#10 fuse is for the cruise control and tach. My cruise control works, my tach doesn't.

Several people have spoke of the MAF and ECU. I have found a listing for the ECU but not the MAF.

Any idea where the MAF is located. I do not have a turbo. It is a 190D 2.5 non-turbo. Does my car even have one?

OK, I think I have made some progress.

With the car off, I removed the ELR idle switch located in the engine compartment passenger side next to the Over Voltage Protection relay, located behind the small plastic shield behind the battery.

With the car running, I reconnected the ELR switch and the tach worked and the idle smoothed out.

I turned off the car and removed that switch. Started car and the tach did not work and the car had a rough idle....as expected.

I reinserted the switch again just to be sure and the tach worked again and idle smoothed out beautifully. Fixed....or so I thought.

Thinking I located my problem with the ELR switch, I went back out to start the car after letting it sit in the cold for 1/2 hour and now the tach doesn't work and idle is more rough.

The ELR part # 005 545 4732 BOSCH 0 281 001 038
The part is $450.00 UGH!

I pulled all relays and switches and cleaned the contacts. Tested OVP works as it should.

Once again, I have not tested compression, but I have very little blow-by, little if any oil consumption and loads of power. Only smokes a little on start up when I deal with the rough idle and then it smooths out when it warms up. When warm it starts with just a bump of the key. What would this have to do with the tach and a/c compressor not working anyway?

I am at a loss.

Go with what you have found so far as the most direct cause and effect. This seems to be in the ELR circuit.

Your op/inop condition is seemingly temperature related per your numerous tests, with the last series of test being very short term running duration. With that in mind, it seems like an intermittent electrical issue, temp-related, for me it could indicate a poor solder joint, possibly in the ELR or OVP. If you can solder, opening the ELR is very easy, remove the silicone seal at the bottom of the box, gently pry the sides of the box away from the base and pull out the base (pins side) and withdraw it from the cover. Reflow the solder joints from the main pins to the board, inspect the rest for any damage. Retest.

If your OVP is old, and the condition persists, do the same to the OVP, although it will be a metal case with some dimpled tangs holding the assembly together rather than a plastic enclosure. Retest

Hopefully this will resolve it. No need to throw parts at it just yet. While you are at it, make sure to clean the ground straps to the chassis, should be a bunch of them next to the battery side where the OVP/ELR are, bundle of wires bolted to the strut tower or nearby. Clean, apply dielectric grease and retighten.

__________________
Stable Mates:
1987 300TD 310K mi (Hans)
2008 Jeep Grand Cherokee OM642 165k mi (Benzrokee)

Thanks, you have been a great help. I will try this and hopefully it will fix the issue.

I hope I am not premature on this declaration but..... FIXED! (For Now)

I did as 87tdwagen recommended. Removed the ELR and opened it up to check the connections. It was dirty so I lightly cleaned the contacts with a brass brush.
After looking it over with a magnifying glass and strong light, I noticed one of the connections had lost its solder completely. I soldered a repair. They are very small solders in there so I had to be careful not to join to another connection. It was more or less just a drip to fill the hole where the connection was.

I took the car out and it started a little rough with no tach. About 4 miles in the tach came on. I parked the car and finished my shopping, started it up and the tach was dead. It came on once again at about the 4 mile mark of driving.

I started the car several more times when I returned home and the idle was very smooth and the tach continued to work.

In addition to the ELR, I decided to pull the EGR, Klima and OVP relays/switches and inspect them. I
used the magnifying glass and brass brush to lightly clean all of the contacts.

I re-soldered all of the contacts in the OVP. I didn't add solder, just reheated what was in there. It is a little easier to solder in there because the contacts are further apart and the chance that you are going to join anything is much less. The EGR and Klima relay/switch contacts looked good.

I reinserted all of the relays/switches and everything is working fine. I let the car cool down and it started with just a bump of the key after 1 glow of the plugs. The idle is smooth.

Hopefully things will stay this way and it is not a gremlin that is taking some time off.

Thanks everyone for your help.

Glad to hear that it is working out for you. These electrics are getting long in tooth, but luckily are still serviceable and often the cause of erratic symptoms, so worth going thru and checking or reflowing the connections.

Best of luck and hope she stays smooth for quite a while

__________________
Stable Mates:
1987 300TD 310K mi (Hans)
2008 Jeep Grand Cherokee OM642 165k mi (Benzrokee)

Yeah I’ve saved a lot of modules reflowing contacts with my soldering iron. The solder gets that cracked up gray dusty look. I think it is a result of thermal stress cycling. Some of these old analog circuits draw a bit of power on startup and get hot. Then they get shut down and are allowed to cool. After thirty years the joints get the classic ring failure. A lot of people call it cold solder here but it isn’t really cold solder which is a contact that wasn’t heated sufficiently enough during manufacturing. It’s a fracture in the solder bump.

I actually like adding fresh solder as the old stuff looks really degraded. Some rosin flux liquid also helps freshen it up before reflowing.

__________________
79 300TD “Old Smokey” AKA “The Mistake” (SOLD)
82 240D stick shift 335k miles (SOLD)
82 300SD 300k miles
85 300D Turbodiesel 170k miles