What would cause this overheating symptom, W123 300D - Page 3 - PeachParts Mercedes-Benz Forum

Mxfrank · #31 11-04-2015, 08:57 PM

Quote:

Originally Posted by funola

I've looked at the W123 wiring diagrams and do not remember seeing a voltage regulator in the instr cluster either. I think the W123 instr cluster gauges is a bridge circuit and that is how they achieve their accuracy w/o an on board voltage regulator.

You won't find a separate IVR on anything built after about 1970, it's going to be a chip. Bridge circuit? Explain that. I checked the 124 dash from my junk bin. There's a voltage divider chip and supporting circuitry on the left side of the pod. I'm sure it regulates to 5V. I will try to verify the gauge voltage on my 190D, by measuring the voltage to the oil gauge. There must be something similar in a 123 pod, but I don't have one here. Anyway, you can see why they don't bother putting it in the schematic...it's not replaceable.

vstech · #32 11-04-2015, 09:13 PM

Quote:

Originally Posted by Mxfrank

Not happening. If it was, he'd be complaining about the lights, radio, blower, gas gauge, etc. And if voltage was that low, both the gas and temp gauges would be reading low, or more likely, not reading at all. In addition, his battery would be drained in short order. And bear in mind that the instruments ground through the sensors.

I am attempting to explain what causes the bad ground to cause a high temp readout.

It was a bad ground strap on my vehicle.

I'm not saying the battery voltage was low, I'm saying the lights, blower etc may have used up the minimal ground, leaving nothing for the temp... The temp is the ONLY sender that uses engine for ground... Aside from the oil pressure sender, and it's default when ground is lost is to show high pressure... Which if the motor is running, it would anyway...

I would love to have an explanation for why it does this... I only know that it does.

vstech · #33 11-04-2015, 09:19 PM

Quote:

Originally Posted by Mxfrank

Not happening. If it was, he'd be complaining about the lights, radio, blower, gas gauge, etc. And if voltage was that low, both the gas and temp gauges would be reading low, or more likely, not reading at all. In addition, his battery would be drained in short order. And bear in mind that the instruments ground through the sensors.

I disagree that he would be complaining of this... The only thing affected would be engine grounded items... Like the alternator, oil pressure sender, and temp sender... Everything else is grounded by the body of the car. And the battery is grounded to the body of the car... W123 doesn't have an oil pressure sender, only 124 and 126...

Hmmm. It's possible the ground strap issue could be affecting the alternator, causing a high voltage scenario due to lack of signal return from the battery light... Not sure, I just know it happens.

funola · #34 11-04-2015, 10:20 PM

Here's a thread with more info and to add to the confusion.

617 instrument cluster question

One forum member says the coolant temp gauge is a Wheatstone bridge type, another says it's electromagnetic with 2 coils. I have not verified which is correct. In order to do so, I'd have had to butcher the temp gauge which I declined to do. One thing I am sure of is the W123 cluster does not have an on board voltage regulator.

funola · #35 11-04-2015, 10:33 PM

My suggestion to the OP is try another known good cluster if one is available. If not, I'd suggest taking the cluster out and clean the connections on the temp and fuel gauge (they are the same type and share connections for power and ground). There are 2 rivets and a stud with a nut for each gauge and those are the electrical connections to the gauges. Take the nuts out and wire brush the stud clean, also wire brush the rivets and then solder them. See if that helps.

sleepstar · #36 11-05-2015, 01:53 AM

the ground strap is a few months old. theres no other electrical problems. i'll look into all this when i get the car back. it was spewing coolant at idle like a garden sprayer. the two things could be unrelated, of course.

Graham · #37 11-05-2015, 09:16 AM

Quote:

Originally Posted by sleepstar

the ground strap is a few months old. theres no other electrical problems. i'll look into all this when i get the car back. it was spewing coolant at idle like a garden sprayer. the two things could be unrelated, of course.

This discussion has gone all over the map. First thing I would do, is check if the car is actually overheating or not. Repeating myself, but easy to do with a cheap IR thermometer or heat sensitive tape.

If it is the gauge (which with coolant spewing, sounds unlikely), then this link of how someone diagnosed and repaired a gauge error might help:
OZBENZ - Australian & New Zealand Mercedes-Benz Forums • View topic - Temperature gauge - easy repair
Bad ground between gauge housing and cluster circuit board.

Mxfrank · #38 11-05-2015, 09:26 AM

Quote:

Originally Posted by vstech

I disagree that he would be complaining of this... The only thing affected would be engine grounded items... Like the alternator, oil pressure sender, and temp sender... Everything else is grounded by the body of the car. And the battery is grounded to the body of the car... W123 doesn't have an oil pressure sender, only 124 and 126...

Hmmm. It's possible the ground strap issue could be affecting the alternator, causing a high voltage scenario due to lack of signal return from the battery light... Not sure, I just know it happens.

Once an alternator bootstraps, the light is irrelevant, because the regulator becomes self powered. An internally regulated alternator is limited to it's design voltage...depending on which generation of Bosch alternator and whether it has a temperature-sensitive regualtor, somewhere between 13.25-14.5V, all within normal operating range. It simply can't go any higher. And as we've discussed, this is electronically limited to 5V at the temperature gauge, which is grounded directly to the motor. Enough.

funola · #39 11-05-2015, 09:32 AM

......... And as we've discussed, this is electronically limited to 5V at the temperature gauge, which is grounded directly to the motor. Enough.[/QUOTE]

This is factually incorrect for the OP's W123 cluster.

Mxfrank · #40 11-05-2015, 09:43 AM

Quote:

Originally Posted by sleepstar

the ground strap is a few months old. theres no other electrical problems. i'll look into all this when i get the car back. it was spewing coolant at idle like a garden sprayer. the two things could be unrelated, of course.

Begin with the thermostat, let's see where it goes after that.

Graham · #41 11-05-2015, 10:51 AM

Quote:

Originally Posted by funola

This is factually incorrect for the OP's W123 cluster.

If car IS overheating this may all be irrelevant.

. But to continue.....

In Jay_Bob's post in the link you provided, he said that the meter on his 123 had three connections.

The Autometer link below talks of two types of gauge. (Ok, we don't have Autometer gauges, but perhaps same descriptions apply? Couldn't find equivalent from VDO) Anyway, it is a useful link for trouble shooting.

http://www.autometer.com/resources/index/faq_view/id/11

The link says that if the sender has a single terminal, it is a short sweep gauge. With these, the signal from the gauge is grounded through the sensor thermistor. The voltage range for the sender is stamped on as being 6-24V.

If the sensor has two wires, then it is a full sweep gauge, the gauge sends a 5V signal to the sender and the gauge measure the drop in voltage after passing through the sender thermistor.

The sender on a W123 has one connection, so that would indicate we do not have a 5V signal to the sender.
Don't know if this clears this up, but at least I tried

Two pin sensor might be like this W124 one? Some later ones have 4 pins and serve two purposes.

Again - If car IS overheating this may all be irrelevant.

sleepstar · #42 11-07-2015, 03:33 PM

so this wasnt an electrical problem. it was really overheating. whatever condition arises by having a faulty "freeze plug" at the drivers side rear (faulty enough to spray fluid) causes running hot at idle (close to red) but rapid cooling when off idle at higher RPM