Im troubleshooting some climate control issues with my 86 300sdl, I went to test the aux water pump and found some strange things going on. For one with the connector plugged in the water pump is not working, even though the wire coming from the connector that goes directly to the pump is getting 12v. Oddly, when I hook up my own 12v power source to that same connector I can hear the pump running. It seems the pump works, but only when I power it, even though it is getting power from the connector. How does this make any sense?

Yes, most of the switching is done on the ground side in MB circuitry.

Are you using a digital volt meter to test? If so, they are deceiving because they don't load the circuit down like an analog meter or a incandescent test light.

Are you checking for power and ground?

Loading the circuit is important when testing a circuit that has a bad connection. A circuit with a bad connection can show good voltage when unloaded but when you apply a load, it can drop to near zero.

Two local shops got caught off guard on the same car using a digital meter to test for power, this lead to lots of parts being changed in an attempt to get the car running.

Car has an occasional stall / no run condition that eventually turned into a hard failure. The check engine light would not illuminate and scan tool not communicate. They checked for power and ground at the engine computer and it appeared to be OK. The computer was changed and still no run. Fuse panel junction block , relays, bunches of probing around netted nothing across two shops.

I do some consulting work for the 2nd shop and was called in to have a look. My first step is always: Give me the history / what has been done to the car up to this point.

Second is: Give me all the old parts that have been removed from the car. ( I've seen too many parts with subtle differences installed on the wrong car. )

Third: Give me access to wiring diagrams / tech info. ( Wiring diagrams are absolutely critical as are component locators. Electronic boxes can be stuffed in many places and it can take forever to find them. )

Fourth: Turn off that darn radio blaring in the shop. ( They know by now that if I'm working in their shop, the radio goes away but they haven't quite put things together that they will have a easier time investigating complicated problems if they were not listening to it blare away too. )

All the old parts went back on the car and I started from zero. Studying the wiring diagram showed where power and ground entered the engine computer. They tested good with the computer unplugged and with the computer plugged in. However, with the computer plugged in and key turned _on_, direct battery feed to the computer went to near zero.

The battery feed wire was good enough to keep the computer memory alive, but when the key was turned on and more of the computer woke up, the load was too great for the failing connection. Using a paper clip as a jumper from key power to battery power and the car now runs. Remove paper clip and the engine stalls.

Probing around the under hood harness showed no visible breaks. Can't see all of the in car harness side because it goes behind the HVAC box. Inspecting the connectors where the battery feed originates and the computer side netted no bad areas. I directed the shop to cut the wire at each end and run a new wire. I don't like doing this without finding the bad spot because if one wire is rubbing through, others might be too. Given the HVAC box ( and dash ) would need to come out, I decided to risk it as long as the customer was informed that future failures may require $$ to pull the HVAC box.

Total time spent on the car less running a new wire? Sub 1 hr. No they are not all this easy but it goes to show that test equipment must give proper results otherwise much time will be lost.

From the thread title, we might have thought this was about a pump.

If the above is directed towards me, the OP was having difficulty testing an electrical device. I provided an example of the perils of inaccurate / misused test equipment.

Regardless, your post helps the OP how?

Just out of curiosity, roughly how much current does an analog multimeter draw?

I've never measured it but in a circuit that has very low amperage capability, it can affect the reading. This is typically expressed as " Input Impedance " , higher = less loading of the circuit.

Generically, an analog volt meter means no power native to the meter is required for it to function. In the days of predigital, for low amperage capability circuits, you could get a vacuum tube volt meter ( VTVM ),Vacuum Tube Volt Meters (VTVM) Later Field Effect Transistor ( FET ) meters were closer to modern digital but still used a needle for display. These were more akin to the very light loading an oscilloscope would produce.

A moving needle is handy to gauge voltage fluctuations where a digital is a blur of numbers making it somewhat difficult to judge are they increasing or decreasing. ( Think speedo or tach that is digital only, many displays have an additional bar graph to help visualization. )

There has always been a "don't use an analog meter to test oxygen sensors because the present too great of a amperage load and can damage the sensor" In this case you want to know the actual voltage reading without any / very low current flow.

There is a way around the digital doesn't load the circuit and can produce false readings. I regularly use a stray voltage eliminator on AC powered industrial equipment so the meter isn't fooled by capacitive coupling of the wiring or component leakage. Fluke SV225

For automotive 12 volt systems I stick with an incandescent test light because it is quick and I've even gone so far as to use a old glass headlight for larger loads.

A low cost, field use analog VM (volt meter) may be expected to have an internal resistance, on the scale of about 20VDC, of 1/2 - 1Mohm (500,000 - 1,000,000 ohm). At 1/2Mohm, when measuring a voltage of ~12VDC, the current thru the meter would be .000024A. In automotive power circuits (a pump electric motor for instance) that is a rather small current.

Good to know! Thanks. I have always used a DMM, and usually try to grab resistance readings if anything seems fishy, but I will keep a test lamp around as a back up. I am also pretty careful to clean any contacts I open when I am troubleshooting, but that doesn't mean there couldnt be a worn wire somewhere.
The more you know (star)

Some reading for Frank, see link. Dual impedance digital multimeters - What's the point?

Fluke is a major manufacturer of meters and electrical test equipment. The article does not specifically state AC circuits but capacitive coupling only occurs on AC circuits.

It does talk about leakage and feeding around an open fuse. In theory one could measure battery voltage then any reading below battery could be considered stray voltage but that complicates things since battery voltage is variable based on load. Applying a decent load ( like with an incandescent test light ) to the circuit is always better to test circuit integrity.

Correct, and most circuits in modern cars also use such "low side switching", because transistors are better at that. Most mechanical relay contacts and manual switches use "high side switching". This endlessly confuses people, especially when external voltage regulators changed to transistor type.

Some here claim the aux pump is essential. I no longer have one in both my 300D's, and get heat fine in the winter. Perhaps in northern Minnesota one could appreciate it. If you don't add a fuse in the wiring, it can melt the push-button switch in your dash, which is a suspect in several cabin fires.

__________________
1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans

Bill,
Good info. What amperage fuse would you recommend?

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Yes it can.
http://www.peachparts.com/shopforum/diesel-discussion/241899-i-lost-all-electrical-fire-high-up-dash.html


Charlie

__________________
there were three HP ratings on the OM616...

1) Not much power
2) Even less power
3) Not nearly enough power!! 240D w/auto

Anyone that thinks a 240D is slow drives too fast.

80 240D Naturally Exasperated, 4-Spd 388k DD 150mph spedo 3:58 Diff

We are advised to NOT judge ALL Muslims by the actions of a few lunatics, but we are encouraged to judge ALL gun owners by the actions of a few lunatics. Funny how that works

I am running a 2amp fuse for the aux water pump. I think I was trying to find a 1.5amp but couldnt find any locally. I grabbed both sides of the connector with some wire attached from a junkyard car and built a little extension cable with a fuseholder on the 12v side by dismantling and desoldering the pins from the connector and resoldering the holder inline.

Someone was selling these adapters at one point, but they are really easy to build if you have soldering skills. Or just add a fuseholder inline in the motors harness or your car wiring, depending on how picky you are about keeping your car able to be reset to stock condition.

Feel free to let me know if you have any questions.

Edit: Here is a thread that shows my custom holder (part way down)
http://www.peachparts.com/shopforum/diesel-discussion/377619-85-w123-ac-clutch-relay-diagnostics.html

pilot . Did you ever solved your problem?