...The A/C and Heater functions have suddenly ceased in my '87 300D...

...Both were in good working order until yesterday morning... ...I started the car and the blower should have come on but did not... ...As I drove the car and rolled the thermostat from hot to cool,... hot air continued to come from the registers,...but no blower function???... ...In checking the fuses,...# 7 was blown... ...A replaced fuse blew immediately???...and hot non blown air continues to flow from the registers......Anyone have any advice or direction???...
Thanks

Something's blowing the fuse. Suspects are auxiliary pump, monovalve, blower and AC compressor. Disconnect these components and reattach one at a time until you find the culprit. I don't remember where the auxiliary pump is in a 124. Hopefully someone will chime in or you can follow the ~1" hose from the aft end of the water pump to the monovalve. It's along that line somewhere. The monovalve is ahead of the battery, the blower is under the wiper but there's a junction block just inboard of the brake master cylinder. You can disable the compressor by removing one of the connectors at the low pressure switch on the AC drier behind the left headlight. There are two pressure switches. The low pressure switch is the one with two spade connectors directly on it. The other pressure switch has connectors on pigtails.

BTW, when that happened to me, it was the AC clutch drawing more than the 4 amps or so that it's rated at. Fuse was fine until the compressor kicked in.

Could also be bad wiring.

Sixto
87 300D

The aux pump is on the left (driver's) side of the engine bay, near the intermediate firewall, down under the w-washer reservoir. The aux pump should not cause a fuse to blow, however, as the PBU circuitry contains its own internal protection. Overloads should cause the PBU to simply shut itself off until the problem is fixed.

Fuse #7 also powers the aux fan (the one in front of the radiator) and the windshield wiper/washer. Either of those could be causing the fuse to blow. As Sixto says, disconnect everything on that fuse until you can get the fuse to not blow. Then reconnect the loads one by one until you find the culprit.

Jeremy

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"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
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...I disconnected all items mentioned above with the exception of the aux pump... ...I now cannot start the car and the fuse still blows with the ignition switch in the on position???...

...Items disconnected...

...aux fan by seperating the connector behind the radiator core support on the drivers side...

...windshield washer motor...by way of unplugging conn. to motor...


...monovalve...by way of unplugging the conn. just forward of the battery...

...blower...by way of unplugging the conn. just above and inboard of the master cylinder...

...compresser...by way of the forward most spade conn. on the sensor connected directly to the receiver dryer...


...What could I have missed???...

Thanks

Crumb! I'm not near a wiring diagram but the connector by the brake master cylinder might be for the starter rather than the blower motor. Reconnect that and I'm pretty sure you'll get the starter back.

Sixto
87 300D

Does your car have a 30 amp strip fuse in a little plastic box attached to the driver side suspension strut tower? If so, it's a separate fuse for the blower.

I forgot about the aspirator motor for the cabin temp sensor. It's tucked under the passenger side AC vent. IIRC easiest access is through the glove box cavity after removing the glove box liner.

It could be a bad key switch. Has it been worked on recently? Perhaps an off brand replacement?

Sixto
87 300D

...It's all coming back to me now... ...Some time ago I ran a temporary jumper wire from the battery to the 30 amp strip fuse because of a suspected weak ignition switch???... ...Like happens,...temporary became permanent...It may very well be that, again, the ignition s/w is at issue... ...I will still go through the process of elimination on the other #7 fused components,...and then come to grips with the ignition s/w...

...I'm getting the feeling that I should stay away from aftermarket ignition switches???... ...Is this true of all aftermarket ig. switches???... ...I can buy aftermarket @ jobber prices... ...Don't get much of a discount from Merz...
Thanks again

I don't know if all aftermarket switches are bad but there was a thread about a poorly made Uro brand switch recently.

Sixto
87 300D

...As luck would have it,...I loaded the ignition s/w with Blaster and whala'...I have a/c and heat again... ...I don't have time right now to install a new s/w as I have been editing video 10 to 12 hours a day... ...Chances are I won't do it until it malfunctions again???... ...Should probably at least carry a new one in the glove box...
Thanks again

...I finally got around to installing a new ignition s/w...Genuine Merz part... ...Everything came back to functioning with the exception of the left AC/heat register no longer blows any air and the R-12 has apparently leaked out ...

...I have found the R-12 leak and will fix it but???,... ...the next day when I started the car again no AC/Heater blower... ...In other words the number 7 fuse is blowing again and I am back to where I was before instaling the new ig/switch???...

...Anybody have any ideas???... ...Thanks...