Engine temp gauge says 100 - infrared says 80 - PeachParts Mercedes-Benz Forum

rhodes2010 · #1 11-17-2009, 10:47 PM

My dashboard engine temp gauge which for years stayed on 80 all the time
started sitting at 90 all the time.

I stuck a mercury thermometer in the radiator water and also used my infrared
and got a solid 80 on both.

This week the dash gauge sits at an even 100 all the time. My oil cooler is
cold - actually cold, and the infrared shows the engine head at 73
other various parts like upper and lower hose and thermostat housing all read
between 47 to 63 C.

I am convinced the sending unit is going haywire. I tried testing it with VOM
and could not get a reading that made sense.

So my question to you all is.... have any of you had dash temp gauge issues, and if so, what was the most frequent culprit ?

I was worried that I was frying the engine, but I am pretty sure I am safe driving it.

Thanks everyone.

vstech · #2 11-17-2009, 10:58 PM

a bad sender is very common, but I would not assume you are safe driving it. a faulty sender won't warn you of a blown hose, or other maladies...

Jeremy5848 · #3 11-17-2009, 11:34 PM

Agreed, the sender is dying. BTW, which car?

nhdoc · #4 11-18-2009, 07:55 AM

Senders are cheap and easy to replace so why not just order one and do it? If it turns out to not be the problem you have at least eliminated it. $15± and 10 minutes of your time for peace of mind is priceless.

Paulc66t · #5 11-18-2009, 08:00 AM

While you are inthe process of replacing the sending unit you can test it. Pull it out and leave it connected and submerge it in some boiling water. Should read 100C.

jdh122 · #6 11-18-2009, 10:50 AM

I had a similar problem with my '83 300D. The temp gauge routinely hovered around 100, but the IR showed the temp was closer to 82. Also, I removed the temp sender and tested the resistance at various temperatures in a pan of hot water. It checked out fine, so I knew that wasn't the problem.

Having ruled out everything but the gauge, I bought about $3 worth of wire and terminals and ran an extra ground from the back of the temp/fuel/oil cluster to a metal grounding point near the floor and just forward of the driver's side door, and, voila! That fixed the problem and my gauge hasn't crept past about 85 since, even in traffic during the hot Houston summer. It also fixed my occasionally jumpy fuel gauge.

charmalu · #7 11-18-2009, 11:48 AM

There are 2 senders on the side of the block. switch the wires from one to the other. that would be the simplest to do first. both senders are the same. this is something my indy mentioned to me when I was having problems.

I used my IR temp gun, changed senders and the gauge and still had a problem reading high.

make sure the ground strap between the engine and body are clean, and tight.

Charlie

Dobroman · #8 11-23-2009, 03:16 AM

I'm having the same problem on my 84 300D. First I had very low engine temp , then checked the thermostat and it was broken in half. Replaced that and now the gage is running 100 or more. Got heat in the cabin now but it's either all the way hot or cold , no in between. I haven't checked the actual engine temp but need peace of mind that I'm not running too hot.
What does it do to switch the wires on the two sending units on the block?
I am gonna have to start to figure this out. I try checking the sending units when I can get the time.
And I still have some minor vacuum issues to resolve too.

charmalu · #9 11-23-2009, 10:45 AM

Both sending units are the same. By switching the wire connectors, and the gauge stays the same, it could mean both sensors are bad, or another problem.

chances of both sensors not working is nill. I would do this simple check.

remove the ground strap between the engine and body to make sure it is clean and tight. this was my whole problem from the start.

with the new engine, at first I was thinking it being tight and new was the cause of a higher temp. I had used the old temp senders, then changed to a new one, and still the same. changed the gauge a couple times, IR temp the engine. everything normal

.

It wasn`t until I had a no start problem, no click, nothing. lights worked horn etc.... turned off the key, and the batt light stayed on

the temp gauge had been jumpy, would spike, and would jump when turning on a switch.

Finally removed the ground strap, and there was paint on the adapter plate not giving me a good ground. I knew it was good and tight and clean, just never gave it a second thought.

All the way cold or all the way hot heat er temp, is probably in the push button CCU. I only get heat when the wheel is in the red detent position. come out of detent, then is get cold. at least I have heat. something else to chase down

Charlie

#10 11-23-2009, 10:49 AM

AFAIK only the 85 has two similar sensors in the head.

71inka02 · #11 11-23-2009, 12:47 PM

I had strange issue back in late September - Temp gauge started to jump around and appeared that car was running hot...

By process of elimination - being that the radiator and water pump had been replaced within the last year...I first changed out the thermostat, and continued to have issue occasionally. I then changed out the temp sender on the side of the block...

Still had an occasional issue - THEN I noticed...One morning when I turned the key to glow position, the temp gauge pegged to max hot position. The car had been parked in my garage overnight, and had not been driven or cranked in over 18 hours --- so I knew the engine was not "overheating".

I talked to my Indy -- he did the infrared temp check and a couple of other things and determined my issue is in the temp gauge itself or with the ground. The gauge will occasionally do the max hot peg when I crank cold - but once I drive a couple of miles, and the thermostat opens, the gauge operates properly.

I haven't had opportunity to have cluster removed and repaired but plan to around the first of December when my schedule slows, and time allows.

?? Where is the ground strap between engine and body??

rhodes2010 · #12 01-03-2010, 04:42 PM

Original Poster here, Finally spent some time trying to get the 1980 300D temp
gauge to read correctly.

So this is what I have tried in the last two weeks....

1. I swapped sensors with the 1983 300SD.
The 1983 300SD continued to read a steady 80 C.
The 1980 300D continues to read 90 to 100C, with no overheating.

2. I have a spare instrument cluster from the donor car that provided
the engine that is in the 1980. Thinking that I must be having a guage
problem I just switched the clusters. Problem of temp guage reading high
still exists.

3. Did confirm, as many have said, that one sensor lead under hood is
specifically for temp gauge, other is specifically for glow plug timer.

I am now going to check the ground strap. I keep thinking that if the
ground strap was a culprit the starter would show signs of trouble.

The wire going to the connector temp sensor is looking pretty weak after 30
years and 318,000 miles. But a loss of connection there would cause
the gauge to read low, not high. As an increase in ground causes gauge to
jump up (I tested this).

I'll check back after getting into the ground strap.
I only have 25,000 miles since putting this engine in, can't beleive that
strap is the problem.

compress ignite · #13 01-03-2010, 06:24 PM

Behind the cluster...

I'm not familiar with your Wiring Harness...
But I'm assured there is a Grounding Wire (MB LOVES "brown for ground")
connecting the Instrument Cluster to a Ground.

MS Fowler · #14 01-03-2010, 07:51 PM

Remember that IR only reads the surface temperature. You should probably find some way to check its calibration--boiling water on the stove is fine. My IR reads 7 degrees high at 212 F.

i-osprey · #15 01-03-2010, 08:51 PM

Quote:

Originally Posted by MS Fowler

Remember that IR only reads the surface temperature. You should probably find some way to check its calibration--boiling water on the stove is fine. My IR reads 7 degrees high at 212 F.

Are you at sea level? Using distilled water?

If not, I guess it could be even more off.