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98 S70 T5M: Diagnosing Cruise Control

Help, Advice and DIY Tutorials on Volvo's extremely popular car line -- Volvo's 1990s "bread and butter" cars -- powered by the ubiquitous and durable Volvo inline 5-cylinder engine.

1992 - 1997 850, 850 R, 850 T5-R, 850 T5, 850 GLT
1997 - 2000 S70, S70 AWD
1997 - 2000 V70, V70 AWD
1997 - 2000 V70-XC
1997 - 2004 C70

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WhatAmIDoing
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98 S70 T5M: Diagnosing Cruise Control

Post by WhatAmIDoing » Wed Jun 12, 2019 4:20 pm

I have a 1000+ mi roadtrip coming up and would really like to have cruise control (CC) in one of my Volvos. After admitting defeat on the 99, I've started diagnosing the "simpler" vacuum operated 98. I am following the Volvo 850 service manual for 1995-96 that can be found uploaded all over the internet.

Since I do not have the diagnostic flashing light port under the hood, and I have no codes via OBDII, I went ahead with testing as described in the manual as if codes were present. I could not find my small wire probes, so I did not do any backprobing as described.

Selector Switch Test - not tested

Brake & Clutch Pedal Check
1) not tested
2) ground to 9T, battery voltage should be present - battery voltage present

Selector SET+ Check
1) not tested
2) ground to 4T, switch ON, pressing SET+ - battery voltage present - replace CC selector switch if battery voltage present

Selector SET- Check
1) not tested
2) ground to 8T, switch ON, pressing SET- - battery voltage present - replace CC selector switch if battery voltage present

Selector RESUME Check
1) not tested
2) ground to 6T, switch ON, pressing RESUME - battery voltage present - replace CC selector switch if battery voltage present

Neutral Start Switch Circuit Disengagement - not tested

Vacuum System Check
1) made jumper wires and connected as described
2) can hear vacuum regulator click when connecting jumper
2) pump turns on, accelerator presses in slowly at first than more quickly, only presses accelerator down about 2/3 of full travel
3) turn pump off, pedal immediately releases (servo not holding vacuum?)
4) voltmeter connected between 3S and 1S - battery voltage present when switch ON key in POSII
5) ohm meter between 7T and 7S for vacuum motor, 6.5 ohms present, should be 17 ohms
5) ohm meter between 7T and 9S, 101.4 ohms, in spec with 100 ohms
5) ohm reading not taken at CC pump
6) jumper wires reinstalled, pump stopped, no leaks heard, pedal / servo retracts immediately (vacuum leak somewhere?)
7) with pump on, pressing brake pedal in a fair bit causes accelerator to release (vacuum switch needs adjusting)
8) with pump on, just barely tapping clutch pedal causes accelerator to release (vacuum switch needs adjusting)

No other tests performed. When I find my probes, will probe wires as described in manual and report findings.


'98 S70 T5M - 270,000+mi - forever a project
'99 S70 "AWD" - 218,000+mi - project
Knows enough to be dangerous :wink:

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Re: 98 S70 T5M: Diagnosing Cruise Control

Post by WhatAmIDoing » Wed Jun 12, 2019 9:51 pm

So I need some input on this one. Based on the diagnostic manual, It sounds like the CC selector switch is bad. One question I have though is what voltage should I see when I activated the switches, since battery voltage seems to be the wrong answer?

It also appears I have a small vacuum leak affecting the servo, since it should remain contracted (engaged) when the pump is turned off? Is there a check valve somewhere? what is the proper routing for all the vacuum lines for the CC? What else should I look at if the valves for the brake and clutch are good?


'98 S70 T5M - 270,000+mi - forever a project
'99 S70 "AWD" - 218,000+mi - project
Knows enough to be dangerous :wink:

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Re: 98 S70 T5M: Diagnosing Cruise Control

Post by Azik09 » Thu Jun 13, 2019 5:40 am

At first, check the electrical switches above brake and clutch pedals. I don't remember which contact of CC module they are connected to. CC module will not activate vacuum pump until it "see" pedals fully released.


If my English is bad, please correct me :oops:

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Re: 98 S70 T5M: Diagnosing Cruise Control

Post by WhatAmIDoing » Thu Jun 13, 2019 8:23 pm

Azik09 wrote:
Thu Jun 13, 2019 5:40 am
At first, check the electrical switches above brake and clutch pedals. I don't remember which contact of CC module they are connected to. CC module will not activate vacuum pump until it "see" pedals fully released.
I believed I checked the right contacts, and the CC was seeing both pedals as fully released, but I will re-check.

I have a sinking feeling this is going to turn into me replacing the whole CC system and then giving up because it still doesn't work.


'98 S70 T5M - 270,000+mi - forever a project
'99 S70 "AWD" - 218,000+mi - project
Knows enough to be dangerous :wink:

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Re: 98 S70 T5M: Diagnosing Cruise Control

Post by WhatAmIDoing » Fri Jun 14, 2019 10:31 pm

Not sure what I did, but I got CC to kinda work for a little while. Managed to maintain 45mph for a couple miles (with help). Had to keep tapping the gas pedal. I definitely have a vacuum leak somewhere. CC switch also seems to be kinda iffy.

Does anyone know what the vacuum line routing is for the CC? I still haven't been able to figure it out while contorted under the dash.


'98 S70 T5M - 270,000+mi - forever a project
'99 S70 "AWD" - 218,000+mi - project
Knows enough to be dangerous :wink:

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Re: 98 S70 T5M: Diagnosing Cruise Control

Post by abscate » Sat Jun 15, 2019 4:00 am

I have spare stalks with CC free. Paying forward from Evan.
Shipping free , also paying forward

If you have bucks pay it back to Evan for his upcoming engine swap.
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