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Old 10-22-2004, 11:40 PM
Join Date: Aug 2000
Location: Holland, MI
Posts: 1,316
Intermittent idle problem, 86 300SDL

I have an intermittent idle speed problem. 1986 300SDL, OM603, 245k miles

Symptoms: sometimes at idle, the tach will erratically jump up and down from 700 to 1,000 rpm and waver, never steady at any one speed. It's not just the tach reading funny, the engine speed is changing. I can feel the engine rpm surge, there is chassis torque reaction in gear, and I can hear (and watch) engine rpm variation in neutral

Sometimes the idle steadies out by itself. Other times, after fluctuating for a bit, the tach drops to 0 rpm and the idle slows, probably to the mechanical limit. I assume at this point the EDS has given up and disconnected itself. When I apply throttle, at about 1,000-1,500 rpm the tach again registers. The engine will then idle at 700 rpm and the jumpy idle may, or may not return.

Cold light-off is a steady 700 rpm with the few hiccups normal to the 603.

Question: is the EDS adjusting the IP in response to an erroneous idle rpm signal, or is producing the adjustment on its own? I have not been able to deduce from the wiring diagrams on the CDs if the EDS drives the tachometer, if the tachometer drives the EDS, or how they're related.

Related symptoms? The speed control will occasionally pulse, sometimes a speed-up pulse, sometimes a slow-down pulse, sometimes erratic up/down movement. This may occur a few times during my 3 1/2 hour commute, and occasionally the up/down pulses will cause the speed control to disconnect.

I usually set 65-70mph as cruise.

When it is acting unstable, if I disconnect by tapping the brake, the Resume will function and the pulsing may, or may not be back.

If it disconnects itself, the Resume will usually not work.

Once, Resume produced an erroneously high resumed speed. I don't know how high it wanted to go, since I decided when we reached 90+ with no sign of limiting that it just wanted to air itself out... The car does like to run, though...

I don't know if this is related, but I have to wonder if both systems have a common speed/rpm sensor problem. Again, I have not found the answer yet in the CDs.

Please note: these problems are intermittent, so nothing has completely failed at this point. I don't want to start removing and replacing unless I can prove it's failed...

The car remains completely driveable, it just acts funny at times...

I love intermittent electrical problems...

Thanking all in advance for help.


Last edited by Jim H; 10-22-2004 at 11:50 PM.
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Old 10-22-2004, 11:48 PM
sixto's Avatar
smoke gets in your eyes
Join Date: Jun 2000
Location: SF Bay Area
Posts: 20,806
The engine speed sensor at the back of the engine provides a signal to the EDS and tach separately.

A couple of things to try:

- disconnect the EDS signal to the IP. Idle will be low but if the engine doesn't stall, take it out and see if the surging occurs.

- remove the speed sensor, check the wires and connections, and clean it. I've never seen the speed sensor but it's somewhere behind the oil filter.

95 S420
87 300SDL
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Old 10-23-2004, 05:28 AM
Registered User
Join Date: Jul 2004
Location: Sydney, Australia
Posts: 197
Also check the idle speed control unit for it's part number. If it starts with A004........... then it has a known fault. The modifed unit is A006.........

I've got the service info on this in my toolbox at work, i'll get the exact part numbers on monday.

MB Tech
Sydney, Australia
Volvo 122S
W201 190D 2.5 manual
W202 C240
W203 C32
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Old 10-23-2004, 10:45 AM
Join Date: Aug 2000
Location: Holland, MI
Posts: 1,316
Thanks for both replies.

Once when it was surging, I disconnected the three-wire connector on the IP, which dropped the rpm to low steady idle. I saw and felt nothing obviously wrong with wires and connector, don't know about internal. I plugged it back in, high (700 rpm) idle resumed, and so did the surging after a bit. Then the problem disappeared and did not reappear for a day or so.

It's raining heavily now, and I don't have garage space, so I'll try to find and check the sensor and wiring later. It's probably hidden well, and there is a fine 18-year old layer of oil from the PO who didn't clean the engine, and I've not had the courage so far, being a bit fearful of blasting connections loose or worse, especially doing so with no way to get to spare parts... Engine cleaning is on my list, just not at the top.

I'll grab a flash light and reading glasses to see which control model I have. Fortunately, I have removed the kick panels. Unfortunately, it's because the same PO had the sound shop put speakers on the panels, and they used fiberglass resin to hold the speaker housing... What a mess, but the control module is easily accessable, in fact my wife has to be careful not to kick it.

Thanks again, and I'll post results later.

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Old 10-23-2004, 12:12 PM
Registered User
Join Date: Aug 2001
Location: Evansville, Indiana
Posts: 8,150
Replace the OVP relay while you're at it, it can cause idle control problems as well...

1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!
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Old 10-23-2004, 12:54 PM
Posts: n/a

Originally Posted by psfred
Replace the OVP relay while you're at it, it can cause idle control problems as well...


I agree with peter

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