temp gauge going haywire - PeachParts Mercedes-Benz Forum

84300DT · #1 01-18-2004, 02:41 PM

my dash temp gauge was going haywire today 40-80-40-60 etcetc - jumping around a lot. then it just was stuck on 40.
engine was fully warmed up and heat working well.

do i need to replace the sender ? is that the one on the block
next to the IP?

i need a working temp gauge ..for peace of mind , ya know

300SDog · #2 01-19-2004, 01:13 AM

Yeah but whats the outside temperature been in Rockport these days??? .......figure whenever the guage reachest its highest point, then the radiator flow is shut down by thermostat..... and when it dips dramatically, a sudden surge of cooled water is going through the galleries when thermostat opened......... and uh, coolant travelling through various passages will carry eddies of cooler temp coolant as thermostat does its job.

Otherwise, try blocking the radiator with cardboard.... this *might* have an effect at stabilizing fluctuations on that dangerously erratic (finely tuned?) temperture guage you are worried about.

BoostnBenz · #3 01-19-2004, 02:50 PM

The lowest temperature thermostat is 80*C so it really shouldn't go any lower than that unless the heat is on max and motor is just idling. I don't think it is very possible for it to be that erratic unless we are talking over long periods of time (minutes) even with a bad thermostat.

Have you done anything to your car lately which could have brought this on? I would make sure the connection on the sender is clean otherwise just replace it, they don't cost very much anyway. If it still persists then perhaps your dash is having grounding problems.

84300DT · #4 01-19-2004, 03:06 PM

Quote:

Originally posted by BoostnBenz
The lowest temperature thermostat is 80*C so it really shouldn't go any lower than that unless the heat is on max and motor is just idling. I don't think it is very possible for it to be that erratic unless we are talking over long periods of time (minutes) even with a bad thermostat.

Have you done anything to your car lately which could have brought this on? I would make sure the connection on the sender is clean otherwise just replace it, they don't cost very much anyway. If it still persists then perhaps your dash is having grounding problems.

thanks jeff. well the only thing that may have brought it on is sitting outside in sub zero temperatures,, hehe,. but i don't think so. it ran after that and warmed up with 'normal' temperature guage operation (it usually sits pinned on top of the top loop of the '8' in 80. ) the sender tip is clean.
no other issues i can think of like low or no heat or (god forbid) steam running out of the engine in an overheat situation

i was thinking t-stat or aux pump but am quite sure there would not be any heat at all if that were the case, right?

the other guages work in the dash ok so i'm down to the sender,
the one on the block right next to the IP. are there any other sensors that are hooked up to the guage?

lrg · #5 01-19-2004, 04:01 PM

The aux pump shouldn't be at all an issue here so I think you can safely ignore that. Given the fact you have good heat I'd say 99% probability it's the sending unit. If the coolant temp actually is at 40 degrees Cent. then the heat you'd be getting from the heater would be feeble at best. Before just replacing the sending unit though I'd check the wiring since from my (limited) experience when the sending units fail they just flat out don't work. It may be a very simple loose wire that vibrated off on start up when it was really stiff from the cold.

84300DT · #6 01-19-2004, 04:53 PM

lrg,
yeah the wire was the first thing i checked - it was on. i pulled it and scraped the sender part but i did not clean the wire end. i will try that. being i had no helper i could not 'jiggle' the wire on the sender and see if that had an effect on the dash.
plus it was freaking snowing like mad out.

grrrr, i hate winter...

P'bert · #7 01-19-2004, 05:32 PM

Erratic temp gauge behavior can be caused by a loose ground for the gauge on the cluster.
I was so advised when the temp gauge on my former 240D bounced around. I removed the cluster, tightened here and there on the back, and never was bothered again.

Phil
'84 Euro 300D
'85 300D

84300DT · #8 01-19-2004, 05:36 PM

phil i pulled the cluster but its kinda tight working in there + was afraid that i would pull the fitting to the oil pressure gauge off and then there would be an even bigger mess

anyone know where the ground to the cluster should be ??

BoostnBenz · #9 01-19-2004, 09:55 PM

The actual ground is in the harness but I'm not sure which pin it is, I was having an odd instrument problem a little ways back so I just added my own ground wire to the dash just to be certain. I just backed out of the little screws a little bit on the tach amp side and ran a wire from there to the chassis.

I think you have to pull incredibly hard for that fitting to come apart, it'd be much more realistic if it were bent to far and snapped. I just use the smallest adjustable I have to tighten it so I know it isn't to much and is easy to undo when I don't have much room to work with.

P'bert · #10 01-20-2004, 12:50 AM

Well, it's not that difficult to remove the oil pressure tube, the speedo cable, and the harness, leaving you with the cluster to tote to your bench. There, you can have a good look at things and test and fiddle to your heart's content.

However, lazy me, I think I tightened the necessary nut behind the temp gauge with a 1/4 ratchet and the appropriate tiny socket, without removing either the tube or the cable.

The suggestion to simply connect an additional ground for the cluster may help, but it won't if your problem is isolated to the temp gauge. Mine was.

Good luck.

Phil
'84 & '85 300D's

84300DT · #11 01-21-2004, 06:53 PM

temp guage was working as if nothing happened today.pinned just on the top of the 8 in 80.

no flickering or jumping around like before. ambient temp around 20 deg F.

guess it's the old 'self healing' system at work again.

or maybe i jostled something by messing with the wire at the sensor and behind the cluster. whatever , i'm glad to have the thing working again as it is a diesel obsession.. checking the temp. every 5 seconds, hehe.

BoostnBenz · #12 01-22-2004, 10:28 AM

Wow, good for you.... Yea, I'm always watching my temperature gauge as well, mainly because I think the new thermostat I have is crap. The car has been trying to run 99*C lately, that is unless I have the heat on....

84300DT · #13 01-22-2004, 10:44 AM

Quote:

Originally posted by BoostnBenz
Wow, good for you.... Yea, I'm always watching my temperature gauge as well, mainly because I think the new thermostat I have is crap. The car has been trying to run 99*C lately, that is unless I have the heat on....

yea jeff know the feelling - it was less of a panic because i knew i had a good bosch t-stat in there otherwise i would have been checking under the hood forthwith..

99 deg C would be a little on the high side for my car unless maybe in the middle of summer and going up a steep incline.
if i were you i would change it just to see what happened assuming there's plenty of coolant and the coolant tested good.
t-stats aren't expensive luckily.

BoostnBenz · #14 01-22-2004, 10:49 AM

Yea I will on the next parts order, as is with the heat going it really holds it down to a fair range. It is a Whaler which I thought was decent, I guess not.... Before this thermostat the car ran to cool now it does this!

lrg · #15 01-22-2004, 11:57 AM

You can always test your current Tstat in a pot of water on the stove. Start with cool water and heat it while watching for the tstat to open. It should open smoothly at just below the designed in proper temp. I used a basic oven meat thermometer and it worked great. Needless to say SWMBO was out at the time.