It looks like pressing the gold button will cycle thru the various ranges. Push it and watch the decimal point position move. Millivolts, volts, tens of volts etc. There is a legend to the lower right of the big dial. Looks like press and hold causes it to revert to auto range.

Try that - if all else fails - read the book.

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I got too many cars!! Insurance eats me alive. Dave

78 Corvette Stingray - 3k
82 242 Turbo Volvo - Manual - 270k
86 300e 5 speed manual - 210k
87 420sel - 240k
89 560sl - 78k
91 420sel - 205k
91 560sel - 85k
94 GMC Suburban - 90k
97 Harley Davidson Heritage Softail - 25k
00 GMC Silverado 1 ton 30k

Tried everything, the unit is shot. I bought another multimeter will check everything tomorrow. Thanks

I tried the new multimeter and was able to successfully adjust the potentiometer to the proper range and test the sensor. How do I test the continuity on the for the coolant temperature sensor? I didn't replace the OVP on this car it is original but I did notice it is different from the unit that I have on my 300SE and 560SEL It only has one 10A Fuse, is this the correct unit and can I just switch that with one of the others safely to test it. I did not replace the wiring harness from the ECU.

Out of curiosity I swapped the OVP, no change at all. Hopefully you can walk me through that test to the sensor. Thanks Everone.

Put the meter into the mode to measure resistance. Put the probes on the terminals of the sensor and read the resistance on the meter. Very straight forward. As the temperature of the engine changes so the resistance changes. I don't remember the resistance values for the various temperatures but i know there is a chart in the service manual for the car.

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I got too many cars!! Insurance eats me alive. Dave

78 Corvette Stingray - 3k
82 242 Turbo Volvo - Manual - 270k
86 300e 5 speed manual - 210k
87 420sel - 240k
89 560sl - 78k
91 420sel - 205k
91 560sel - 85k
94 GMC Suburban - 90k
97 Harley Davidson Heritage Softail - 25k
00 GMC Silverado 1 ton 30k

Does your meter have a beep setting at which when the two probes connect to each other there is a beep sound ? If not then you can set the meter to the lowest resistance range.

I dont know if your ECU is the same as the one in my car (87, 260E). But you can try even if it is not the same, one of the ECU pins must be connected to the temp sensor.

Disconnect the ECU connector from the ECU.
Disconnect temp sensor connector from the temp sensor.

Connect one end of the probe to ECU connector terminal 21. And connect the other end of the probe to one of the terminals of the temp sensor connector. Try all the sensor connector terminals because I dont know the exact layout of the sensor connector. For one of the terminals, you should get close to zero ohms, i.e the meter should display something like what it would display when the two probes are shorted to each other. If you found one such terminal, then your continuity from the coolant sensor to the ECU is o.k.

If you did not find one such terminal then it is possible that your ECU is different from mine (unless soneome else can verify that they are the same), so then it becomes a more tedious task, but doable. Just try all the terminals on the ECU's connector one by one. There should be one connection out of all possible combinations of connections (worst case 25 x 4) that should give you zero ohms.

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Saumil S. Patel

I tested the resistance as you instructed, the multimeter did have the beep feature. What I tested was the wiring on the two prong green coolant sensor to every terminal on the ECU connector since I was unsure which one was terminal #21. I didn't get a beep from either probe or any of the terminals. It would appear that the sensor readings are not reaching the ECU?

Are you sure this is the coolant sensor for the ECU ? I am not familiar with this car. And you tried both terminals of the sensor connector, none of them is connected with any of the terminals on the ECU connector, right ? Then this is one of your main problems, without the temp sensor connection, the ECU puts the car in a limp mode.

One of the terminals of the coolant sensor in my 87, 260E also goes to the Ignition control unit. You can check if any of the terminals is connected to any of the terminals of the ICU connector (the same beep test). If you do find a connection to ICU and do not find a connection to ECU, you are now sure that you are looking at the correct coolant temp sensor. And the wire is for sure cut somewhere, then just replace the harness.

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Saumil S. Patel

Here is the sensor I tested, I tried it to all of the terminals and got nothing.

Attached Thumbnails

 

Yes, that looks like the correct sensor. Still, check if one of the terminals of this sensor's cable connector goes to one of the terminals on the ignition control unit's connector.

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Saumil S. Patel

Yes, one of the terminals from the sensor cable does go to one of the terminals on the connector cable to the ICU's Left Lower Cable.

So one of the terminals go to the ICU but the other one does not go to the ECU. Thats your problem then and it is a big one. But it would be great if someone (David, Arthur, Brian ??) confirms the similarity between my 87 260E and your 88 420 SEL as far as the temp sensor connection to ECU is concerned.

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Saumil S. Patel

So what do you think the problem is, the wire is broken somewhere? If we can find out to what terminal on the ECU connector, I suppose I could just run a new one.

Thats the only logical conclusion. Your car is a w126, maybe this ECU connector pin diagram could help. There is one engine that is excluded, make sure it is not your car.

Attached Thumbnails

 

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Saumil S. Patel

That is great, I can't wait to try this out. How do I know which engine I have. The car is the W126 1988 420SEL. Again, thanks. This will help me out big time.