I had weak heat on my 300D. Engine temp fine, plenty of coolant, blower fine, and heat consistent irrespective of engine speed (i.e. didn't drop when idling). But it was weak. So had to be the monovalve.

So I had the monovalve replaced. Now the heat is great....BUT, the heat now drops at idle (whereas it didn't before). From what I understand, that would be symptomatic of a bad aux water pump. But, like I say, what heat I had was perfectly consistent before I replaced the monovalve. Plus the aux water pump isn't all that old.

Any ideas?

Thanks!

__________________
14 E250 Bluetec 4Matic "Sinclair", Palladium Silver on Black, 154k miles
06 E320 CDI "Rutherford", Black on Tan, 172k mi, Stage 1 tune, tuned TCU
91 300D "Otis", Smoke Silver on Tan, 142k mi, wastegate conversion, ALDA delete

19 Honda CR-V EX 61k mi
Fourteen other MB's owned and sold
1961 Very Tolerant Wife

Yes, I have an idea.

How about you go down and test the pump with your hand and see if it's operating..........then, you don't need for us to guess whether the pump is good or not.............

How do I do that?

__________________
14 E250 Bluetec 4Matic "Sinclair", Palladium Silver on Black, 154k miles
06 E320 CDI "Rutherford", Black on Tan, 172k mi, Stage 1 tune, tuned TCU
91 300D "Otis", Smoke Silver on Tan, 142k mi, wastegate conversion, ALDA delete

19 Honda CR-V EX 61k mi
Fourteen other MB's owned and sold
1961 Very Tolerant Wife

Turn the key to the #2 position, reach in and find the aux. pump, and see if it's running.

That engine might need to be above a certain temperature so make the test on a warm engine.

While you are in there, make sure that whomever replaced the monovalve did not knock the wires off the pump.

__________________
"I have no convictions ... I blow with the wind, and the prevailing wind happens to be from Vichy"

Current
Monika '74 450 SL
BrownHilda '79 280SL
FoxyCleopatra '99 Chevy Suburban
Scarlett 2014 Jeep Cherokee
Krystal 2004 Volvo S60
Gone
'74 Jeep CJ5
'97 Jeep ZJ Laredo
Rudolf ‘86 300SDL
Bruno '81 300SD
Fritzi '84 BMW
'92 Subaru
'96 Impala SS
'71 Buick GS conv
'67 GTO conv
'63 Corvair conv
'57 Nomad

Get a 1A fuse hooked up to that aux coolant pump...

__________________
$60 OM617 Blank Exhaust Flanges
$110 OM606 Blank Exhaust Flanges
No merc at the moment

OK. I know what it looks like. Where is it in the engine compartment?

It almost sounds as if the AWP failed/got disconnected (if that's possible) when the monovalve was installed.

I don't really know much about this stuff (obviously), but I want to have as much info as possible before I take it back to the tech.

__________________
14 E250 Bluetec 4Matic "Sinclair", Palladium Silver on Black, 154k miles
06 E320 CDI "Rutherford", Black on Tan, 172k mi, Stage 1 tune, tuned TCU
91 300D "Otis", Smoke Silver on Tan, 142k mi, wastegate conversion, ALDA delete

19 Honda CR-V EX 61k mi
Fourteen other MB's owned and sold
1961 Very Tolerant Wife

That's what I'm wondering about.

__________________
14 E250 Bluetec 4Matic "Sinclair", Palladium Silver on Black, 154k miles
06 E320 CDI "Rutherford", Black on Tan, 172k mi, Stage 1 tune, tuned TCU
91 300D "Otis", Smoke Silver on Tan, 142k mi, wastegate conversion, ALDA delete

19 Honda CR-V EX 61k mi
Fourteen other MB's owned and sold
1961 Very Tolerant Wife

I'm having trouble finding the auxiliary water pump. I have picture of it, but don't know how large it is. Where is it in relation to battery, monovalve, and reservoir tank? It's a 1992 300D.

Thanks so much.

__________________
14 E250 Bluetec 4Matic "Sinclair", Palladium Silver on Black, 154k miles
06 E320 CDI "Rutherford", Black on Tan, 172k mi, Stage 1 tune, tuned TCU
91 300D "Otis", Smoke Silver on Tan, 142k mi, wastegate conversion, ALDA delete

19 Honda CR-V EX 61k mi
Fourteen other MB's owned and sold
1961 Very Tolerant Wife

If its anything like a 77-85 W123, its on the passenger side of the engine, about 10" behind the passenger headlight assembly.

__________________
-------------------------------
'85 300D, 'Lance',250k, ... winter beater (100k on franken-Frybrid 3 Valve Kit)
'82 300D, 'Tex', 228k body / 170k engine ... summer car
'83 300TD Cali Wagon 210k, wife's car

I believe I've located mine, and that's one way to describe the location.

Another way to say it would be to say it it mounted on the passenger side front "frame rail" between the engine and the radiator. At least that's where mine is...

__________________
1984 300 Coupe TurboDiesel
Silver blue paint over navy blue interior
2nd owner & 2nd engine in an otherwise
99% original unmolested car
~210k miles on the clock

1986 Ford F250 4x4 Supercab
Charcoal & blue two tone paint over burgundy interior
Banks turbo, DRW, ZF-5 & SMF conversion
152k on the clock - actual mileage unknown

It's under the Windshield Washer Tank!

You're gonna be amazed at how much and how many heater hoses are on this
chassis!

__________________
'84 300SD sold
124.128

It is indeed a marginally functioning auxiliary water pump that is the problem. I clearly needed a new monovalve, but I'm guessing the bad monovalve was masking the fact that I also had a bad AWP. Surprised that it only lasted 4-5 years. Oh well...at least these are cheap fixes.

__________________
14 E250 Bluetec 4Matic "Sinclair", Palladium Silver on Black, 154k miles
06 E320 CDI "Rutherford", Black on Tan, 172k mi, Stage 1 tune, tuned TCU
91 300D "Otis", Smoke Silver on Tan, 142k mi, wastegate conversion, ALDA delete

19 Honda CR-V EX 61k mi
Fourteen other MB's owned and sold
1961 Very Tolerant Wife

I'm not.

The typical replacement is the usual aftermarket junk. The SD unit started to give problems after six months.

Please correct me if I'm wrong, but the '86 and newer models have a fuse for the aux water pump.
Alan

__________________
DO IT RIGHT THE FIRST TIME, DON'T BE A HACK
Political Correctness is NOT part of my vocabulary
and finally FIGHT CRIME...SHOOT BACK

'82 240D Stick
'85 300D Auto
Drove my first MB at age 16 1960 300SL W190