http://mechatronics.mech.northwestern.edu/mechatronics/design_ref/tools/multimeter.html

Current measurement section is at the bottom, but if you're not that familiar with a multimeter I'd suggest reading the whole thing.

If the connector to the auxiliary water pump is the typical two-wire plug-socket configuration, you will need to jumper one connection and measure in-series with the other. Typically, this means unplugging the connection, and jumpering what you determine to be the ground connection. This means it is effectively the same as if the connectors were plugged together.

Then, you use your multimeter in the current setting to complete the circuit between the two terminals that normally carry the battery supply (B+).

Hopefully they are obviously color coded. Fortunately, current is going to be the same whether you choose to measure the B+ line or the ground return line (if there is a ground return line). (So essentially, it is necessary to jumper one pair of terminals to their normally connected configuration, and measure in series with the other pair.)

You do not take a current measurement across the two terminals of the same connector. That is like taking the current measurement of the battery - very high current - will blow the meter's fuse or something in it.

Hope this helps,
Ken300D

You are correct that I DO NOT know how to check the current draw. I went to an Auto Zone and asked a counter man to help. He said he did not know too much either. We located the plug and separated the ends partway. I was not holding the ammeter but started the car. I came back and saw the reading 2.3. I was not astute enough at the time to see how he hooked it up. I also did not ask enough questions to satisfy myself that the machine was accurate and set correctly. I tried to test with another ammeter last PM and got no reading by a parallel method. Tonight I will pull the plug, twist it so only one male piece enters and complete the circuit with the leads.

If you put the ammeter in parallel, one of two senarios probably happened. 1) The climate control computer is fried, or 2) the ammeter is fried. Ammeters are always put in series with the load. Never parallel.

__________________
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

I went to a parts store and the counter guy could not give much info except he noted my aux pump had continuity. I went to Radio Shack and got an ammeter from them. In series I got .04 amps. The amps were + when one plug was bypassed and - when the other was. This does not seem to be much current. Is it low because the aux pump is toasted or because the cc unit is not asking for any. The cc unit just puts out hot air out of the side vents when on.

Earlier I did not get any current flow when I tested in parallel. Does this mean I did not toast the cc?

I checked my old records and in Nov 1991 my aux pump had trash in it and needed cleaning.

So- do I try soldering the cc, buy a new cc and aux pump or?

Ok a short lesson in electronics.

In your case, it sounds to me like you shorted the output of the climate control unit (hereafter refered to as the CCU) when you put the ammeter across the motor terminals. The ammeter has very low resistance and looks to the CCU as a short circuit. The motor normally draws about 1.5 amps at 12 volts so it has a resistance of about 8 ohms when it is running. But when you put the ammeter across the motor terminals (in parallel) the resistance seen by the CCU was almost zero which made the current go very high. This high current most certainly destroyed the switching element (normally a transistor) inside the CCU. Or it may have vaporized a PCB trace or both. Anyway, it is most likely that the CCU has been damaged. So now when you try to measure the current to the motor there is none because the CCU cannot supply any voltage since the circuits that do that have been destroyed.

I suspect that when you connected the ammeter across the motor terminals the first time, the needle on the meter would have jumped and pegged to the right for a very short time. You may not have noticed since it only takes miliseconds to destroy the circuit. If something fused open then there would be no current at all which is the case now. The current that you read now (40 millamps) is virtually nothing.

There is nothing that you could have done to hurt the motor. Either the motor is working properly or it is not. If the motor is not able to turn then it will draw a higher current than normal and put the CCU at risk. If you want to test the motor, unplug it from the CCU and then using clip leads connect it to the battery and see if it turns. If it does you are OK. If not it needs to be replaced. But the CCU needs professional help.

If you would like me to take a look at it, let me know. I have repaired several of them.

__________________
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

So do this;

1. Test the pump by applying 12 volts (proper polarity) and measure the running current. It should be about 1.3 amps. If it is significantly higher, replace the pump.

2. Replace or repair the CCU.

Make sure the pump is OK before you turn things on since it can destroy a good CCU.

__________________
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

After looking at Ken300Ds thread to see how to measure current, I measured current by plugging one male part into one female part of the aux coolant pump wire plugs. Then I touched one lead of the ammeter to the other male part and the other ammeter lead into the other female part. This is when I got .04 amps.

Before reading dpetryk comment I went back to the first store that originally measured 2.3 amps. The original guy was not there but another guy came out( I had to make sure the dial was set to amps and the leads were plugged in correctly...) to help me. He set up the measurement differently. He partially inserted both male parts of the connection in. Then he ran one ammeter lead to one male plug and another to a ground on the car. He got an amp reading of 2,4-2.6 amps. Is this the correct way to measure amps?

No. The meter must be in series with the motor leads. The ammeter must not be connected to ground. Ken's post assumes that you know which wire is going to ground. Which I suspect you dont. So the safe way is to put the meter in series with either lead of the motor and connect them to the original wires.

__________________
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

Thanks to all for the advice.

Dpetryk-I do not know which spade is ground on the plug as the wire away from the aux pump goes into a cover and I cannot visually trace it.

I am not sure where 1.3 amps came from. I called a someone with a shop manual and he said to put ccu in defrost mode and the reading should be less than 0.8 amps.

I will most likely take my car to a shop. I had climate control probs in Nov 1991 and the shop had much trouble figuring out the trouble. Will report what happens.

Ok I appreciate the report on what you find.

__________________
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

Today I looked at my old Suntune Dwell meter and noted it is really a multimeter. I will not have to borrow at Auto Zone/Radio Shack.. when it is dark.

The parts guy at a good shop who gave me the 0.8 amp defrost current draw info also told me the resistance on the mono valve should be 11-19 ohms. Mine tested at 15. I also got a super high amp load on the ccu-not sure if I trust it.

1.3 Amps is the MAXIMUM current that it can draw without blowing the CCU. If it is drawing anything MORE than that, replace it, or try flushing it and retest current draw. 1.4 Amps will blow the CCU.

Good luck,

Took my 300E in today. Mechanics-two actually- first thought it was ccu. After replacing "all worked well until car hits some road bumps." Now they want to replace inside and outside temp sensors. If that works they will put the old ccu in to see if it was just temp sensors.

Lead mechanic feels new ccu better than rebuilt.

See also new thread on need for timing chain rails on 300Es with 296K miles. Should I replace the timing chain(first changed at 170K miles) while the over is off?

Got car back from mechanic. He felt monovalve and evaporator temp sensor were bad. Unfortunately problem not fixed. It does respond somewhat to temperatures now but no air comes out of the bottom/foot vents and sometimes comes out of the central ones-not supposed to happen in heat mode. Also temp regulation is too hot and tends to "hunt." I guess I will have to go back.

Mechanic also said "aux coolant pump just helps the car get heat sooner-not a big deal."

I may try to solder the ccu joints myself. How do I get the pushbuttons out? 2 screws and the wood is off. 2 more screws and the plastic mounting board comes off the dash. How do I get the pushbuttons out of the mounting board? Are there screws?

You have it right Lizem, to remove the buttons, just pop them out with a small screwdriver, you may need one on each side for the real tight ones.

Make sure you mark or note the orientation of everything you remove, because, some of these pieces (maybe all) have reflectors to carry light from the bulbs and your dash will end up dark. Don't ask me how I know this.

Harry
86 300 SDL