Gauges on instrument panel swing wildly

[Marvin Smith]

I have a 1986 mercedes model 300SD and am having some strange problems:

1. My outside temperature sensor reads 125 degrees or more independent of the outside ambient temperature - what's wrong?

2. My instrument panel is driving me wild. The symptoms are:

a. The coolant temperature gauge normally reads approx. 83 degrees C and is steady once the car has warmed up. However,
lately after normal engine warm up, it jumps up and down every second or so between maximum permissable operating engine temperature and the normally operating value of 83 degrees. The engine never overheats and there is plenty of coolant. Is it possible that the thermostat is thermally cycling between open and closed and needs to be changed?

b. When this happens, the fuel gauge, which would normally be steady, jumps up and down in sync with the temperature gauge movements.

c. If either turn signal is turned on, the temperature and fuel gauges jump up and down wildly in sync with the turn signal flashing signal intervals.

d. If the headlights are turned on, both turn signals flash in sync similar to the way the hazard lights would perform. The dashboard lights go out and the speedometer lights go off. The headlights stay on during all of this weird activity.

e. I was told previously that it looked as if I had a grounding problem. I had my mechanic check the car from one end to the other and he could not find any grounding problems. Help!

[sixto]

I can't offer any help but where did you find an 86 300SD?

Sixto
95 S420
91 300SE
87 300SDL
83 300SD

[Marvin Smith]

To Sixto:

The car that I referred to was a 1986 300SDL. Is there any possibility that the water pump is not providing enough coolant and is about to fail thus causing the system to give me a warning wih flashing llights, gauges swinging wildly, etc. to replace the water pump before it fails completely? I checked the engine noise
with the engine running this evening and it is making a squealing noise similar to other cars that I have had to replace water pumps on. What do you think?

Thanks for your help.

Marvin

[sixto]

I don't think the car has cooling system diagnostics as sophisticated as that. I don't think it has any cooling system diagnostics other than your eyes and the temp gauge.

It sure sounds like a bad ground, but I'd start by replacing, not just inspecting but replacing all the fuses if you haven't done so in the last 10 years. You're looking at a $5 investment if you get fuses from FastLane. Next is to loosen, clean and tighten every ground point under the dash. You might have to pull the gauge cluster for this but that's an easy task with your telescoping steering wheel.

If I had the original head on my SDL, it would scare me to have an inaccurate temp gauge. These heads are prone to cracking when overheated. If the head's been replaced in the last 5 years or so, you probably have the tougher 4th or 5th redesign which is more resilient to overheating. Not impervious to overheating, just tougher than the original head.

It's easy to test the water pump. There's a weep hole to tell you if it's going bad. You can also loosen the belt and spin the water pump pulley by hand. Be sensitive to noise and roughness. There should be neither. Again, cooling system service is not something to postpne on in a 300SDL.

Sixto
95 S420
91 300SE
87 300SDL
83 300SD

[xp190]

I had a very similar problem on my 94 acclaim. All the gauges on the instrument cluster would start going up and down all over the place, it was related to turn signals, lights. If I smakcked the dashboard a few times it would fix itself, but only temporairly. It turned out to be a bad solder joint on the back of the cluster. I had to pull it out and resolder it all. Problem solved.

I would suspect electronics and not the engine itself first.

xp

[Marvin Smith]

This message is for Sixto:

Thank you for your prompt reply. Exactly where is this weep hole located that you mentioned for testing the water pump and what does the test consist of? It sounds like the pump would be discharging coolant onto the ground? I will replace all fuses as you suggest and try to eliminate any possibility that improper grounding is the problem.

Please respond.

thanks,

marvin Smith

[sixto]

It's too dark to check right now but I'm pretty sure it's on some top surface of the water pump behind the pulley. It's a hole about 4mm in diameter just ahead of the pump drive shaft seal. As the seal quits, coolant seeps out and leaks out that hole as a warning that major seal failure is imminent.

The test consists of nothing more than inspecting the weep hole for coolant. Your eyes and nose will tell you if there's coolant or try a q-tip.

Sixto
95 S420
91 300SE
87 300SDL
83 300SD

[Rockman59]

It turned out to be a bad solder joint on the back of the cluster
++++++++++++++++++++++++++++++++++++++++++++++
I had the "wildly swinging gauges" problem and it too turned out to be bad electrical connections. That's the place to start checking first.