My mom's 79 240D temp and gas guage is not working. It was working fine until yesterday. Mysteriously both failed simutaneously for no reason at all. All other guages are working fine.

Where should I start first to trouble shoot this problem?

Did a "search" and people mentioned the fuel and temp sending unit. My question is where are they located on the car and is this something I can replace myself?

On behalf of my mom...thanks in advance,

jrmd01@yahoo.com

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JR
1986 Gold 300E 4Dr automatic

ok i found the fuel sending unit underneath the first aid kit. I cleaned the wire connectors and now the fuel guage seems to be working. If it fails again, I'll take out the fuel sending unit (46mm?) and clean it.

I still can't find the temperature sending unit. Can anyone point me in the right direction?

thanks.

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JR
1986 Gold 300E 4Dr automatic

I'm really surprised. I full expected that the round connector on the back of the instrument cluster was loose, this would be the common denominator.

The early engines such as yours have two coolant sensors, one for the glow plug system and the other for the gauge. If I remember correctly the rear one is for the gauge. They are in the side of the head near the injection pump.

Good luck,

When both gauges fail simultaneously it is usually due to the failure of the voltage regulator on the back of the instrument cluster. This regulator reduces the voltage to the two gauges to 10 volts. The reason for reducing the voltage is to prevent fluctuations in the gauge readings as the cars system voltage fluctuates during normal driving. 10 volts will give constant readings.

found the temp sending unit but there are 2 wires. One comming from the temp sending unit, and another wire (which was disconnected). I asked my dad's friend what that was and he said that it goes to the guage. Although the fuse between the wires was missing. Does anyone know what I'm talking about? I'll try to take a pic of it today.

In the mean time, I'll call the MB dealers to see if they carry this "fuse".

To: Mr. LarryBible - I don't know how I fixed the fuel guage, all I did was disconnect the fuel sending unit, blew away the dust, then reconnected it, and like magic the fuel guage was working again. ??? P.S. thanks for pointing out the temp sending unit.

To: Mr. Goodwrench - If my mom continues to have problems after I reconnect the loose wires I found near the temp sending unit, I'll double check that regulator that you mentioned. Although I'd hate to have to take that insrument cluster out. I did more damage than good when mine (86 300E) was removed to change one light bulb. Now the RPM guage, clock, and economy, and both lights don't work....DAMN IT!

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JR
1986 Gold 300E 4Dr automatic

If you got the fuel gauge to work it probably isn't the voltage regulator.

well, I also got the temp guage to work. I can't seem to attach a pic to this post.

Anyway, there was a wire that was disconnected. One of my dad's friend told me that a fuse was missing. I contacted the MB dealer (parts dept) they had no clue what I was talking about. Disappointed, I just put in 8amp fuse and taped it in to both ends (temporary fix). I'll look for an inline fuse holder and put it in later.

When I figure out how to put a pic on this website, I hope someone can tell me EXACTLY belongs between these disconnected wires.

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JR
1986 Gold 300E 4Dr automatic