RPM/Engine speed sensor - Page 2 - PeachParts Mercedes-Benz Forum

funola · #16 05-10-2017, 10:30 AM

Quote:

Originally Posted by Diseasel300

So it is the EGR computer in the '85 that needs to be looked into then. Question for those who know: If the EGR computer is bypassed as suggested above, will the Klima relay still function, or is it looking for that square-wave signal generated by the EGR computer? Important info since the OP wants to get the A/C working

I believe the AC and tach will still work as reported by people who have done it.

funola · #17 05-10-2017, 10:39 AM

Here's a thread with more info. AC compressor won't come on- 85 300D

I get a headache every time I have to look at those schematics to figure out how it works. What's needed is one schematic that shows the entire AC system control. It does not exist!

Be careful if you make bypass jumpers. Jumping to the wrong pins i.e power and ground can potentially smoke parts.

dcraigk · #18 05-10-2017, 02:07 PM

I took off the EGR connection after I replaced the blown OVP fuse and all my connections are clean.

So if I jump those pins and the tach works, then I have a problem with the pass through at the EGR black box?

dcraigk · #19 05-10-2017, 10:18 PM

OK, the jumper wires worked. I had a tach for the few seconds I let the engine run.

So, I don't really want to leave it with jumper wires. Do I need a new black box? Or find someone who can test the pins on the circuit board?

Diseasel300 · #20 05-10-2017, 10:58 PM

Quote:

Originally Posted by funola

I get a headache every time I have to look at those schematics to figure out how it works. What's needed is one schematic that shows the entire AC system control. It does not exist!

Try the wiring diagrams for the early-'86 300SDL if you want your brain to go into meltdown. I'm still amazed I got the HVAC system working in that car. There are pins that go to things that are completely undocumented. The late-'86 and '87 cars have things documented, but use slightly different setups. To make it worse, my car was built in June '86, so it has some "late" systems and some "early" systems. NIGHTMARE

funola · #21 05-11-2017, 02:18 PM

Quote:

Originally Posted by dcraigk

OK, the jumper wires worked. I had a tach for the few seconds I let the engine run.

So, I don't really want to leave it with jumper wires. Do I need a new black box? Or find someone who can test the pins on the circuit board?

Only if you want the EGR to work. Is your AC working now?

dcraigk · #22 05-11-2017, 02:31 PM

No AC. The compressor clutch did not engage when the wires were jumped. There may be something else going on with the AC. I'll need to have someone look at that next week.

I had an electronics repair shop look at the EGR black box. He re-soldered a few connections but he said it was out of his area of expertise.

I can buy a used one but I'd rather have this one fixed. Does anyone on here fix these?

BillGrissom · #23 05-11-2017, 11:03 PM

I would first focus on the signals to the AC clutch. Are you getting 12 V from the Klima box to turn it on? As I recall, the refrigerant pressure switch is in series with the direct clutch wiring (not via relay), I forgot if the upstream or ground side, but check the schematic. Regardless, the clutch needs 12 V across its two terminals to actuate. The pressure switch only turns on if the pressure is within bounds, something like 10 psig < P < 300 psig. You can just remove the 2 spade lugs from the switch and short them together to bypass. Low refrigerant pressure is the most common reason the AC clutch won't engage.

Re replacing the EGR box, just take your time and you will find a 1985 in a junkyard in a year or two, and real cheap. I am sure you will want to head west to the coast a few times this summer to cool off, which will put you in Orange County or San Diego which is the M-B heaven and graveyard. I would hit a few Ecology yards when we visited our son when he was at UCSD.

dcraigk · #24 05-11-2017, 11:14 PM

I'll have the refrigerant checked next week. If that's ok then I'll look at the compressor clutch.

I found a used box on eBay from a dismantler in the San Fernando Valley. It will be here Saturday. I'm excited to have a tach.

This weekend I have a lot of work to do. Lower ball joints, tie rods and axles, front calipers, new hand pump and Diesel Purge

funola · #25 05-12-2017, 12:07 PM

Are just going to keep replacing parts till it works? So far you've replaced the RPM sensor and OVP? which did not fix the problem. Next you are replacing the EGR computer? There are cheaper ways by doing some simple tests. For example, an ohm meter will tell you if the RPM sensor is good / bad. Its resistance reading is posted somewhere.

dcraigk · #26 05-12-2017, 02:26 PM

If you read my very first post in this thread, you will see that I tested the RPM sensor per the repair manual and it did not pass, so it was replaced.

When I got the car 2 weeks ago the OVP fuse was blown. That was the first thing I fixed. No change. The RPM sensor didn't pass the test, so it was changed. From what I read on here there is no good test for the OVP, so it was replaced. No change. Next came jumping the tach wires at the EGR connection. Now I got the tach to work. So it looks like the connections on the EGR are bad, so yes, I did buy a used one.

Do you have a suggestion?

funola · #27 05-12-2017, 03:14 PM

You did not says what the rpm sensor test was, only that you can't find the test anymore. It'd be interesting to test both rpm sensors and see if they give different or the same results. Some people will argue the current RPM sensor is working so it's a waste of time to test the old one. Maybe the old one was fine but had too large a gap and not producing sufficient voltage?

At this point, I would suggest removing the jumpers, check that you have power supply voltage to the EGR computer, install the EGR computer and adjust the RPM sensor gap to the lowest value of the spec. Take a DVM, set it on the lowest AC volts scale and measure the EG computer tach output. You should see 3 to 4 volts.

The AC compressor clutch is easy to test. With engine off. supply 12 V to the clutch coil. You should hear a click as the clutch gap close which you can also see. Take the Schrader valve cap off the suction side (near front of engine) and depress it momentarily. If you do not hear any pressure release, there is no refrigerant and the pressure switch on the dryer will not close thus the compressor will not engage. If you have enough refrigerant in the system and the dryer pressure switch is working, then it's a Klima relay control issue.

dcraigk · #28 05-12-2017, 05:50 PM

This is my 3rd '85 300CD but I sold the last one 15 years ago. The problems now are completely different than they were when the car was only 15 years old and I'm a bit rusty on my knowledge.

IIRC from the service manual, the RPM Sensor reading was supposed to be above at or above 4 AC volts and mine wasn't. Than manual said to replace it so I did.

Where on the EGR connection plug do I test for 12V? I guess I can try them all.

I didn't know the RPM sensor could be adjusted.

I don't have an external power source to test the AC clutch. Is the Schrader valve under the air filter housing (CA version)? I see an AC line with a red cap on it.

I'm chasing a lot of issues. I thought I would take it to the local independent MB shop for the lower ball joints and tie rods but after a near $1000 estimate, I'm going to have to do them myself. I'm still trying to find instructions on removing the spindle.

funola · #29 05-13-2017, 01:19 PM

Quote:

Originally Posted by dcraigk

...................

IIRC from the service manual, the RPM Sensor reading was supposed to be above at or above 4 AC volts and mine wasn't. Than manual said to replace it so I did.

Where on the EGR connection plug do I test for 12V? I guess I can try them all.

I didn't know the RPM sensor could be adjusted.

I don't have an external power source to test the AC clutch. Is the Schrader valve under the air filter housing (CA version)? I see an AC line with a red cap on it.
..............

The gap between the rpm sensor and the flywheel teeth is adjustable afaik. Smaller the gap, higher the output.

Look on the schematics to see what pins on EGR connector is the supply voltage.

Your battery is an excellent power source to test the compressor clutch.

Red cap is the high pressure side, blue is low. Either one can be used to check for pressure.

dcraigk · #30 05-18-2017, 11:42 PM

Well I'm just finished installing new axles, all the steering components, having the rims painted, new tires and several other things, I can focus on the AC again.

Having just installed the rpm sensor and looking at the old one, I can't see how there would be an adjustment.

Today I had a look at the AC compressor. I noticed something that looked like green coolant on the bottom. I asked my mechanic while I was there making an appointment for an alignment. He said that's dye to check for leaks. That's probably why my AC isn't working. I'll get a good idea on the whole system next week when he checks for leaks.

I did check for pressure on the low (blue) side. There was pressure but not a lot. A yellow/green foam came out when I pushed the pin in the valve.

Hopefully it just needs O rings.