is high resistance in the fan motor circuits (dirty connections or grounds) Another is just hardened and dried up old grease in the fan bearings causing drag. Either of those can slow fan speed and efficiency.

 

OEM Lucas fans found in 308s did not have bearings. They have bushings. No grease goes in the Lucas fan.
You have to disassemble the fan motor, wipe the bushings, and place a couple of drops in the upper and lower bushings. Use Zoom Oil for electric motors. Can be found at ACE hardware or on online.
Image Unavailable, Please Login

 

FWIW replacement of the brushes at that stage is simple and about $6

 

My 78 308 was acting similarly to the OP's. After lots of time with an IR gun I replaced the gauge- it indeed was the problem.