Micha1

Hi all,

My 1989 928S4 keeps telling me it is too hot; could it be a faulty gauge reading? Has anyone ever experienced one?

I have installed a new thermostat, seal in behind the thermostat, and a new gauge sensor in the last couple of weeks.

I have cleaned out any debris in behind the radiator, my fans work properly, the thermostat works fine. I have movement of coolant in the overflow tank, so seems like the water pump is working fine. I have no leaks anywhere in the cooling system. Flaps are permanently open.

When I use multiple infared thermometers on the car, the highest reading I get anywhere on the motor is 169 degrees farenheit.

Both the digital readout says car is hot and red square at end of gauge is lit. However, the needle is just a bit above the halfway point in the gauge.

Any advice? Previous experience with this type of problem?

Thanks in advance for your help.

MainePorsche

Welcome to the Tank on your first post.
This is a Bump for you.
You seem to have covered many bases with the matter but as I am an 87 with analog, I can't comment at this point to help you.
Hang in there - you will get assistance.
Post some pics of your car.

mickster

Silly question-when the car is first turned on, where is the needle on the temp gauge?

You probably just have a bad temp. gauge (not sensor).

MainePorsche

169*F is not 'hot' - where are you measuring this ?
169*F might go along with the needle in mid-postion, but the light and the digital warning has me perplexed.

Micha1

Needle is at the zero position when the car is started up or not running from a cold start.

The 169 degrees is on the head. Any other temperatures I get are lower. The temperature sensor on the water bridge is 135, the top rad hose is 145, bottom rad hose is 135, front of rad is 94, intake is around 125, cap on radiator overflow cap is 110...can't recall all the others off hand, but I point pretty much every where.

I know 210 is boiling point and am never near there, anywhere on the engine.

Thanks

Fogey1

I once spent a lot of time, effort and some $$ chasing an overheat shown on the temp gauge - until I tested the temp of the water stream returning to the overflow bottle and found it was OK.

That car had two temp senders ~ side by side, one that talked to the fuel injection system and one that talked to the temp gauge on the dash. The car had always run fine and
replacing the sender to the dash fixed my "overheating problem."

Your car is running cold.

Bilal928S4

There are two sensors above the water bridge, IIRC. The sensor on the driver side is what gives the signal to the gauge. The one on the passenger side might be the one that triggers the light and also runs the computer.

Micha1

Hi Fogey1 and Bilal928S4,

Appreciate your responses.

In regard to the two sensors. Yes, have spent time with both these sensors, and actually have changed both over the last year. Actually just changed the temperature gauge sensor on Monday (the one with the brown top on the driver's side of the water bridge), as that was what I was hoping was giving me the (hopefully) erroneous messages in my dash. Note, my 1989 has the digital readout and the actual gauge as well. Digital system gives me the words, and the gauge gives me the red square at the end of the gauge...which the needle is nowhere near.

As well, I think my car is running cool. The temperature readings I specified were from last evening when it was about 80 farenheit with about 40% humidity. I know the humidity is not an issue as I get the messages when it is low humidity as well.

I have also added some water wetter to release the surface tension of the coolant (50/50 split) in hopes of help as well. Did this Monday as well.

I think perhaps it is some kind of electrical gremlin giving a bad message, but I am no expert mechanic, just a brave backyard 928 lover. My brother in law is a licensed mechanic and he is stumped as well.

Anyway, if anyone else has experienced this or has suggestions, I am open and willing to check them out.

I love my black pristine 928, and do not like seeing any potential problems .

Mrmerlin

where did you get the thermostat, and temp sensor?
They may be defective.

for your car you should be seeing temps on the top rad inlet of about 185 degrees and the return hose should be close to 170.

I would refit the old thermostat and the old temp sensor see if this changes anything.

from your report the thermostat isnt working as it should and it sounds like its open

Micha1

Thanks for the reply mrmerlin.

We have confirmed the thermostat is working. We have actually used a stethoscope to hear it work, and we can feel the temperature change in the hoses.

Is there any other kind of sensor other than the one in the water bridge that speaks to the gauge and digital readout?

Don't mean to be a negative Nellie, but I really don't think the thermostat or sensor are a problem. I will revisit however.

Much appreciated

Mrmerlin

you can confirm thermostat operation by using the IR gun you should see 185 at the inlet and about 170 at the outlet of the radiator,
if these are not what your seeing the thermostat should be swapped out first.

There is only one sender for the gauge and warning lamp,
the other sender is the temp 2 and its for the computers

Micha1

Hi Mrmerlin,

Can you tell me where the temp2 sensor is located....the one for the computers, and where is the sender for the gauge?

I was lead to believe the sensor for the gauge is the one located in the water bridge with the brown top. Is this correct? If so, where is the temp2 sensor located.

Thanks

mickster

Here:

http://members.rennlist.com/v1uhoh/underthe.htm

#2 in the top picture.

Get yourself the shop manual CD (Morehouse) or hard manuals.

You can download them (search will find them) but the Morehouse set has a lot of extras and incredible linking and such.

donaheyzhen

The sensor on the driver side is what gives the signal to the gauge.

Leon Speed

Weird problem. Some suggestions:

1. Clear the fault memory by disconnecting the battery for a few minutes.
2. Check the temperature sender resistance values. At 158F it should be about 97 Ohm, at 176F about 70 Ohm. I think you have to measure each pin to ground, not across the pins.
3. Check if the digidash connectors are clean, seated and locked. This requires removal of the pod.

Afterthought: the temp sender has two pins. Maybe one pin is for the temp needle and the other one for temp (too hot) warning? Check the connector and the wires to the sender, maybe one of the wires is corroded/broken.

Afterthought 2: maybe not . See here:https://rennlist.com/forums/928-foru...i-testing.html