Temperature reading problems [1997 SL500]

[Benzy Boy]

My gauge cluster cooalant temperature reading hasn't gone above 80C for the last 5 drives. It used to steadily go up to 96C and stay there but now it fluctuates around 50-60C and stays there more or less.

Can you help me troubleshoot this problem? Do you think it's the temperature sensor or the thermostat or something else?
I ordered a replacement temperature sensor because it's the easiest to replace and will see what effect it has.

Heat blows hot/very hot even when it's showing 50C after a 30 minute drive.

 

[gsxr]

Normally I'd say a bad thermostat, but you wouldn't have full heat at an indicated 50°C. Might be a bad sending unit or gauge.

You have a 1997 SL500, correct? If so, I'm not sure how to test / diagnose this, the senders & gauge are completely different than a 124.

 

[Benzy Boy]

Normally I'd say a bad thermostat, but you wouldn't have full heat at an indicated 50°C. Might be a bad sending unit or gauge.

You have a 1997 SL500, correct? If so, I'm not sure how to test / diagnose this, the senders & gauge are completely different than a 124.

My bad I didn't know. Mods can move this to off-topic then.
In the meantime I bought the gauge cluster temp sensor and I'll replace it and see what happens.
I just want to make sure I won't be damaging my engine driving it.

Heat blowing hot would indicate the engine is running at its normal operating temperature correct?

 

[gsxr]

My bad I didn't know. Mods can move this to off-topic then.
In the meantime I bought the gauge cluster temp sensor and I'll replace it and see what happens.
I just want to make sure I won't be damaging my engine driving it.

If engine temp really is 50C you shouldn't drive it like that.

Heat blowing hot would indicate the engine is running at its normal operating temperature correct?

Correct. If engine temp is only 50C, the air with HVAC set to max heat would be lukewarm at best.


If you HVAC controls look like the photo below, you MAY be able to view data from that controller by pressing a button combination, and scrolling through the menu. The 210 has similar controls which allow this but I'm not sure about the 129. You need a decoder sheet to translate what you are seeing. Probably need to search on an R129 forum somewhere:

 

[Benzy Boy]

Changing my thermostat this week. I just got the part from the dealership and it makes a sound like there's a little metal ball inside it when I shake it. Is that normal? Also is it imperative I change the 90 degree L hose as well?

 

[gsxr]

Changing my thermostat this week. I just got the part from the dealership and it makes a sound like there's a little metal ball inside it when I shake it. Is that normal? Also is it imperative I change the 90 degree L hose as well?

Yup, the metal ball is normal. If the 90° hose is original, it's a good idea to replace it proactively, but if it's in good condition this isn't required.